Hypothetical types of biochemistry

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False-color Cassini radar mosaic of Titan's north polar region; the blue areas are lakes of liquid hydrocarbons.

"The existence of lakes of liquid hydrocarbons on Titan opens up the possibility for solvents and energy sources that are alternatives to those in our biosphere and that might support novel life forms altogether different from those on Earth."—NASA Astrobiology Roadmap 2008

Several forms of biochemistry are agreed to be scientifically viable, but are not proven to exist at this time. The kinds of living organisms known on Earth, as of 2026, all use carbon compounds for basic structural and metabolic functions, water as a solvent, and deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) to define and control their form. If life exists on other celestial bodies (planets, moons), it may be chemically similar, though it is also possible that there are organisms with quite different chemistries – for instance, involving other classes of carbon compounds, compounds of another element, and/or another solvent in place of water.

The possibility of life-forms being based on "alternative" biochemistries is the topic of an ongoing scientific discussion, informed by what is known about extraterrestrial environments and about the chemical behaviour of various elements and compounds. It is of interest in synthetic biology and is also a common subject in science fiction.

The element silicon has been much discussed as a hypothetical alternative to carbon. Silicon is in the same group as carbon on the periodic table and, like carbon, it is tetravalent. Hypothetical alternatives to water include ammonia, which, like water, is a polar molecule, and cosmically abundant; and non-polar hydrocarbon solvents such as methane and ethane, which are known to exist in liquid form on the surface of Titan.

Overview of hypothetical types of biochemistry

Overview of hypothetical types of biochemistry

Type	Basis	Brief description	Details
Alternative-chirality biomolecules	Alternative biochemistry	Mirror image biochemistry	Alternative-chirality biomolecules refer to biomolecules with reflected chirality. In known Earth-based life, amino acids are almost universally of the L form and sugars are of the D form; however, the mirror-image forms could equally form the basis for alternative biochemistries. Synthetic biologists have proposed creating mirror-image versions of existing organisms, using entirely mirror-image biochemistry; these would behave identically to their template organisms except when interacting with existing biomolecules. Mirror-image microorganisms would be resistant to the immune systems of existing organisms. Scientists have stated concern over this risk and discouraged the creation of them.
Alternative nucleic acids	Alternative biochemistry	Different genetic storage	Xeno nucleic acids (XNA) may possibly be used in place of RNA or DNA. XNA is the general term for a nucleic acid with an altered sugar backbone. Examples of XNA are: TNA, which uses threose; HNA, which uses 1,5-anhydrohexitol; GNA, which uses glycol; CeNA, which uses cyclohexene; LNA, which utilizes a form of ribose that contains an extra linkage between its 4' carbon and 2' oxygen; FANA, which uses arabinose, but with a single fluorine atom attached to its 2' carbon; PNA, which uses, in place of sugar and phosphate, N-(2-aminoethyl)-glycine units connected by peptide bonds. In comparison, Hachimoji DNA changes the base pairs instead of the backbone. These new base pairs are P (2-Aminoimidazo[1,2a][1,3,5]triazin-4(1H)-one), Z (6-Amino-5-nitropyridin-2-one), B (Isoguanine), and S (rS=Isocytosine for RNA, dS=1-Methylcytosine for DNA).
Ammonia biochemistry	Non-water solvents	Ammonia-based life	Ammonia is relatively abundant in the universe and has chemical similarities to water. The possible role of liquid ammonia as an alternative solvent for life is an idea dating back to 1954 at least, when J. B. S. Haldane raised the topic at a symposium about life's origin.
Arsenic biochemistry	Alternative biochemistry	Arsenic-based life	Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms.
Borane biochemistry (Organoboron chemistry)	Alternative biochemistry	Borane-based life	Boranes are dangerously explosive in Earth's atmosphere but would be more stable in a reducing atmosphere (environment), one with no oxygen or other oxidizing gases, and which may contain actively reductant gases such as hydrogen, carbon monoxide, methane, and hydrogen sulfide. Molecular structures containing alternating boron and nitrogen atoms share some properties with hydrocarbons. However, boron is far rarer in the universe than its neighbours of carbon, nitrogen, and oxygen.
Cosmic necklace-based biology	Nonplanetary life	Non-chemical life	In 2020, Luis A. Anchordoqu and Eugene M. Chudnovsky hypothesized that life composed of magnetic semipoles connected by cosmic strings could evolve inside stars.
Dusty plasma-based biology	Nonplanetary life	Non-chemical life	In 2007, Vadim N. Tsytovich and colleagues proposed that lifelike behaviours could be exhibited by dust particles suspended in a plasma, under conditions that might exist in space.
Extremophiles	Alternative environment	Life in variable environments	It would be biochemically possible to sustain life in environments that are only periodically consistent with life as we know it, such as extremely high or low temperatures, pressures, or pH; or the presence of high levels of salt or nuclear radiation.
Heteropoly acid biochemistry	Alternative biochemistry	Heteropoly acid-based life	Various metals can form complex structures with oxygen, such as heteropoly acids.
Hydrogen fluoride biochemistry	Non-water solvents	Hydrogen fluoride-based life	Hydrogen fluoride has been considered as a possible solvent for life by scientists such as Peter Sneath.
Hydrogen sulfide biochemistry	Non-water solvents	Hydrogen sulfide-based life	Hydrogen sulfide is a chemical analog of water but is less polar and a weaker inorganic solvent.
Methane biochemistry (Azotosome)	Non-water solvents	Methane-based life	Methane is relatively abundant in the Solar System and the Universe and is known to exist in liquid form on Titan, the largest moon of Saturn. Though highly unlikely, it is considered to be possible for Titan to harbour life. If so, it will most likely be methane-based life.
Non-green photosynthesizers	Other speculations	Alternate plant life	Physicists have noted that, although photosynthesis on Earth generally involves green plants, a variety of other-coloured plants could also support photosynthesis, essential for most life on Earth, and that other colours might be preferred in places that receive a different mix of stellar radiation than Earth. In particular, retinal is capable of, and has been observed to, perform photosynthesis. Bacteria capable of photosynthesis are known as microbial rhodopsins. A plant or creature that uses retinal photosynthesis is always purple.
Shadow biosphere	Alternative environment	A hidden life biosphere on Earth	A shadow biosphere is a hypothetical microbial biosphere of Earth that uses radically different biochemical and molecular processes than currently known life. It could exist, for example, deep in the crust or sealed in ancient rocks.
Silicon biochemistry (Organosilicon)	Alternative biochemistry	Silicon-based life	Like carbon, silicon can create molecules that are sufficiently large to carry biological information; however, the scope of possible silicon chemistry is far more limited than that of carbon.
Silicon dioxide biochemistry	Non-water solvents	Silicon dioxide-based life	Gerald Feinberg and Robert Shapiro have suggested that molten silicate rock could serve as a liquid medium for organisms with a chemistry based on silicon, oxygen, and other elements such as aluminium.
Sulfur biochemistry	Alternative biochemistry	Sulfur-based life	The biological use of sulfur as an alternative to carbon is purely hypothetical, especially because sulfur usually forms only linear chains rather than branched ones.

Shadow biosphere

Main article: Shadow biosphere

The Arecibo message (1974) transmitted information into space about basic chemistry of Earth life.

A shadow biosphere is a hypothetical microbial biosphere of Earth that uses radically different biochemical and molecular processes than currently known life. Although life on Earth is relatively well-studied, the shadow biosphere may still remain unnoticed because the exploration of the microbial world targets primarily the biochemistry of the macro-organisms.

Alternative-chirality biomolecules

Perhaps the least unusual alternative biochemistry would be one with differing chirality of its biomolecules. In known Earth-based life, amino acids are almost universally of the L form and sugars are of the D form. Molecules using D amino acids or L sugars may be possible; molecules of such a chirality, however, would be incompatible with organisms using the opposing chirality molecules. Amino acids which chirality is opposite to the norm are found on Earth, and these substances are generally thought to result from decay of organisms of normal chirality. However, physicist Paul Davies speculates that some of them might be products of "anti-chiral" life.

It is questionable, however, whether such a biochemistry would be truly alien. Although it would certainly be an alternative stereochemistry, molecules that are overwhelmingly found in one enantiomer throughout the vast majority of organisms can nonetheless often be found in another enantiomer in different (often basal) organisms such as in comparisons between members of Archaea and other domains, making it an open topic whether an alternative stereochemistry is truly novel.

Non-carbon-based biochemistries

On Earth, all known living things have a carbon-based structure and system. Scientists have speculated about the advantages and disadvantages of using elements other than carbon to form the molecular structures necessary for life, but no one has proposed a theory employing such atoms to form all the necessary structures. However, as Carl Sagan argued, it is very difficult to be certain whether a statement that applies to all life on Earth will turn out to apply to all life throughout the universe. Sagan used the term "carbon chauvinism" for such an assumption. He regarded silicon and germanium as conceivable alternatives to carbon (other plausible elements include but are not limited to palladium and titanium); but, on the other hand, he noted that carbon does seem more chemically versatile and is more abundant in the cosmos. Norman Horowitz devised the experiments to determine whether life might exist on Mars that were carried out by the Viking Lander of 1976, the first U.S. mission to successfully land a probe on the surface of Mars. Horowitz argued that the great versatility of the carbon atom makes it the element most likely to provide solutions, even exotic solutions, to the problems of survival on other planets. He considered that there was only a remote possibility that non-carbon life forms could exist with genetic information systems capable of self-replication and the ability to evolve and adapt.

Silicon biochemistry

See also: Organosilicon

Structure of silane, analogue of methane

Structure of the silicone polydimethylsiloxane (PDMS)

Marine diatoms – carbon-based organisms that extract silicon from sea water, in the form of its oxide (silica) and incorporate it into their cell walls

The silicon atom has been much discussed as the basis for an alternative biochemical system, because silicon has many chemical similarities to carbon and is in the same group of the periodic table. Like carbon, silicon can create molecules that are sufficiently large to carry biological information.

However, silicon has several drawbacks as a carbon alternative. Carbon is ten times more cosmically abundant than silicon, and its chemistry appears naturally more complex. By 1998, astronomers had identified 84 carbon-containing molecules in the interstellar medium, but only 8 containing silicon, of which half also include carbon. Even though Earth and other terrestrial planets are exceptionally silicon-rich and carbon-poor (silicon is roughly 925 times more abundant in Earth's crust than carbon), terrestrial life bases itself on carbon. This might be due to the comparatively lower diversity of functional groups observed in naturally-occurring Silicon-based polymers.

Relative to carbon, silicon has a much larger atomic radius, and forms much weaker covalent bonds to atoms — except oxygen and fluorine, with which it forms very strong bonds. Almost no multiple bonds to silicon are stable, although silicon does exhibit varied coordination number. Silanes, silicon analogues to the alkanes, react rapidly with water, and long-chain silanes spontaneously decompose. Consequently, most terrestrial silicon is "locked up" in silica, and not a wide variety of biogenic precursors.

Silicones, which alternate between silicon and oxygen atoms, are much more stable than silanes, and may even be more stable than the equivalent hydrocarbons in sulfuric acid-rich extraterrestrial environments. Alternatively, the weak bonds in silicon compounds may help maintain a rapid pace of life at cryogenic temperatures. Polysilanols, the silicon homologues to sugars, are among the few compounds soluble in liquid nitrogen.

All known silicon macromolecules are artificial polymers, and so "monotonous compared with the combinatorial universe of organic macromolecules". Even so, some Earth life uses biogenic silica: diatoms' silicate skeletons. A. G. Cairns-Smith hypothesized that silicate minerals in water played a crucial role in abiogenesis, in that biogenic carbon compounds formed around their crystal structures. Although not observed in nature, carbon–silicon bonds have been added to biochemistry under directed evolution (artificial selection): a cytochrome c protein from Rhodothermus marinus has been engineered to catalyse new carbon–silicon bonds between hydrosilanes and diazo compounds.

Other exotic element-based biochemistries

See also: Organoboron chemistry

• Boranes are dangerously explosive in Earth's atmosphere, but would be more stable in a reducing atmosphere. However, boron's low cosmic abundance makes it less likely as a base for life than carbon.
• Various metals, together with oxygen, can form very complex and thermally stable structures rivalling those of organic compounds; the heteropoly acids are one such family. Some metal oxides are also similar to carbon in their ability to form both nanotube structures and diamond-like crystals (such as cubic zirconia). Titanium, aluminium, magnesium, and iron are all more abundant in Earth's crust than carbon. Metal-oxide-based life could therefore be a possibility under certain conditions, including those (such as high temperatures) at which carbon-based life would be unlikely. The Cronin group at Glasgow University reported self-assembly of tungsten polyoxometalates into cell-like spheres. By modifying their metal oxide content, the spheres can acquire holes that act as porous membrane, selectively allowing chemicals in and out of the sphere according to size.
• Sulfur is also able to form long-chain molecules, but suffers from the same high-reactivity problems as phosphorus and silanes. The biological use of sulfur as an alternative to carbon is purely hypothetical, especially because sulfur usually forms only linear chains rather than branched ones. (The biological use of sulfur as an electron acceptor is widespread and can be traced back 3.5 billion years on Earth, thus predating the use of molecular oxygen. Sulfur-reducing bacteria can utilize elemental sulfur instead of oxygen, reducing sulfur to hydrogen sulfide.)

Arsenic as an alternative to phosphorus

See also: GFAJ-1

While arsenic, which is chemically similar to phosphorus, is poisonous for most life forms on Earth, it is incorporated into the biochemistry of some organisms. Some marine algae incorporate arsenic into complex organic molecules such as arsenosugars and arsenobetaines. Fungi and bacteria can produce volatile methylated arsenic compounds. Arsenate reduction and arsenite oxidation have been observed in microbes (Chrysiogenes arsenatis). Additionally, some prokaryotes can use arsenate as a terminal electron acceptor during anaerobic growth and some can utilize arsenite as an electron donor to generate energy.

It has been speculated that the earliest life forms on Earth may have used arsenic biochemistry in place of phosphorus in the structure of their DNA. A common objection to this scenario is that arsenate esters are so much less stable to hydrolysis than corresponding phosphate esters that arsenic is poorly suited for this function.

The authors of a 2010 geomicrobiology study, supported in part by NASA, have postulated that a bacterium named GFAJ-1, collected in the sediments of Mono Lake in eastern California, can employ such 'arsenic DNA' when cultured without phosphorus. They proposed that the bacterium may employ high levels of poly-β-hydroxybutyrate or other means to reduce the effective concentration of water and stabilize its arsenate esters. This claim was heavily criticized almost immediately after publication for the perceived lack of appropriate controls. Other authors were unable to reproduce their results and showed that the study had issues with phosphate contamination, suggesting that the low amounts present could sustain extremophile lifeforms. The 2010 paper was retracted in 2025.

Non-water solvents

In addition to carbon compounds, all currently known terrestrial life also requires water as a solvent. This has led to discussions about whether water is the only liquid capable of filling that role. The idea that an extraterrestrial life-form might be based on a solvent other than water has been taken seriously in recent scientific literature by the biochemist Steven Benner, and by the astrobiological committee chaired by John A. Baross. Solvents discussed by the Baross committee include ammonia, sulfuric acid, formamide, hydrocarbons, and (at temperatures much lower than Earth's) liquid nitrogen, or hydrogen in the form of a supercritical fluid.

Water as a solvent limits the forms biochemistry can take. For example, Steven Benner, proposes the polyelectrolyte theory of the gene that claims that for a genetic biopolymer such as DNA to function in water, it requires repeated ionic charges. If water is not required for life, these limits on genetic biopolymers are removed.

Carl Sagan once described himself as both a carbon chauvinist and a water chauvinist; however, on another occasion he said that he was a carbon chauvinist but "not that much of a water chauvinist". He speculated on hydrocarbons, hydrofluoric acid, and ammonia as possible alternatives to water.

Some of the properties of water that are important for life processes include:

• A complexity which leads to a large number of permutations of possible reaction paths including acid–base chemistry, H⁺ cations, OH⁻ anions, hydrogen bonding, van der Waals bonding, dipole–dipole and other polar interactions, aqueous solvent cages, and hydrolysis. This complexity offers a large number of pathways for evolution to produce life, many other solvents have dramatically fewer possible reactions, which severely limits evolution.
• Thermodynamic stability: the free energy of formation of liquid water is low enough (−237.24 kJ/mol) that water undergoes few reactions. Other solvents are highly reactive, particularly with oxygen.
• Water does not combust in oxygen because it is already the combustion product of hydrogen with oxygen. Most alternative solvents are not stable in an oxygen-rich atmosphere, so it is highly unlikely that those liquids could support aerobic life.
• A large temperature range over which it is liquid.
• High solubility of oxygen and carbon dioxide at room temperature supporting the evolution of aerobic aquatic plant and animal life.
• A high heat capacity (leading to higher environmental temperature stability).
• Water is a room-temperature liquid leading to a large population of quantum transition states required to overcome reaction barriers. Cryogenic liquids (such as liquid methane) have exponentially lower transition state populations which are needed for life based on chemical reactions. This leads to chemical reaction rates which may be so slow as to preclude the development of any life based on chemical reactions.
• Spectroscopic transparency allowing solar radiation to penetrate several meters into the liquid (or solid), greatly aiding the evolution of aquatic life.
• A large heat of vaporization leading to stable lakes and oceans.
• The ability to dissolve a wide variety of compounds.
• The solid (ice) has lower density than the liquid, so ice floats on the liquid. This is why bodies of water freeze over but do not freeze solid (from the bottom up). If ice were denser than liquid water (as is true for nearly all other compounds), then large bodies of liquid would slowly freeze solid, which would not be conducive to the formation of life.

Water as a compound is cosmically abundant, although much of it is in the form of vapor or ice. Subsurface liquid water is considered likely or possible on several of the outer moons: Enceladus and Europa (where geysers have been observed), Titan, and Ganymede. Earth and Titan are the only worlds currently known to have stable bodies of liquid on their surfaces.

Not all properties of water are necessarily advantageous for life, however. For instance, water ice has a high albedo, meaning that it reflects a significant quantity of light and heat from the Sun. During ice ages, as reflective ice builds up over the surface of the water, the effects of global cooling are increased.

There are some properties that make certain compounds and elements much more favourable than others as solvents in a successful biosphere. The solvent must be able to exist in liquid equilibrium over a range of temperatures the planetary object would normally encounter. Because boiling points vary with the pressure, the question tends not to be does the prospective solvent remain liquid, but at what pressure. For example, hydrogen cyanide has a narrow liquid-phase temperature range at 1 atmosphere, but in an atmosphere with the pressure of Venus, with 91 standard atmospheres (92 bar) of pressure, it can indeed exist in liquid form over a wide temperature range.

Ammonia

The ammonia molecule (NH₃), like the water molecule, is abundant in the universe, being a compound of hydrogen (the simplest and most common element) with another very common element, nitrogen. The possible role of liquid ammonia as an alternative solvent for life is an idea dating back to 1954 at least, when J. B. S. Haldane raised the topic at a symposium about life's origin.

Many chemical reactions can occur in an ammonia solution, and liquid ammonia has chemical similarities with water. Ammonia dissolves most organic molecules at least as well as water does, and many elemental metals. Haldane indicated that various common water-related organic compounds have ammonia-related analogues; for instance, the ammonia-related amine group (−NH₂) is analogous to the water-related hydroxyl group (−OH).

Ammonia, like water, can either accept or donate an H⁺ ion. When ammonia accepts an H⁺, it forms the ammonium cation (NH₄⁺), analogous to hydronium (H₃O⁺). When it donates an H⁺ ion, it forms the amide anion (NH₂⁻), analogous to the hydroxide anion (OH⁻). Compared to water, however, ammonia is more inclined to accept an H⁺ ion, and less inclined to donate one; it is a stronger nucleophile. Ammonia added to water functions as an Arrhenius base: it increases the concentration of the anion hydroxide. Conversely, using a solvent system definition of acidity and basicity, water added to liquid ammonia functions as an acid, because it increases the concentration of the cation ammonium. The carbonyl group (C=O), which is much used in terrestrial biochemistry, would not be stable in ammonia solution, but the analogous imine group (C=NH) could be used instead.

However, ammonia has some problems as a basis for life. The hydrogen bonds between ammonia molecules are weaker than those in water, causing ammonia's heat of vaporization to be half that of water, its surface tension to be a third, and reducing its ability to concentrate non-polar molecules through a hydrophobic effect. Gerald Feinberg and Robert Shapiro have questioned whether ammonia could hold prebiotic molecules together well enough to allow the emergence of a self-reproducing system. Ammonia is also flammable in oxygen and could not exist sustainably in an environment suitable for aerobic metabolism.

Titan's theorized internal structure, subsurface ocean shown in blue

A biosphere based on ammonia would likely exist at temperatures or air pressures that are extremely unusual in relation to life on Earth. Life on Earth usually exists between the melting point and boiling point of water, at a pressure designated as normal pressure, between 0 and 100 °C (273 and 373 K). When also held to normal pressure, ammonia's melting and boiling points are −78 °C (195 K) and −33 °C (240 K) respectively. Because chemical reactions generally proceed more slowly at lower temperatures, ammonia-based life existing in this set of conditions might metabolize more slowly and evolve more slowly than life on Earth. On the other hand, lower temperatures could also enable living systems to use chemical species that would be too unstable at Earth temperatures to be useful.

A set of conditions where ammonia is liquid at Earth-like temperatures would involve it being at a much higher pressure. For example, at 60 atm ammonia melts at −77 °C (196 K) and boils at 98 °C (371 K).

Ammonia and ammonia–water mixtures remain liquid at temperatures far below the freezing point of pure water, so such biochemistries might be well suited to planets and moons orbiting outside the water-based habitability zone. Such conditions could exist, for example, under the surface of Saturn's largest moon Titan.

Methane and other hydrocarbons

Methane (CH₄) is a simple hydrocarbon: that is, a compound of two of the most common elements in the cosmos: hydrogen and carbon. It has a cosmic abundance comparable with ammonia. Hydrocarbons could act as a solvent over a wide range of temperatures but would lack polarity. Isaac Asimov, the biochemist and science fiction writer, suggested in 1981 that poly-lipids could form a substitute for proteins in a non-polar solvent such as methane. Lakes composed of a mixture of hydrocarbons, including methane and ethane, have been detected on the surface of Titan by the Cassini spacecraft.

There is debate about the effectiveness of methane and other hydrocarbons as a solvent for life compared to water or ammonia. Water is a stronger solvent than the hydrocarbons, enabling easier transport of substances in a cell. However, water is also more chemically reactive and can break down large organic molecules through hydrolysis. A life-form which solvent was a hydrocarbon would not face the threat of its biomolecules being destroyed in this way. Also, the water molecule's tendency to form strong hydrogen bonds can interfere with internal hydrogen bonding in complex organic molecules. Life with a hydrocarbon solvent could make more use of hydrogen bonds within its biomolecules. Moreover, the strength of hydrogen bonds within biomolecules would be appropriate to a low-temperature biochemistry.

Astrobiologist Chris McKay has argued, on thermodynamic grounds, that if life does exist on Titan's surface, using hydrocarbons as a solvent, it is likely also to use the more complex hydrocarbons as an energy source by reacting them with hydrogen, reducing ethane and acetylene to methane. Possible evidence for this form of life on Titan was identified in 2010 by Darrell Strobel of Johns Hopkins University; a greater abundance of molecular hydrogen in the upper atmospheric layers of Titan compared to the lower layers, arguing for a downward diffusion at a rate of roughly 10²⁵ molecules per second and disappearance of hydrogen near Titan's surface. As Strobel noted, his findings were in line with the effects Chris McKay had predicted if methanogenic life-forms were present. The same year, another study showed low levels of acetylene on Titan's surface, which were interpreted by Chris McKay as consistent with the hypothesis of organisms reducing acetylene to methane. While restating the biological hypothesis, McKay cautioned that other explanations for the hydrogen and acetylene findings are to be considered more likely: the possibilities of yet unidentified physical or chemical processes (e.g. a non-living surface catalyst enabling acetylene to react with hydrogen), or flaws in the current models of material flow. He noted that even a non-biological catalyst effective at 95 K would in itself be a startling discovery.

Azotosome

A hypothetical cell membrane termed an azotosome, able to function in liquid methane in Titan conditions was computer-modelled in an article published in February 2015. Composed of acrylonitrile, a small molecule containing carbon, hydrogen, and nitrogen, it is predicted to have stability and flexibility in liquid methane comparable to that of a phospholipid bilayer (the type of cell membrane possessed by all life on Earth) in liquid water. An analysis of data obtained using the Atacama Large Millimeter / submillimeter Array (ALMA), completed in 2017, confirmed substantial amounts of acrylonitrile in Titan's atmosphere. Later studies questioned whether acrylonitrile would be able to self-assemble into azotosomes. However, in 2025 a new mechanism was proposed by scientists Christian Mayer and Conor Nixon to overcome the previous barriers to self-assembly of azotosomes in liquid methane, based on 'splashing' of a methane lake surface film by a hydrocarbon raindrop.

An artist's concept of the proposed mechanism for vesicle formation on Titan

Hydrogen fluoride

Hydrogen fluoride (HF), like water, is a polar molecule, and due to its polarity it can dissolve many ionic compounds. At atmospheric pressure, its melting point is 189.15 K (−84.00 °C), and its boiling point is 292.69 K (19.54 °C); the difference between the two is a little more than 100 K. HF also makes hydrogen bonds with its neighbour molecules, as do water and ammonia. It has been considered as a possible solvent for life by scientists such as Peter Sneath and Carl Sagan.

HF is dangerous to the systems of molecules that Earth-life is made of, but certain other organic compounds, such as paraffin waxes, are stable with it. Like water and ammonia, liquid hydrogen fluoride supports an acid–base chemistry. Using a solvent system definition of acidity and basicity, nitric acid functions as a base when it is added to liquid HF.

However, hydrogen fluoride is cosmically rare, unlike water, ammonia, and methane.

Hydrogen sulfide

Hydrogen sulfide is the closest chemical analog to water, but is less polar and is a weaker inorganic solvent. Hydrogen sulfide is quite plentiful on Jupiter's moon Io and may be in liquid form a short distance below the surface; astrobiologist Dirk Schulze-Makuch has suggested it as a possible solvent for life there. On a planet with hydrogen sulfide oceans, the source of the hydrogen sulfide could come from volcanoes, in which case it could be mixed in with a bit of hydrogen fluoride, which could help dissolve minerals. Hydrogen sulfide life might use a mixture of carbon monoxide and carbon dioxide as their carbon source. They might produce and live on sulfur monoxide, which is analogous to oxygen (O₂). Hydrogen sulfide, like hydrogen cyanide and ammonia, suffers from the small temperature range where it is liquid, though that, like that of hydrogen cyanide and ammonia, increases with increasing pressure.

Silicon dioxide and silicates

Silicon dioxide, also known as silica and quartz, is very abundant in the universe and has a large temperature range where it is liquid. However, its melting point is 1,600 to 1,725 °C (2,912 to 3,137 °F), so it would be impossible to make organic compounds in that temperature, because all of them would decompose. Silicates are similar to silicon dioxide and some have lower melting points than silica. Feinberg and Shapiro have suggested that molten silicate rock could serve as a liquid medium for organisms with a chemistry based on silicon, oxygen, and other elements such as aluminium.

Other solvents or cosolvents

Sulfuric acid (H₂SO₄)

Other solvents sometimes proposed:

• Supercritical fluids: supercritical carbon dioxide and supercritical hydrogen.
• Simple hydrogen compounds: hydrogen chloride.
• More complex compounds: sulfuric acid, formamide, methanol.
• Very-low-temperature fluids: liquid nitrogen and hydrogen.
• High-temperature liquids: sodium chloride.

Sulfuric acid in liquid form is strongly polar. It remains liquid at higher temperatures than water, its liquid range being 10 °C to 337 °C at a pressure of 1 atm, although above 300 °C it slowly decomposes. Sulfuric acid is known to be abundant in the clouds of Venus, in the form of aerosol droplets. In a biochemistry that used sulfuric acid as a solvent, the alkene group (C=C), with two carbon atoms joined by a double bond, could function analogously to the carbonyl group (C=O) in water-based biochemistry.

A proposal has been made that life on Mars may exist and be using a mixture of water and hydrogen peroxide as its solvent. A 61.2% (by mass) mix of water and hydrogen peroxide has a freezing point of −56.5 °C and tends to super-cool rather than crystallize. It is also hygroscopic, an advantage in a water-scarce environment.

Supercritical carbon dioxide has been proposed as a candidate for alternative biochemistry due to its ability to selectively dissolve organic compounds and assist the functioning of enzymes and because "super-Earth"- or "super-Venus"-type planets with dense high-pressure atmospheres may be common.

Other speculations

Non-green photosynthesizers

Physicists have noted that, although photosynthesis on Earth generally involves green plants, a variety of other-colored plants could also support photosynthesis, essential for most life on Earth, and that other colors might be preferred in places that receive a different mix of stellar radiation than Earth.^[88][89] These studies indicate that blue plants would be unlikely; however yellow or red plants may be relatively common.

Variable environments

Many Earth plants and animals undergo major biochemical changes during their life cycles as a response to changing environmental conditions, for example, by having a spore or hibernation state that can be sustained for years or even millennia between more active life stages. Thus, it would be biochemically possible to sustain life in environments that are only periodically consistent with life as we know it.

For example, frogs in cold climates can survive for extended periods of time with most of their body water in a frozen state, whereas desert frogs in Australia can become inactive and dehydrate in dry periods, losing up to 75% of their fluids, yet return to life by rapidly rehydrating in wet periods. Either type of frog would appear biochemically inactive (i.e. not living) during dormant periods to anyone lacking a sensitive means of detecting low levels of metabolism.

Alanine world and hypothetical alternatives

Early stage of the genetic code (GC-Code) with "alanine world" and its possible alternatives

The genetic code may have evolved during the transition from the RNA world to a protein world. The alanine world hypothesis postulates that the evolution of the genetic code (the so-called GC phase) started with only four basic amino acids: alanine, glycine, proline and ornithine (now arginine). The evolution of the genetic code ended with 20 proteinogenic amino acids. From a chemical point of view, most of them are Alanine-derivatives particularly suitable for the construction of α-helices and β-sheets – basic secondary structural elements of modern proteins. Direct evidence of this is an experimental procedure in molecular biology known as alanine scanning.

A hypothetical proline world would create a possible alternative life with the genetic code based on the proline chemical scaffold as the protein backbone. Similarly, a glycine world and ornithine world are also conceivable, but nature has chosen none of them. Evolution of life with Proline, Glycine, or Ornithine as the basic structure for protein-like polymers (foldamers) would lead to parallel biological worlds. They would have morphologically radically different body plans and genetics from the living organisms of the known biosphere.

Nonplanetary life

Main article: Non-planetary abiogenesis

Dusty plasma-based

See also: Dusty plasma

In 2007, Vadim N. Tsytovich and colleagues proposed that lifelike behaviors could be exhibited by dust particles suspended in a plasma, under conditions that might exist in space. Computer models showed that, when the dust became charged, the particles could self-organize into microscopic helical structures, and the authors offer "a rough sketch of a possible model of...helical grain structure reproduction".

Cosmic necklace-based

In 2020, Luis A. Anchordoqu and Eugene M. Chudnovsky of the City University of New York hypothesized that cosmic necklace-based life composed of magnetic monopoles connected by cosmic strings could evolve inside stars. This would be achieved by a stretching of cosmic strings due to the star's intense gravity, thus allowing it to take on more complex forms and potentially form structures similar to the RNA and DNA structures found within carbon-based life. As such, it is theoretically possible that such beings could eventually become intelligent and construct a civilization using the power generated by the star's nuclear fusion. Because such use would use up part of the star's energy output, the luminosity would also fall. For this reason, it is thought that such life might exist inside stars observed to be cooling faster or dimmer than current cosmological models predict.

Life on a neutron star

Frank Drake suggested in 1973 that intelligent life could inhabit neutron stars. Physical models in 1973 implied that Drake's creatures would be microscopic.

Planetary engineering

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Planetary_engineering 

Planetary engineering is the development and application of technology for the purpose of influencing the environment of a planet. Planetary engineering encompasses a variety of methods such as terraforming, seeding, and geoengineering.

Widely discussed in the scientific community, terraforming refers to the alteration of other planets to create a habitable environment for terrestrial life. Seeding refers to the introduction of life from Earth to habitable planets. Geoengineering refers to the engineering of a planet's climate, and has already been applied on Earth. Each of these methods are composed of varying approaches and possess differing levels of feasibility and ethical concern.

Historical Context

The idea of humans altering other planet environments dates back before the term terraforming was created. In the early 20th century, there was a period for rapid scientific discovery which included planetary atmospheres. Astronomer Percival Lowell popularized the idea that Mars may have canals which sparked debate on whether Mars had the potential to house life. Most of Lowell's work was later proven incorrect but it got people thinking about altering planets.

By the 1940s and 1950s, the idea for planetary engineering started to appear in works of science fiction. Writers started discussing and exploring humans, changing environments of other planets to make them habitable. They often included ideas for changing entire ecosystems for humans to live on. These fictional explorations reflected humans desire to control the nature they lived in. Humans on earth have often changed their environments to help survive. This can be anywhere from agriculture to large scale infrastructure.

Astrophysicist Carl Sagan first proposed the scientific idea back in a 1961 Science Paper discussing the topic of terraforming the atmosphere of Venus with algae to reduce carbon dioxide and temperatures. This became one of the first times where a leading scientist discussed the idea publicly of altering a planet's environment. Over the next few decades, the idea of planetary engineering changed from just science fiction to more of a scientific discussion, as space exploration advanced. In the 1970s, the space race accelerated and the first made satellites and probes were being made. These groundbreaking satellites and probes were sent out into space to return data from that helped us understand the Moons and Earths ecosystem better.

At the same time geoengineering on earth became widely talked about topics for concerns of industrial pollution, ozone depletion, and global warming. Scientists began to discuss large scale strategies we could combat these changes in earth atmosphere which included stratospheric aerosol injection.

Around the 1980s, the development of modern computing and data collection helped further advance the thinking of planetary engineering. Agencies started to collaborate to build a model of weather patterns on a global scale. Later this helped us to really understand climate change, and this shifted focus of managing just local and regional climates to managing weather on a planetary scale. By the 2000s, geoengineering became the more widely used term for climate intervention. These include strategies like solar radiation management (SRM). Solar radiation management was used to reflect sunlight to slow climate change. The effects would vary based on timing, magnitude, and the type of radiation. They soon installed temporary, moderate, and responsive scenarios. These would be used to monitor the effects of solar radiation management as to not have unexpected consequences like ozone depletion.

Terraforming

Main article: Terraforming

Projected temperature and precipitation changes relative to preindustrial; end-of-century response without (a) and with (b) geoengineering to avoid temperature rise above 1.5C.

A theoretical design for a power station on Mars. Terraforming designs are not yet planned.

Terraforming is the process of modifying the atmosphere, temperature, surface topography or ecology of a planet, moon, or other body in order to replicate the environment of Earth.

Technologies

A common object of discussion on potential terraforming is the planet Mars. To terraform Mars, humans would need to create a new atmosphere, due to the planet's high carbon dioxide concentration and low atmospheric pressure. This would be possible by introducing more greenhouse gases to below "freezing point from indigenous materials". To terraform Venus, carbon dioxide would need to be converted to graphite since Venus receives twice as much sunlight as Earth. This process is only possible if the greenhouse effect is removed with the use of "high-altitude absorbing fine particles" or a sun shield, creating a more habitable Venus.

NASA has defined categories of habitability systems and technologies for terraforming to be feasible. These topics include creating power-efficient systems for preserving and packaging  food for crews, preparing and cooking foods, dispensing water, and developing facilities for rest, trash and recycling, and areas for crew hygiene and rest.

Feasibility

A variety of planetary engineering challenges stand in the way of terraforming efforts. The atmospheric terraforming of Mars, for example, would require "significant quantities of gas" to be added to the Martian atmosphere. This gas has been thought to be stored in solid and liquid form within Mars' polar ice caps and underground reservoirs. It is unlikely, however, that enough CO₂ for sufficient atmospheric change is present within Mars' polar deposits, and liquid CO₂ could only be present at warmer temperatures "deep within the crust". Furthermore, sublimating the entire volume of Mars' polar caps would increase its current atmospheric pressure to 15 millibar, where an increase to around 1000 millibar would be required for habitability. For reference, Earth's average sea-level pressure is 1013.25 mbar.

First formally proposed by astrophysicist Carl Sagan, the terraforming of Venus has since been discussed through methods such as organic molecule-induced carbon conversion, sun reflection, increasing planetary spin, and various chemical means. Due to the high presence of sulfuric acid and solar wind on Venus, which are harmful to organic environments, organic methods of carbon conversion have been found unfeasible. Other methods, such as solar shading, hydrogen bombardment, and magnesium-calcium bombardment are theoretically sound but would require large-scale resources and space technologies not yet available to humans.

Habitability

In planetary engineering, the term habitability is used to describe if a planet has the ability to support life and what the conditions are to make that possible. Scientists distinguish between two types of habitably, short-term and long-term. Short-term refers to an organism's ability to survive on a planet from anywhere to a few days to a few years. Long-term refers to an organism's ability to sustain itself on a planet for multiple decades.

A habitat is described as an environment that supports the activities of at least one known organism. This can mean different things:

• Survival means the organism can stay alive and repair damage by using resources provided by the environment.
• Maintenance means that the organism can continue normal cell activities but not reproduce.
• Growth means the organism is able get larger.
• Reproduction means the organism can multiply or create the next generation.Habitability is important in the field of planetary engineering because it is what most engineers and scientists aim to create when talking about modifying another planet's ecosystem or environment. Understanding habitability helps to guide the engineering strategies used like temperature management and resource distribution.

Ethical considerations

While successful terraforming would allow life to prosper on other planets, philosophers have debated whether this practice is morally sound. Certain ethics experts suggest that planets like Mars hold an intrinsic value independent of their utility to humanity and should therefore be free from human interference. Also, some argue that through the steps that are necessary to make Mars habitable - such as fusion reactors, space-based solar-powered lasers, or spreading a thin layer of soot on Mars' polar ice caps - would deteriorate the current aesthetic value that Mars possesses. This calls into question humanity's intrinsic ethical and moral values, as it raises the question of whether humanity is willing to eradicate the current ecosystem of another planet for their benefit. Through this ethical framework, terraforming attempts on these planets could be seen to threaten their intrinsically valuable environments, rendering these efforts unethical.

Another important ethical consideration for terraforming is the way some humans believe it is their role in our universe. The media also influences how we perceive terraforming. As the media often shows, terraforming is an exciting step or even a necessary step for the survival of humans. In many TV shows and movies space colonization is portrayed as spectacular with pretty images to encourage support from the public. This sparks many debates between philosophers. For example, Paul York suggests that if humans ever have the need to terraform another planets surface because Earths is so deteriorated, then terraforming a different planets surface like Mars could lead to a similar destruction. On the other hand, James Schwartz and other environmental philosophers argue that if we explore and terraform other planets that this could lead to an understanding and solve some environmental problems we face on Earth.

Seeding

NASA's Hubble Space Telescope took the picture of Mars on June 26, 2001, when Mars was approximately 68 million kilometers (43 million miles) from Earth — the closest Mars has ever been to Earth since 1988. Hubble can see details as small as 16 kilometers (10 miles) across. The colors have been carefully balanced to give a realistic view of Mars' hues as they might appear through a telescope. Especially striking is the large amount of seasonal dust storm activity seen in this image. One large storm system is churning high above the northern polar cap (top of image), and a smaller dust storm cloud can be seen nearby. Another large dust storm is spilling out of the giant Hellas impact basin in the Southern Hemisphere (lower right) exploration.

Environmental considerations

Mars is the primary subject of discussion for seeding. Locations for seeding are chosen based on atmospheric temperature, air pressure, existence of harmful radiation, and availability of natural resources, such as water and other compounds essential to terrestrial life.

Developing microorganisms for seeding

Natural or engineered microorganisms must be created or discovered that can withstand the harsh environments of Mars. The first organisms used must be able to survive exposure to ionizing radiation and the high concentration of CO₂ present in the Martian atmosphere. Later organisms such as multicellular plants must be able to withstand the freezing temperatures, withstand high CO₂ levels, and produce significant amounts of O₂.

Microorganisms provide significant advantages over non-biological mechanisms. They are self-replicating, negating the needs to either transport or manufacture large machinery to the surface of Mars. They can also perform complicated chemical reactions with little maintenance to realize planet-scale terraforming.

Climate engineering

Main article: Climate engineering

Impression of the hypothetical phrases of the terraforming of Mars

Climate engineering is a form of planetary engineering which involves the process of deliberate and large-scale alteration of the Earth's climate system to combat climate change. Examples of geoengineering are carbon dioxide removal (CDR), which removes carbon dioxide from the atmosphere, and solar radiation modification (SRM) to reflect solar energy to space. Carbon dioxide removal (CDR) has multiple practices, the simplest being reforestation, to more complex processes such as direct air capture. The latter is rather difficult to deploy on an industrial scale, for high costs and substantial energy usage would be some aspects to address.

Examples of SRM include stratospheric aerosol injection (SAI) and marine cloud brightening (MCB). When a volcano erupts, small particles known as aerosols proliferate throughout the atmosphere, reflecting the sun's energy back into space. This results in a cooling effect, and humanity could conceivably inject these aerosols into the stratosphere, spurring large-scale cooling.

Visible ship tracks in the Northern Pacific, on 4 March 2009. On an overcast day, the clouds look uniform. However, NASA MODIS images' sensor reveals long, skinny trails of brighter clouds hidden within. As ships travel across the ocean, pollution in the ships' exhaust create more cloud drops that are smaller in size, resulting in even brighter clouds.

One proposal for MCB involves spraying a vapor into low-laying sea clouds, creating more cloud condensation nuclei. This would in theory result in the cloud becoming whiter, and reflecting light more efficiently.

Fermi paradox

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Fermi_paradox

Enrico Fermi (Los Alamos 1945)

The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life and the apparently high likelihood of its existence.

The paradox is named after physicist Enrico Fermi, who informally posed the question—remembered by Emil Konopinski as "But where is everybody?"—during a 1950 conversation at Los Alamos with colleagues Konopinski, Edward Teller, and Herbert York. The paradox first appeared in print in a 1963 paper by Carl Sagan and the paradox has since been fully characterized by scientists. Early formulations of the paradox have also been identified in writings by Bernard Le Bovier de Fontenelle (1686) and Jules Verne (1865), and by Soviet rocket scientist Konstantin Tsiolkovsky (1933).

There have been many attempts to resolve the Fermi paradox, such as suggesting that intelligent extraterrestrial beings are extremely rare, that the lifetime of such civilizations is short, or that they exist but (for various reasons) humans see no evidence.

Chain of reasoning

Some of the facts and hypotheses that together serve to highlight the apparent contradiction:

• There are billions of stars in the Milky Way similar to the Sun.
• With high probability, some of these stars have Earth-like planets orbiting in the habitable zone.
• Many of these stars, and hence their planets, are much older than the Sun. If Earth-like planets are typical, some may have developed intelligent life long ago.
• Some of these civilizations may have developed interstellar travel, a step that humans are investigating.
• Even at the slow pace of envisioned interstellar travel, the Milky Way galaxy could be completely traversed in a few million years.
• Since many of the Sun-like stars are billions of years older than the Sun, the Earth should have already been visited by extraterrestrial civilizations, or at least their probes.
• However, there is no convincing evidence that this has happened.

History

Los Alamos conversation

Enrico Fermi posed the paradox to fellow physicists Emil Konopinski (top), Edward Teller (middle), and Herbert York (bottom) at Los Alamos in 1950.

Enrico Fermi was a Nobel Prize-winning physicist who predicted the existence of neutrinos and helped create the first artificial nuclear reactor, an early feat of the Manhattan Project. He was known to pose simple but seemingly unanswerable questions—termed "Fermi questions"—to his colleagues and students, like "How many atoms of Caesar's last breath do you inhale with each lungful of air?"

In 1950, Fermi visited Los Alamos National Laboratory in New Mexico and, while walking to the Fuller Lodge for lunch, conversed with fellow physicists Emil Konopinski, Edward Teller, and Herbert York about reports of flying saucers and the feasibility of faster-than-light travel. When the conversation shifted to unrelated topics at the lodge, Fermi blurted a question variously recalled as: "Where is everybody?" (Teller), "Don't you ever wonder where everybody is?" (York), or "But where is everybody?" (Konopinski). According to Teller, "The result of his question was general laughter because of the strange fact that, in spite of Fermi's question coming out of the blue, everybody around the table seemed to understand at once that he was talking about extraterrestrial life."

According to York, Fermi "followed up with a series of calculations on the probability of earthlike planets, the probability of life given an earth, the probability of humans given life, the likely rise and duration of high technology, and so on. He concluded on the basis of such calculations that we ought to have been visited long ago and many times over." However, Teller recalled that Fermi did not elaborate on his question beyond "perhaps a statement that the distances to the next location of living beings may be very great and that, indeed, as far as our galaxy is concerned, we are living somewhere in the sticks, far removed from the metropolitan area of the galactic center."

Predecessors

Russian rocket scientist Konstantin Tsiolkovsky

Fermi was not the first to note the paradox. In his 1686 book Conversations on the Plurality of Worlds, Bernard Le Bovier de Fontenelle—later the secretary of the French Academy of Sciences—constructs a dialogue in which Fontenelle's claims of "intelligent beings exist in other worlds, for instance the Moon" are refuted by a character who notes that "If this were the case, the Moon's inhabitants would already have come to us before now." This may have inspired a similar discussion in Jules Verne's 1865 novel Around the Moon, which has also been identified as an early conceptualization of the Fermi paradox.

Another early formulation Fermi paradox was presented and dissected in the 1930s writings of Russian rocket scientist Konstantin Tsiolkovsky. Although his rocketry work was embraced by the materialist Soviets, his philosophical writings were suppressed and unknown for most of the 20th century. Tsiolkovsky noted that critics refute the existence of advanced extraterrestrial life as such civilizations would have visited humanity or left some detectable evidence. He posed a solution to the paradox: humanity is quarantined by aliens to protect its independent cultural development, which resembles the zoo hypothesis proposed by John Ball.

Popularization

Carl Sagan, seen here beside a Viking lander, first mentioned the paradox in print.

The Fermi question first appeared in print in a footnote of a 1963 paper by Carl Sagan. Two years later, Stephen Dole noted the dilemma at a symposium—"If there are so many advanced forms of life around, where is everybody?"—but did not attribute it to Fermi. A chapter of Intelligent Life in the Universe, co-authored by Sagan and Iosif Shklovsky, was headlined with the Fermi-attributed "Where are they?" The Fermi question also appeared in NASA's 1970 Project Cyclops report, a 1973 book by Sagan, and a 1975 article in JBIS Interstellar Studies by David Viewing that first described it as a paradox.

Later that year, Michael Hart published a detailed examination of the paradox in the Quarterly Journal of the Royal Astronomical Society. Hart, who concluded that "we are the first civilization in our Galaxy", proposed four broad categories of solutions to the paradox: those that are physical (a space travel limitation), sociological (aliens choose not to visit Earth), temporal (aliens have not had time to travel to Earth), or that extraterrestrials have already visited. His paper sparked significant interest in the paradox among academics and even politicians, with a discussion held in the House of Lords. A seminal response—"Extraterrestrial intelligent beings do not exist"—was written by Frank Tipler, who argued that, if an advanced extraterrestrial civilization existed, their self-replicating spacecraft should have already been detected in the Solar System. The term "Fermi paradox" was coined in a 1977 article by David Stephenson and was widely adopted.

The popularization of the Fermi paradox damaged SETI efforts, and Senator William Proxmire cited Tipler when he spurred the termination of the federally funded NASA SETI program in 1981. According to Robert Gray, the paradox may contribute to a "de facto prohibition on government support for research in a branch of astrobiology".

Criticism

Fermi did not publish anything regarding the paradox, with Sagan once suggesting the quote to be apocryphal. Scientists like Robert Gray have criticized its attribution to Fermi, and alternative terms like the "Hart–Tipler argument" or "Tsiolkovsky–Fermi–Viewing–Hart paradox" have been proposed. According to Gray, the current understanding of the paradox misinterprets Fermi's question and subsequent discussion, which was challenging the feasibility of interstellar travel rather than the existence of advanced extraterrestrial life.

Basis

Enrico Fermi (1901–1954)

The Fermi paradox is a conflict between the argument that scale and probability seem to favor intelligent life being common in the universe, and the total lack of evidence of intelligent life having ever arisen anywhere other than on Earth.

The first aspect of the Fermi paradox is a function of the scale or the large numbers involved: there are an estimated 200–400 billion stars in the Milky Way (2–4 × 10¹¹) and 70 sextillion (7×10²²) in the observable universe. Even if intelligent life occurs on only a minuscule percentage of planets around these stars, there might still be a great number of extant civilizations, and if the percentage were high enough it would produce a significant number of extant civilizations in the Milky Way. This assumes the mediocrity principle, by which Earth is a typical planet.

The second aspect of the Fermi paradox is the argument of probability: given intelligent life's ability to overcome scarcity, and its tendency to colonize new habitats, it seems possible that at least some civilizations would be technologically advanced, seek out new resources in space, and colonize their star system and, subsequently, surrounding star systems. Since there is no significant evidence on Earth, or elsewhere in the known universe, of other intelligent life after 13.8 billion years of the universe's history, there is a conflict requiring a resolution. Some examples of possible resolutions are that intelligent life is rarer than is thought, that assumptions about the general development or behavior of intelligent species are flawed, or, more radically, that the scientific understanding of the nature of the universe is quite incomplete.

The Fermi paradox can be asked in two ways. The first is, "Why are no aliens or their artifacts found on Earth, or in the Solar System?". If interstellar travel is possible, even the "slow" kind nearly within the reach of Earth technology, then it would only take from 5 million to 50 million years to colonize the galaxy. This is relatively brief on a geological scale, let alone a cosmological one. Since there are many stars older than the Sun, and since intelligent life might have evolved earlier elsewhere, the question then becomes why the galaxy has not been colonized already. Even if colonization is impractical or undesirable to all alien civilizations, large-scale exploration of the galaxy could be possible by probes. These might leave detectable artifacts in the Solar System, such as old probes or evidence of mining activity, but none of these have been observed.

The second form of the question is "Why are there no signs of intelligence elsewhere in the universe?". This version does not assume interstellar travel, but includes other galaxies as well. For distant galaxies, travel times may well explain the lack of alien visits to Earth, but a sufficiently advanced civilization could potentially be observable over a significant fraction of the size of the observable universe. Even if such civilizations are rare, the scale argument indicates they should exist somewhere at some point during the history of the universe, and since they could be detected from far away over a considerable period of time, many more potential sites for their origin are within range of human observation. It is unknown whether the paradox is stronger for the Milky Way galaxy or for the universe as a whole.

Drake equation

Main article: Drake equation

The theories and principles in the Drake equation are closely related to the Fermi paradox. The equation was formulated by Frank Drake in 1961 in an attempt to find a systematic means to evaluate the numerous probabilities involved in the existence of alien life. The equation is

${\displaystyle N=R_{\ast }\cdot f_{\mathrm{p}}\cdot n_{\mathrm{e}}\cdot f_{\mathrm{l}}\cdot f_{\mathrm{i}}\cdot f_{\mathrm{c}}\cdot L,}$

where ${\displaystyle N}$ is the number of technologically advanced civilizations in the Milky Way galaxy, and ${\displaystyle N}$ is asserted to be the product of

• ${\displaystyle R_{\ast }}$, the rate of formation of stars in the galaxy;
• ${\displaystyle f_p}$, the fraction of those stars with planetary systems;
• ${\displaystyle n_e}$, the number of planets, per solar system, with an environment suitable for organic life;
• ${\displaystyle f_l}$, the fraction of those suitable planets whereon organic life appears;
• ${\displaystyle f_i}$, the fraction of life-bearing planets whereon intelligent life appears;
• ${\displaystyle f_c}$, the fraction of civilizations that reach the technological level whereby detectable signals may be dispatched; and
• ${\displaystyle L}$, the length of time that those civilizations dispatch their signals.

The fundamental problem is that the last four terms (${\displaystyle f_l}$, ${\displaystyle f_i}$, ${\displaystyle f_c}$, and ${\displaystyle L}$) are entirely unknown, rendering statistical estimates impossible.

The Drake equation has been used by both optimists and pessimists, with wildly differing results. The first scientific meeting on the search for extraterrestrial intelligence (SETI), which had 10 attendees including Frank Drake and Carl Sagan, speculated that the number of civilizations was roughly between 1,000 and 100,000,000 civilizations in the Milky Way galaxy. Conversely, Frank Tipler and John D. Barrow used pessimistic numbers and speculated that the average number of civilizations in a galaxy is much less than one. Almost all arguments involving the Drake equation suffer from the overconfidence effect, a common error of probabilistic reasoning about low-probability events, by guessing specific numbers for likelihoods of events whose mechanism is not understood, such as the likelihood of abiogenesis on an Earth-like planet, with estimates varying over many hundreds of orders of magnitude. An analysis that takes into account some of the uncertainty associated with this lack of understanding has been carried out by Anders Sandberg, Eric Drexler and Toby Ord, and suggests "a substantial ex ante probability of there being no other intelligent life in our observable universe".

Great Filter

Main article: Great Filter

The Great Filter, a concept introduced by Robin Hanson in 1996, represents whatever natural phenomena that would make it unlikely for life to evolve from inanimate matter to an advanced civilization. The most commonly agreed-upon low probability event is abiogenesis: a gradual process of increasing complexity of the first self-replicating molecules by a randomly occurring chemical process. Other proposed great filters are the emergence of eukaryotic cells or of meiosis or some of the steps involved in the evolution of a brain-like organ capable of complex logical deductions.

Astrobiologists Dirk Schulze-Makuch and William Bains, reviewing the history of life on Earth, including convergent evolution, concluded that transitions such as oxygenic photosynthesis, the eukaryotic cell, multicellularity, and tool-using intelligence are likely to occur on any Earth-like planet given enough time. They argue that the Great Filter may be abiogenesis, the rise of technological human-level intelligence, or an inability to settle other worlds because of self-destruction or a lack of resources. Paleobiologist Olev Vinn has suggested that the great filter may have universal biological roots related to evolutionary animal behavior.

Grabby Aliens

Main article: Quiet and loud aliens

In 2021, the concepts of quiet, loud, and grabby aliens were introduced by Hanson et al. The proposed "loud" aliens expand rapidly in a highly detectable way throughout the universe and endure, while "quiet" aliens are hard or impossible to detect and eventually disappear. "Grabby" aliens prevent the emergence of other civilizations in their sphere of influence, which expands at a rate near the speed of light. The authors argue that if loud civilizations are rare, as they appear to be, then quiet civilizations are also rare. The paper suggests that humanity's existing stage of technological development is relatively early in the potential timeline of intelligent life in the universe, as loud aliens would otherwise be observable by astronomers.

Earlier in 2013, Anders Sandberg and Stuart Armstrong examined the potential for intelligent life to spread intergalactically throughout the universe and the implications for the Fermi Paradox. Their study suggests that with sufficient energy, intelligent civilizations could potentially colonize the entire Milky Way galaxy within a few million years, and spread to nearby galaxies in a timespan that is cosmologically brief. They conclude that intergalactic colonization appears possible with the resources of a single planetary system and that intergalactic colonization is of comparable difficulty to interstellar colonization, and therefore the Fermi paradox is much sharper than commonly thought.

Critics such as David Kipping have contended that the "Grabby Aliens" model is reliant on unproven assumptions, lacking enough scientific rigor to be empirically falsifiable, and suggested other explanations for the proposed earliness of humans such as planets in M-dwarf systems being uninhabitable. Robin Hanson has responded to these criticisms.

Anthropics

Anthropic reasoning and the question of why we happen to find ourselves as humans creates a number of potential problems for astrobiology. Walter Barta argues that Hanson's grabby aliens model creates an anthropic dilemma. According to Hanson's model, most observers in our reference class should be grabby aliens themselves. This leads to the question of why we do not find ourselves as grabby aliens, but rather as a species confined to a single planet.

Empirical evidence

Main articles: Search for extraterrestrial intelligence and Technosignature

There are two parts of the Fermi paradox that rely on empirical evidence—that there are many potentially habitable planets, and that humans see no evidence of life. The first point, that many suitable planets exist, was an assumption in Fermi's time, but is since supported by the discovery that exoplanets are common. Existing models predict billions of habitable worlds in the Milky Way.

The second part of the paradox, that humans see no evidence of extraterrestrial life, is also an active field of scientific research. This includes both efforts to find any indication of life, and efforts specifically directed to finding intelligent life. These searches have been made since 1960, and several are ongoing.

Although astronomers do not usually search for extraterrestrials, they have observed phenomena that they could not immediately explain without positing an intelligent civilization as the source. For example, pulsars, when first discovered in 1967, were called little green men (LGM) because of the precise repetition of their pulses. In all cases, explanations with no need for intelligent life have been found for such observations, but the possibility of discovery remains. Proposed examples include asteroid mining that would change the appearance of debris disks around stars, or spectral lines from nuclear waste disposal in stars.

Electromagnetic emissions

Further information: Project Phoenix (SETI), SERENDIP, and Allen Telescope Array

Radio telescopes are often used by SETI projects.

Radio technology and the ability to construct a radio telescope are presumed to be a natural advance for technological species, theoretically creating effects that might be detected over interstellar distances. The careful searching for non-natural radio emissions from space may lead to the detection of alien civilizations. Sensitive alien observers of the Solar System, for example, would note unusually intense radio waves for a G2 star due to Earth's television and telecommunication broadcasts. In the absence of an apparent natural cause, alien observers might infer the existence of a terrestrial civilization. Such signals could be either "accidental" by-products of a civilization, or deliberate attempts to communicate, such as the Arecibo message. It is unclear whether "leakage", as opposed to a deliberate beacon, could be detected by an extraterrestrial civilization. The most sensitive radio telescopes on Earth, as of 2019, would not be able to detect non-directional radio signals (such as broadband) even at a fraction of a light-year away, but other civilizations could hypothetically have much better equipment.

A number of astronomers and observatories have attempted and are attempting to detect such evidence, mostly through SETI organizations such as the SETI Institute and Breakthrough Listen. Several decades of SETI analysis have not revealed any unusually bright or meaningfully repetitive radio emissions.

Direct planetary observation

A composite picture of Earth at night, created using data from the Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS). Large-scale artificial lighting produced by human civilization is detectable from space.

Exoplanet detection and classification is a very active sub-discipline in astronomy; the first candidate terrestrial planet discovered within a star's habitable zone was found in 2007. New refinements in exoplanet detection methods, and use of existing methods from space (such as the Kepler and TESS missions) have detected and characterized Earth-size planets, and determined whether they are within the habitable zones of their stars. Such observational refinements have allowed better estimates of how common these potentially habitable worlds are, typically in the range of 0.5-1.0 potentially habitable planets per star.

Conjectures about interstellar probes

Further information: Hart–Tipler conjecture, Von Neumann probe, and Bracewell probe

The Hart–Tipler conjecture is a form of contraposition which states that because no interstellar probes have been detected, there likely is no other intelligent life in the universe, as such life should be expected to eventually create and launch such probes. Self-replicating probes could exhaustively explore a galaxy the size of the Milky Way in as little as a million years. If even a single civilization in the Milky Way attempted this, such probes could spread throughout the entire galaxy. Another speculation for contact with an alien probe—one that would be trying to find human beings—is an alien Bracewell probe. Such a hypothetical device would be an autonomous space probe whose purpose is to seek out and communicate with alien civilizations (as opposed to von Neumann probes, which are usually described as purely exploratory). These were proposed as an alternative to carrying a slow speed-of-light dialogue between vastly distant neighbors. Rather than contending with the long delays a radio dialogue would suffer, a probe housing an artificial intelligence would seek out an alien civilization to carry on a close-range communication with the discovered civilization. The findings of such a probe would still have to be transmitted to the home civilization at light speed, but an information-gathering dialogue could be conducted in real time.

Direct exploration of the Solar System has yielded no evidence indicating a visit by aliens or their probes. Detailed exploration of areas of the Solar System where resources would be plentiful may yet produce evidence of alien exploration, though the entirety of the Solar System is relatively vast and difficult to investigate. Attempts to signal, attract, or activate hypothetical Bracewell probes in Earth's vicinity have not succeeded.

Searches for stellar-scale artifacts

Further information: Dyson sphere, Stellar engine, and Kardashev scale

A variant of the speculative Dyson sphere. Such large-scale artifacts would drastically alter the spectrum of a star.

In 1959, Freeman Dyson observed that every developing human civilization constantly increases its energy consumption, and he conjectured that a civilization might try to harness a large part of the energy produced by a star. He proposed a hypothetical "Dyson sphere" as a means: a shell or cloud of objects enclosing a star to absorb and utilize as much radiant energy as possible. Such a feat of astroengineering would drastically alter the observed spectrum of the star involved, changing it at least partly from the normal emission lines of a natural stellar atmosphere to those of black-body radiation, probably with a peak in the infrared. Dyson speculated that advanced alien civilizations might be detected by examining the spectra of stars and searching for such an altered spectrum.

There have been attempts to find evidence of Dyson spheres that would alter the spectra of their core stars. Direct observation of thousands of galaxies has shown no explicit evidence of artificial construction or modifications. In October 2015, there was speculation that a dimming of light from star KIC 8462852, observed by the Kepler space telescope, could have been a result of such a Dyson sphere under construction. However, in 2018, further observations determined that the amount of dimming varied by the frequency of the light, pointing to dust, rather than an opaque object such as a Dyson sphere, as the cause of the dimming.

Hypothetical explanations for the paradox

Rarity of intelligent life

Extraterrestrial life is rare or non-existent

Main articles: Rare Earth hypothesis and Firstborn hypothesis

Those who think that intelligent extraterrestrial life is (nearly) impossible argue that the conditions needed for the evolution of life—or at least the evolution of biological complexity—are rare or even unique to Earth. Under this assumption, called the rare Earth hypothesis, a rejection of the mediocrity principle, complex multicellular life is regarded as exceedingly unusual.

The rare Earth hypothesis argues that the evolution of biological complexity requires a host of fortuitous circumstances, such as a galactic habitable zone, a star and planet(s) having the requisite conditions, such as enough of a continuous habitable zone, the advantage of a giant guardian like Jupiter and a large moon, conditions needed to ensure the planet has a magnetosphere and plate tectonics, the chemistry of the lithosphere, atmosphere, and oceans, the role of "evolutionary pumps" such as massive glaciation and rare bolide impacts. Perhaps most importantly, advanced life needs whatever it was that led to the transition of (some) prokaryotic cells to eukaryotic cells, sexual reproduction and the Cambrian explosion.

In his book Wonderful Life (1989), Stephen Jay Gould suggested that if the "tape of life" were rewound to the time of the Cambrian explosion, and one or two tweaks made, human beings probably never would have evolved. Other thinkers such as Fontana, Buss, and Kauffman have written about the self-organizing properties of life.

Extraterrestrial intelligence is rare or non-existent

It is possible that even if complex life is common, intelligence (and consequently civilizations) is not. While there are remote sensing techniques that could perhaps detect life-bearing planets without relying on the signs of technology, none of them have the ability to determine if any detected life is intelligent. This is sometimes referred to as the "algae vs. alumnae" problem.

Charles Lineweaver states that when considering any extreme trait in an animal, intermediate stages do not necessarily produce "inevitable" outcomes. For example, large brains are no more "inevitable", or convergent, than are the long noses of animals such as aardvarks and elephants. As he points out, "dolphins have had ~20 million years to build a radio telescope and have not done so". In addition, Rebecca Boyle points out that of all the species that have evolved in the history of life on the planet Earth, only one—human beings and only in the beginning stages—has ever become space-faring.

Extraterrestrial intelligence is relatively new

Main article: Firstborn hypothesis

Given that the expected lifespan of the universe is at least one trillion years and the age of the universe is around 14 billion years, it is possible that humans have emerged at or near the earliest possible opportunity for intelligent life to evolve. Avi Loeb, an astrophysicist and cosmologist, has suggested that Earth may be a very early example of a life-bearing planet and that life-bearing planets may be more likely trillions of years from now. He has put forward the view that the Universe has only recently reached a state in which life is possible and this is the reason humanity has not detected extraterrestrial life. The firstborn hypothesis posits that humans are the first, or one of the first, intelligent species to evolve. Therefore, many intelligent species may eventually exist, but few, if any, currently do. Moreover, it is possible that said species, even if they already exist, are developing more slowly, or have more limited resources on their home world, meaning that they may take longer than humans have to achieve spaceflight.

Periodic extinction by natural events

See also: Global catastrophic risk and Neocatastrophism

An asteroid impact may trigger an extinction event.

New life might commonly die out due to runaway heating or cooling on their fledgling planets. On Earth, there have been numerous major extinction events that destroyed the majority of complex species alive at the time; the extinction of the non-avian dinosaurs is the best known example. These are thought to have been caused by events such as impact from a large asteroid, massive volcanic eruptions, or astronomical events such as gamma-ray bursts. It may be the case that such extinction events are common throughout the universe and periodically destroy intelligent life, or at least its civilizations, before the species is able to develop the technology to communicate with other intelligent species.

However, the chances of extinction by natural events may be very low on the scale of a civilization's lifetime. Based on an analysis of impact craters on Earth and the Moon, the average interval between impacts large enough to cause global consequences (like the Chicxulub impact) is estimated to be around 100 million years.

Evolutionary explanations

It is the nature of intelligent life to destroy itself

See also: Technological utopianism § Criticisms

A 23-kiloton tower shot called BADGER, fired as part of the Operation Upshot–Knothole nuclear test series

This is the argument that technological civilizations may usually or invariably destroy themselves before or shortly after developing radio or spaceflight technology. The astrophysicist Sebastian von Hoerner stated that the progress of science and technology on Earth was driven by two factors—the struggle for domination and the desire for an easy life. The former potentially leads to complete destruction, while the latter may lead to biological or mental degeneration. Possible means of annihilation via major global issues, where global interconnectedness actually makes humanity more vulnerable than resilient, are many, including war, accidental environmental contamination or damage, the development of biotechnology, synthetic life like mirror life, resource depletion, climate change, or artificial intelligence. This general theme is explored both in fiction and in scientific hypotheses.

In 1966, Sagan and Shklovskii speculated that technological civilizations will either tend to destroy themselves within a century of developing interstellar communicative capability or master their self-destructive tendencies and survive for billion-year timescales. Self-annihilation may also be viewed in terms of thermodynamics: insofar as life is an ordered system that can sustain itself against the tendency to disorder, Stephen Hawking's "external transmission" or interstellar communicative phase, where knowledge production and knowledge management is more important than transmission of information via evolution, may be the point at which the system becomes unstable and self-destructs. Here, Hawking emphasizes self-design of the human genome (transhumanism) or enhancement via machines (e.g., brain–computer interface) to enhance human intelligence and reduce aggression, without which he implies human civilization may be too stupid collectively to survive an increasingly unstable system. For instance, the development of technologies during the "external transmission" phase, such as weaponization of artificial general intelligence or antimatter, may not be met by concomitant increases in human ability to manage its own inventions. Consequently, disorder increases in the system: global governance may become increasingly destabilized, worsening humanity's ability to manage the possible means of annihilation listed above, resulting in global societal collapse.

A less theoretical example might be the resource-depletion issue on Polynesian islands, of which Easter Island is only the best known. David Brin points out that during the expansion phase from 1500 BC to 800 AD there were cycles of overpopulation followed by what might be called periodic cullings of adult males through war or ritual. He writes, "There are many stories of islands whose men were almost wiped out—sometimes by internal strife, and sometimes by invading males from other islands."

Using extinct civilizations such as Easter Island as models, a study conducted in 2018 by Adam Frank et al. posited that climate change induced by "energy intensive" civilizations may prevent sustainability within such civilizations, thus explaining the paradoxical lack of evidence for intelligent extraterrestrial life. Based on dynamical systems theory, the study examined how technological civilizations (exo-civilizations) consume resources and the feedback effects this consumption has on their planets and its carrying capacity. According to Adam Frank "[t]he point is to recognize that driving climate change may be something generic. The laws of physics demand that any young population, building an energy-intensive civilization like ours, is going to have feedback on its planet. Seeing climate change in this cosmic context may give us better insight into what's happening to us now and how to deal with it." Generalizing the Anthropocene, their model produces four different outcomes:

Possible trajectories of anthropogenic climate change in a model by Frank et al., 2018

• Die-off: A scenario where the population grows quickly, surpassing the planet's carrying capacity, which leads to a peak followed by a rapid decline. The population eventually stabilizes at a much lower equilibrium level, allowing the planet to partially recover.
• Sustainability: A scenario where civilizations successfully transition from high-impact resources (like fossil fuels) to sustainable ones (like solar energy) before significant environmental degradation occurs. This allows the civilization and planet to reach a stable equilibrium, avoiding catastrophic effects.
• Collapse Without Resource Change: In this trajectory, the population and environmental degradation increase rapidly. The civilization does not switch to sustainable resources in time, leading to a total collapse where a tipping point is crossed and the population drops.
• Collapse With Resource Change: Similar to the previous scenario, but in this case, the civilization attempts to transition to sustainable resources. However, the change comes too late, and the environmental damage is irreversible, still leading to the civilization's collapse.

Only one intelligent species can exist in a given region of space

See also: Berserker hypothesis, Dark forest hypothesis, Technological singularity, and Von Neumann probe

Another hypothesis is that an intelligent species beyond a certain point of technological capability will destroy other intelligent species as they appear, perhaps by using self-replicating probes. Science fiction writer Fred Saberhagen has explored this idea in his Berserker series, as has physicist Gregory Benford and also, science fiction writer Greg Bear in his The Forge of God novel, and later Liu Cixin in his The Three-Body Problem series.

A species might undertake such extermination out of expansionist motives, greed, paranoia, or aggression. In 1981, cosmologist Edward Harrison argued that such behavior would be an act of prudence: an intelligent species that has overcome its own self-destructive tendencies might view any other species bent on galactic expansion as a threat. It has also been suggested that a successful alien species would be a superpredator, as are humans. Another possibility invokes the "tragedy of the commons" and the anthropic principle: the first lifeform to achieve interstellar travel will necessarily (even if unintentionally) prevent competitors from arising, and humans simply happen to be first.^[123]

Civilizations only broadcast detectable signals for a brief period of time

It may be that alien civilizations are detectable through their radio emissions for only a short time, reducing the likelihood of spotting them. The usual assumption is that civilizations outgrow radio through technological advancement. However, there could be other leakage such as that from microwaves used to transmit power from solar satellites to ground receivers. Regarding the first point, in a 2006 Sky & Telescope article, Seth Shostak wrote, "Moreover, radio leakage from a planet is only likely to get weaker as a civilization advances and its communications technology gets better. Earth itself is increasingly switching from broadcasts to leakage-free cables and fiber optics, and from primitive but obvious carrier-wave broadcasts to subtler, hard-to-recognize spread-spectrum transmissions."

More hypothetically, advanced alien civilizations may evolve beyond broadcasting at all in the electromagnetic spectrum and communicate by technologies not developed or used by mankind. Some scientists have hypothesized that advanced civilizations may send neutrino signals. If such signals exist, they could be detectable by neutrino detectors that are as of 2009 under construction for other goals.

Alien life may be too incomprehensible

Microwave window as seen by a ground-based system. From NASA report SP-419: SETI – the Search for Extraterrestrial Intelligence

Another possibility is that human theoreticians have underestimated how much alien life might differ from that on Earth. Aliens may be psychologically unwilling to attempt to communicate with human beings. Perhaps human mathematics is parochial to Earth and not shared by other life, though others argue this can only apply to abstract math since the math associated with physics must be similar (in results, if not in methods).

In his 2009 book, SETI scientist Seth Shostak wrote, "Our experiments [such as plans to use drilling rigs on Mars] are still looking for the type of extraterrestrial that would have appealed to Percival Lowell [astronomer who believed he had observed canals on Mars]."

Physiology might also be a communication barrier. Carl Sagan speculated that an alien species might have a thought process orders of magnitude slower (or faster) than that of humans. A message broadcast by that species might seem like random background noise to humans, and therefore go undetected.

Paul Davies stated that 500 years ago the very idea of a computer doing work merely by manipulating internal data may not have been viewed as a technology at all. He writes, "Might there be a still higher level [...] If so, this 'third level' would never be manifest through observations made at the informational level, still less the matter level. There is no vocabulary to describe the third level, but that doesn't mean it is non-existent, and we need to be open to the possibility that alien technology may operate at the third level, or maybe the fourth, fifth [...] levels."

Arthur C. Clarke hypothesized that "our technology must still be laughably primitive; we may well be like jungle savages listening for the throbbing of tom-toms, while the ether around them carries more words per second than they could utter in a lifetime". Another thought is that technological civilizations invariably experience a technological singularity and attain a post-biological character.

Sociological explanations

Expansionism is not the cosmic norm

In response to Tipler's idea of self-replicating probes, Stephen Jay Gould wrote, "I must confess that I simply don't know how to react to such arguments. I have enough trouble predicting the plans and reactions of the people closest to me. I am usually baffled by the thoughts and accomplishments of humans in different cultures. I'll be damned if I can state with certainty what some extraterrestrial source of intelligence might do."

Alien species may have only settled part of the galaxy

According to a study by Frank et al., advanced civilizations may not colonize everything in the galaxy due to their potential adoption of steady states of expansion. This hypothesis suggests that civilizations might reach a stable pattern of expansion where they neither collapse nor aggressively spread throughout the galaxy. A February 2019 article in Popular Science states, "Sweeping across the Milky Way and establishing a unified galactic empire might be inevitable for a monolithic super-civilization, but most cultures are neither monolithic nor super—at least if our experience is any guide." Astrophysicist Adam Frank, along with co-authors such as astronomer Jason Wright, ran a variety of simulations in which they varied such factors as settlement lifespans, fractions of suitable planets, and recharge times between launches. They found many of their simulations seemingly resulted in a "third category" in which the Milky Way remains partially settled indefinitely. The abstract to their 2019 paper states, "These results break the link between Hart's famous 'Fact A' (no interstellar visitors on Earth now) and the conclusion that humans must, therefore, be the only technological civilization in the galaxy. Explicitly, our solutions admit situations where our current circumstances are consistent with an otherwise settled, steady-state galaxy."

An alternative scenario is that long-lived civilizations may only choose to colonize stars during closest approach. As low mass K- and M-type dwarfs are by far the most common types of main sequence stars in the Milky Way, they are more likely to pass close to existing civilizations. These stars have longer life spans, which may be preferred by such a civilization. Interstellar travel capability of 0.3 light years is theoretically sufficient to colonize all M-dwarfs in the galaxy within 2 billion years. If the travel capability is increased to 2 light years, then all K-dwarfs can be colonized in the same time frame.

Alien species may isolate themselves in virtual worlds

Avi Loeb suggests that one possible explanation for the Fermi paradox is virtual reality technology. Individuals of extraterrestrial civilizations may prefer to spend time in virtual worlds or metaverses that have different physical law constraints as opposed to focusing on colonizing planets. Nick Bostrom suggests that some advanced beings may divest themselves entirely of physical form, create massive artificial virtual environments, transfer themselves into these environments through mind uploading, and exist totally within virtual worlds, ignoring the external physical universe.

It may be that intelligent alien life develops an "increasing disinterest" in their outside world. Possibly any sufficiently advanced society will develop highly engaging media and entertainment well before the capacity for advanced space travel, with the rate of appeal of these social contrivances being destined, because of their inherent reduced complexity, to overtake any desire for complex, expensive endeavors such as space exploration and communication. Once any sufficiently advanced civilization becomes able to master its environment, and most of its physical needs are met through technology, various "social and entertainment technologies", including virtual reality, are postulated to become the primary drivers and motivations of that civilization.

Artificial intelligence may not be aggressively expansionist

See also: Existential risk from artificial general intelligence

While artificial intelligence supplanting its creators could only deepen the Fermi paradox, such as through enabling the colonizing of the galaxy through self-replicating probes, it is also possible that after replacing its creators, artificial intelligence either doesn't expand or endure for a variety of reasons. Michael A. Garrett has suggested that biological civilizations may universally underestimate the speed that AI systems progress, and not react to it in time, thus making it a possible great filter. He also argues that this could make the longevity of advanced technological civilizations less than 200 years, thus explaining the great silence observed by SETI.

Economic explanations

Lack of resources needed to physically spread throughout the galaxy

See also: Project Daedalus, Project Orion (nuclear propulsion), and Project Longshot

The ability of an alien culture to colonize other star systems is based on the idea that interstellar travel is technologically feasible. While the existing understanding of physics rules out the possibility of faster-than-light travel, it appears that there are no major theoretical barriers to the construction of "slow" interstellar ships, even though the engineering required is considerably beyond existing human capabilities. This idea underlies the concept of the Von Neumann probe and the Bracewell probe as a potential evidence of extraterrestrial intelligence.

It is possible, however, that scientific knowledge cannot properly gauge the feasibility and costs of such interstellar colonization. Theoretical barriers may not yet be understood, and the resources needed may be so great as to make it unlikely that any civilization could afford to attempt it. Even if interstellar travel and colonization are possible, they may be difficult, leading to a more gradual pace of colonization based on percolation.

Colonization efforts may not occur as an unstoppable hyper-aggressive rush, but rather as an uneven tendency to "percolate" outwards, within an eventual slowing and termination of the effort given the enormous costs involved and the expectation that colonies will inevitably develop a culture and civilization of their own. Colonization may thus occur in "clusters", with large areas remaining uncolonized at any one time, and planets only restarting the colonization process when their populations begin to outstrip their world's carrying capacity.

Information is cheaper to transmit than matter is to transfer

If a human-capability machine intelligence is possible, and if it is possible to transfer such constructs over vast distances and rebuild them on a remote machine, then it might not make strong economic sense to travel the galaxy by spaceflight. Louis K. Scheffer calculates the cost of radio transmission of information across space to be cheaper than spaceflight by a factor of 10⁸–10¹⁷. For a machine civilization, the costs of interstellar travel are therefore enormous compared to the more efficient option of sending computational signals across space to already established sites. After the first civilization has physically explored or colonized the galaxy, as well as sent such machines for easy exploration, then any subsequent civilizations, after having contacted the first, may find it cheaper, faster, and easier to explore the galaxy through intelligent mind transfers to the machines built by the first civilization. However, since a star system needs only one such remote machine, and the communication is most likely highly directed, transmitted at high-frequencies, and at a minimal power to be economical, such signals would be hard to detect from Earth.

By contrast, in economics the counter-intuitive Jevons paradox implies that higher productivity results in higher demand. In other words, increased economic efficiency results in increased economic growth. For example, increased renewable energy has the risk of not directly resulting in declining fossil fuel use, but rather additional economic growth as fossil fuels instead are directed to alternative uses. Thus, technological innovation makes human civilization more capable of higher levels of consumption, as opposed to its existing consumption being achieved more efficiently at a stable level.

Other species' home planets cannot support industrial economies

Amedeo Balbi and Adam Frank propose the concept of an "oxygen bottleneck" for the emergence of the industrial production necessary for spaceflight. The "oxygen bottleneck" refers to the critical level of atmospheric oxygen necessary for fire and combustion. Earth's atmospheric oxygen concentration is about 21%, but has been much lower in the past and may also be on many exoplanets. The authors argue that while the threshold of oxygen required for the existence of complex life and ecosystems is relatively low, industrial processes which are necessary precursors to spaceflight, particularly metal smelting and many forms of electricity generation, require higher oxygen concentrations of at least some 18%. A planet with oxygen sufficient to support intelligent life but not to develop advanced metallurgy would be technologically gated by its extremely limited industrial capabilities at a level likely incapable of supporting spaceflight. Thus, the presence of high levels of oxygen in a planet's atmosphere is not only a potential biosignature but also a critical factor in the emergence of detectable technological civilizations.

Another hypothesis in this category is the "waterworlds hypothesis". According to author and scientist David Brin: "it turns out that our Earth skates the very inner edge of our sun's continuously habitable—or 'Goldilocks'—zone. And Earth may be anomalous. It may be that because we are so close to our sun, we have an anomalously oxygen-rich atmosphere, and we have anomalously little ocean for a water world. In other words, 32 percent continental mass may be high among water worlds..." Brin continues, "In which case, the evolution of creatures like us, with hands and fire and all that sort of thing, may be rare in the galaxy. In which case, when we do build starships and head out there, perhaps we'll find lots and lots of life worlds, but they're all like Polynesia. We'll find lots and lots of intelligent lifeforms out there, but they're all dolphins, whales, squids, who could never build their own starships. What a perfect universe for us to be in, because nobody would be able to boss us around, and we'd get to be the voyagers, the Star Trek people, the starship builders, the policemen, and so on."

Intelligent alien species have not developed advanced technologies

Le Moustier Neanderthals (Charles R. Knight, 1920)

It may be that while alien species with intelligence exist, they are primitive or have not reached the level of technological advancement necessary to communicate. Along with non-intelligent life, such civilizations would also be very difficult to detect from Earth. A trip using conventional rockets would take hundreds of thousands of years to reach the nearest stars.

To skeptics, the fact that over the history of life on the Earth, only one species has developed a civilization to the point of being capable of spaceflight, and this only in the early stages, lends credence to the idea that technologically advanced civilizations are rare in the universe.

Developing practical spaceflight technology is very difficult or expensive

The rapid increase of scientific and technological progress seen in the 18th to 20th centuries (the Industrial Revolution), compared to earlier eras, led to the common assumption that such progress will continue at exponential rates as time goes by, eventually leading to the progress level required for space exploration. The "universal limit to technological development" (ULTD) hypothesis proposes that there is a limit to the potential growth of a civilization, and that this limit may be placed well below the point required for space exploration. Such limits may be based on the enormous strain spaceflight may put on a planet's resources, physical limitations (such as faster-than-light travel being impossible), and even limitations based on the species' own biology.

Discovering extraterrestrial life is very difficult

Humans are not listening properly

There are some assumptions that underlie the SETI programs that may cause searchers to miss signals that exist. Extraterrestrials might, for example, transmit signals that have a very high or low data rate, or employ unconventional (in human terms) frequencies, which would make them hard to distinguish from background noise. Signals might be sent from non-main sequence star systems that humans search with lower priority; our programs assume that most alien life will be orbiting Sun-like stars.

Radio signals cannot be straightforwardly detected at interstellar distances

The greatest challenge is the sheer size of the radio search needed to look for signals (effectively spanning the entire observable universe), the limited amount of resources committed to SETI, and the sensitivity of modern instruments. SETI estimates, for instance, that with a radio telescope as sensitive as the Arecibo Observatory, Earth's television and radio broadcasts would only be detectable at distances up to 0.3 light-years, less than 1/10 the distance to the nearest star. A signal is much easier to detect if it consists of a deliberate, powerful transmission directed at Earth. Such signals could be detected at ranges of hundreds to tens of thousands of light-years distance. However, this means that detectors must be listening to an appropriate range of frequencies, and be in that region of space to which the beam is being sent. Many SETI searches assume that extraterrestrial civilizations will be broadcasting a deliberate signal, like the Arecibo message, in order to be found. Moreover, as human communication technology has advanced, humans have reduced the use of broadband radio transmissions in favor of more efficient and higher-bandwidth methods such as satellite communication and fibre optics. It may be that alien civilizations, having, as we have, largely moved past high-power radio broadcasting, producing very few, if any, detectable transmissions.

Thus, to detect alien civilizations through their radio emissions, Earth observers need very sensitive instruments, and moreover must hope that:

1) Aliens have developed radio technology, and,

2) Aliens use radio as a primary means of communication, and,

3) For reasons unknown, their transmitters are orders of magnitude more powerful than ours, or they are deliberately broadcasting high-power radio signals towards Earth as part of their own efforts to contact other civilizations, and,

4) We are listening at the right frequency, at the right time, and,

5) We recognize their transmission as an attempt at communication.

Humans have not listened for long enough

Humanity's ability to detect intelligent extraterrestrial life has existed for only a very brief period—from 1937 onwards, if the invention of the radio telescope is taken as the dividing line—and Homo sapiens is a geologically recent species. The whole period of modern human existence to date is a very brief period on a cosmological scale, and radio transmissions have only been propagated since 1895. Thus, it remains possible that human beings have neither existed long enough nor made themselves sufficiently detectable to be found by extraterrestrial intelligence.

Intelligent life may be too far away

NASA's conception of the Terrestrial Planet Finder

It may be that non-colonizing technologically capable alien civilizations exist, but that they are simply too far apart for meaningful two-way communication. Sebastian von Hoerner estimated the average duration of civilization at 6,500 years and the average distance between civilizations in the Milky Way at 1,000 light years. If two civilizations are separated by several thousand light-years, it is possible that one or both cultures may become extinct before meaningful dialogue can be established. Human searches may be able to detect their existence, but communication will remain impossible because of distance. It has been suggested that this problem might be ameliorated somewhat if contact and communication is made through a Bracewell probe. In this case at least one partner in the exchange may obtain meaningful information. Alternatively, a civilization may simply broadcast its knowledge, and leave it to the receiver to make what they may of it. This is similar to the transmission of information from ancient civilizations to the present, and humanity has undertaken similar activities like the Arecibo message, which could transfer information about Earth's intelligent species, even if it never yields a response or does not yield a response in time for humanity to receive it. It is possible that observational signatures of self-destroyed civilizations could be detected, depending on the destruction scenario and the timing of human observation relative to it.

A related speculation by Sagan and Newman suggests that if other civilizations exist, and are transmitting and exploring, their signals and probes simply have not arrived yet, i.e. that Humans are a relatively early civilization. However, critics have noted that this is unlikely, since it requires that humanity's advancement has occurred at a very special point in time, while the Milky Way is in transition from empty to full. This is a tiny fraction of the lifespan of a galaxy under ordinary assumptions, so the likelihood that humanity is in the midst of this transition is considered low in the paradox. In 2021, Hanson et al. reconsidered this likelihood and concluded it is indeed plausible when assuming that many civilizations are "grabby", i.e. displace other civilizations. Under this assumption there is a selection effect of the sort that provided we exist and are not (yet) destroyed by grabby aliens, we are very unlikely to observe aliens. Specifically, grabby aliens imply a typical civilizational expansion rate at nearly the speed of light because otherwise many other civilizations would be visible. The transition time between detection of an alien technosignature and extinction would be vanishingly short in cosmological timeframes, making it likely we are before that time period.

Some SETI skeptics may also believe that humanity is at a very special point of time—specifically, a transitional period from no space-faring societies to one space-faring society, namely that of human beings.

Intelligent life exists buried below the surfaces of ice planets

Planetary scientist Alan Stern put forward the idea that there could be a number of worlds with subsurface oceans (such as Jupiter's Europa or Saturn's Enceladus). The surface would provide a large degree of protection from such things as cometary impacts and nearby supernovae, as well as creating a situation in which a much broader range of orbital configurations are capable of supporting life. Life, and potentially intelligence and civilization, could evolve below the surface of such a planet, but be very hard to detect, insofar as it is generally only possible to observe the surface of planets from space. Stern states, "If they have technology, and let's say they're broadcasting, or they have city lights or whatever—we can't see it in any part of the spectrum, except maybe very-low-frequency [radio]."Moreover, such a civilization may have great difficulty getting to space, insofar as even getting to the surface of their world could present a considerable engineering challenge involving tunneling through many kilometres of ice. This may severely hamper their ability to communicate with us.

Advanced civilizations may limit their search for life to technological signatures

If life is abundant in the universe but the cost of space travel is high, an advanced civilization may choose to focus its search not on signs of life in general, but on those of other advanced civilizations, and specifically on radio signals. Since humanity has only recently began to use radio communication, its signals may have yet to arrive to other inhabited planets, and if they have, probes from those planets may have yet to arrive on Earth.

Willingness to communicate

Everyone is listening but no one is transmitting

Alien civilizations might be technically capable of contacting Earth, but could be only listening instead of transmitting. If all or most civilizations act in the same way, the galaxy could be full of civilizations eager for contact, but everyone is listening and no one is transmitting. This is the so-called SETI Paradox. The only civilization known, humanity, does not explicitly transmit, except for a few small efforts.

Alien governments are choosing not to respond

Even these limited efforts, and certainly any attempt to expand them, are controversial. It is not even clear humanity would respond to a detected signal—the official policy within the SETI community is that "[no] response to a signal or other evidence of extraterrestrial intelligence should be sent until appropriate international consultations have taken place". However, given the possible impact of any reply, it may be very difficult to obtain any consensus on whether to reply, and if so, who would speak and what they would say. It is therefore quite possible that an alien civilization led by cautious decision-makers might conclude that not responding is the soundest option. Moreover, as the only observed civilization does not have a planetary central government capable of making a binding decision about a response, alien civilizations, themselves divided into various political units without a central decision-making authority, may be aware of our existence and technically capable of responding, but cannot agree on whether and/or how to do so.

Communication is dangerous

See also: Dark forest hypothesis

An alien civilization might feel it is too dangerous to communicate, either for humanity or for them. It is argued that when very different civilizations have met on Earth, the results have often been disastrous for one side or the other, and the same may well apply to interstellar contact. Even contact at a safe distance could lead to infection by computer code or even ideas themselves. Perhaps prudent civilizations actively hide not only from Earth but from everyone, out of fear of other civilizations.

Perhaps the Fermi paradox itself, however aliens may conceive of it, is the reason for any civilization to avoid contact with other civilizations, even if no other obstacles existed. From any one civilization's point of view, it would be unlikely for them to be the first ones to make first contact. According to this reasoning, it is likely that previous civilizations faced fatal problems upon first contact and doing so should be avoided. So perhaps every civilization keeps quiet because of the possibility that there is a real reason for others to do so.

In 1987, science fiction author Greg Bear explored this concept in his novel The Forge of God. In The Forge of God, humanity is likened to a baby crying in a hostile forest: "There once was an infant lost in the woods, crying its heart out, wondering why no one answered, drawing down the wolves." One of the characters explains, "We've been sitting in our tree chirping like foolish birds for over a century now, wondering why no other birds answered. The galactic skies are full of hawks, that's why. Planetisms that don't know enough to keep quiet, get eaten."

In Liu Cixin's 2008 novel The Dark Forest, the author proposes a literary explanation for the Fermi paradox in which countless alien civilizations exist, but are both silent and paranoid, destroying any nascent lifeforms loud enough to make themselves known. This is because any other intelligent life may represent a future threat. As a result, Liu's fictional universe contains a plethora of quiet civilizations which do not reveal themselves, as in a "dark forest"...filled with "armed hunter(s) stalking through the trees like a ghost". This idea has come to be known as the dark forest hypothesis.

Earth is deliberately being avoided

Main article: Zoo hypothesis

The zoo hypothesis states that intelligent extraterrestrial life exists and does not contact life on Earth to allow for its natural evolution and development as a sort of cosmic closed nature reserve. A variation on the zoo hypothesis is the laboratory hypothesis, where humanity has been or is being subject to experiments, with Earth or the Solar System effectively serving as a laboratory. The zoo hypothesis may break down under the uniformity of motive flaw: all it takes is a single culture or civilization (or even a faction or rogue actor within one) to decide to act contrary to the interplanetary consensus, and the probability of such a violation of hegemony increases with the number of civilizations, tending not towards a galactic league with a single policy towards Earth, but towards multiple competing factions. However, if artificial superintelligences are paramount in galactic politics, and such intelligences tend towards consolidation behind a central authority, then this would at least partially address the uniformity of motive flaw by dissuading rogue behavior.

Analysis of the inter-arrival times between civilizations in the galaxy based on common astrobiological assumptions suggests that the initial civilization would have a commanding lead over the later arrivals, inasmuch as it has had time to assert control over resources, and settle the best planets (assuming similar biological needs to competitors). As such, it may have established what has been termed the zoo hypothesis through force or as a galactic or universal norm and the resultant "paradox" by a cultural founder effect with or without the continued activity of the founder. Some colonization scenarios predict spherical expansion across star systems, with continued expansion coming from the systems just previously settled. It has been suggested that this would cause a strong selection process among colonists, favoring cultural, biological, or political adaptation to living aboard spacecraft or space habitats for long periods of time; as a result, they may only settle a very limited number of the highest-quality planets, or simply stay aboard their ships and forgo planets entirely. This may result in a lack of interest in colonization, instead focusing on planets only as a destructible source of non-renewable resources. Alternatively, they may have an ethic of protection for "nursery worlds", and protect them without intervening. Moreover, having developed spaceborne habitation sufficient to support their needs, they may obtain resources through asteroid mining and mostly ignore terrestrial worlds insofar as they require a much greater expenditure of fuel and resources to make it to land on for mining compared to smaller objects.

It is possible that a civilization advanced enough to travel between planetary systems could be actively visiting or observing Earth while remaining undetected or unrecognized. Following this logic, and building on arguments that other proposed solutions to the Fermi paradox may be implausible, Ian Crawford and Dirk Schulze-Makuch have argued that technological civilisations are either very rare in the Galaxy or are deliberately hiding from us.

Earth is deliberately being isolated

Main article: Planetarium hypothesis

A related idea to the zoo hypothesis is that, beyond a certain distance, the perceived universe is a simulated reality. The planetarium hypothesis speculates that beings may have created this simulation so that the universe appears to be empty of other life.

Conspiracy theories: alien life is already here, unacknowledged and/or deliberately concealed

Main article: Extraterrestrial UFO hypothesis

Further information: UFO conspiracy theories

A significant fraction of the population believes that at least some UFOs (Unidentified Flying Objects) are spacecraft piloted by aliens. While most of these are unrecognized or mistaken interpretations of mundane phenomena, some occurrences remain puzzling even after investigation. The scientific consensus is that although they may be unexplained, they do not rise to the level of convincing evidence.

Similarly, it is theoretically possible that SETI groups are not reporting positive detections, or governments have been blocking signals or suppressing publication. This response might be attributed to security or economic interests from the potential use of advanced extraterrestrial technology. It has been suggested that the detection of an extraterrestrial radio signal or technology could well be the most highly secret information that exists. Claims that this has already happened are common in the popular press, but the scientists involved report the opposite experience—the press becomes informed and interested in a potential detection even before a signal can be confirmed.

Regarding the idea that aliens are in secret contact with governments, David Brin writes, "Aversion to an idea, simply because of its long association with crackpots, gives crackpots altogether too much influence."