1579 drawing of the Great Chain of Being from Didacus Valades [es], Rhetorica Christiana
The great chain of being is a hierarchical structure of all matter and life, thought by medieval Christianity to have been decreed by God. The chain begins with God and descends through angels, humans, animals and plants to minerals.
The chain of being hierarchy has God at the top, above angels, which like him are entirely spirit, without material bodies, and hence unchangeable. Beneath them are humans, consisting both of spirit and matter; they change and die, and are thus essentially impermanent. Lower are animals and plants. At the bottom are the mineral materials
of the earth itself; they consist only of matter. Thus, the higher the
being is in the chain, the more attributes it has, including all the
attributes of the beings below it. The minerals are, in the medieval mind, a possible exception to the immutability of the material beings in the chain, as alchemy promised to turn lower elements like lead into those higher up the chain, like silver or gold.
The Great Chain
The Great Chain of being links God, angels, humans, animals, plants, and minerals. The links of the chain are:
God
God is the creator of all things. Many religions, such as Judaism, Christianity, and Islam
believe he created the entire universe and everything in it. He has
spiritual attributes found in angels and humans. God has unique
attributes of omnipotence, omnipresence, and omniscience. He is the model of perfection in all of creation.
In the New Testament, the Epistle to the Colossians
sets out a partial list: "everything visible and everything invisible,
Thrones, Dominations, Sovereignties, Powers – all things were created
through him and for him." The Epistle to the Ephesians
also lists several entities: "Far above all principality, and power,
and might, and dominion, and every name that is named, not only in this
world, but also in that which is to come".
In the 5th and 6th centuries, Pseudo-Dionysius the Areopagite set out a more elaborate hierarchy, consisting of three lists, each of three types:
Humans uniquely share spiritual attributes with God and the angels above them, love
and language, and physical attributes with the animals below them, like
having material bodies that experienced emotions and sensations such as
lust and pain, and physical needs such as hunger and thirst.
Charles Bonnet's chain of being from Traité d'insectologie, 1745
Animals
Animals have senses, are able to move, and have physical appetites. The apex predator like the lion,
could move vigorously, and has powerful senses like keen eyesight and
the ability to smell their prey from a distance, while a lower order of
animals might wiggle or crawl, or like oysters were sessile, attached to the sea-bed. All, however, share the senses of touch and taste.
Plants
Plants
lacked sense organs and the ability to move, but they could grow and
reproduce. The highest plants have important healing attributes within
their leaves, buds, and flowers. Lower plants included fungi and mosses.
Minerals
At
the bottom of the chain, minerals were unable to move, sense, grow, or
reproduce. Their attributes were being solid and strong, while the
gemstones possessed magic. The king of gems was the diamond.
The basic idea of a ranking of the world's organisms goes back to Aristotle's biology. In his History of Animals,
where he ranked animals over plants based on their ability to move and
sense, and graded the animals by their reproductive mode, live birth
being "higher" than laying cold eggs, and possession of blood,
warm-blooded mammals and birds again being "higher" than "bloodless"
invertebrates.
Aristotle's non-religious concept of higher and lower organisms was taken up by natural philosophers during the Scholastic period to form the basis of the Scala Naturae. The scala
allowed for an ordering of beings, thus forming a basis for
classification where each kind of mineral, plant and animal could be
slotted into place. In medieval times, the great chain was seen as a
God-given and unchangeable ordering. In the Northern Renaissance, the scientific focus shifted to biology; the threefold division of the chain below humans formed the basis for Carl Linnaeus's Systema Naturæ from 1737, where he divided the physical components of the world into the three familiar kingdoms of minerals, plants and animals.
In alchemy
Alchemy
used the great chain as the basis for its cosmology. Since all beings
were linked into a chain, so that there was a fundamental unity of all matter,
the transformation from one place in the chain to the next might,
according to alchemical reasoning, be possible. In turn, the unit of the
matter enabled alchemy to make another key assumption, the philosopher's stone, which somehow gathered and concentrated the universal spirit found in all matter along the chain, and which ex hypothesi might enable the alchemical transformation of one substance to another, such as the base metal lead to the noble metal gold.
The set nature of species, and thus the absoluteness of creatures'
places in the great chain, came into question during the 18th century.
The dual nature of the chain, divided yet united, had always allowed for
seeing creation as essentially one continuous whole, with the potential
for overlap between the links. Radical thinkers like Jean-Baptiste Lamarck
saw a progression of life forms from the simplest creatures striving
towards complexity and perfection, a schema accepted by zoologists like Henri de Blainville. The very idea of an ordering of organisms, even if supposedly fixed, laid the basis for the idea of transmutation of species, whether progressive goal-directed orthogenesis or Charles Darwin's undirected theory of evolution.
The chain of being continued to be part of metaphysics in 19th-century education, and the concept was well known. The geologist Charles Lyell used it as a metaphor in his 1851 Elements of Geology description of the geological column, where he used the term "missing links" about missing parts of the continuum. The term "missing link" later came to signify transitional fossils, particularly those bridging the gulf between man and beasts.
Allenby
and Garreau propose that the Catholic Church's narrative of the great
chain of being kept the peace in Europe for centuries. The very concept
of rebellion simply lay outside the reality within which most people
lived, for to defy the King was to defy God. King James I himself wrote,
"The state of monarchy is the most supreme thing upon earth: for kings
are not only God's Lieutenants upon earth, and sit upon God's throne,
but even by God himself they are called Gods."
Adaptations and similar concepts
The American philosopher Ken Wilber described a "Great Nest of Being" which he claims to belong to a culture-independent "perennial philosophy" traceable across 3000 years of mystical and esoteric writings. Wilber's system corresponds with other concepts of transpersonal psychology. In his 1977 book A Guide for the Perplexed, the economist E. F. Schumacher described a hierarchy of beings, with humans at the top able mindfully to perceive the "eternal now".
Extraterrestrial life, or alien life (colloquially, aliens), is life that originates from another world rather than on Earth. No extraterrestrial life has yet been scientifically conclusively detected. Such life might range from simple forms such as prokaryotes to intelligent beings, possibly bringing forth civilizations that might be far more, or far less, advanced than humans. The Drake equation speculates about the existence of sapient life elsewhere in the universe. The science of extraterrestrial life is known as astrobiology.
Speculation about the possibility of inhabited worlds beyond Earth dates back to antiquity. Early Christian writers discussed the idea of a "plurality of worlds" as proposed by earlier thinkers such as Democritus; Augustine references Epicurus's idea of innumerable worlds "throughout the boundless immensity of space" in The City of God.
Pre-modern writers typically assumed extraterrestrial "worlds" were inhabited by living beings. William Vorilong, in the 15th century, acknowledged the possibility Jesus could have visited extraterrestrial worlds to redeem their inhabitants.Nicholas of Cusa
wrote in 1440 that Earth is "a brilliant star" like other celestial
objects visible in space; which would appear similar to the Sun,
from an exterior perspective, due to a layer of "fiery brightness" in
the outer layer of the atmosphere. He theorized all extraterrestrial
bodies could be inhabited by men, plants, and animals, including the
Sun. Descartes
wrote that there were no means to prove the stars were not inhabited by
"intelligent creatures", but their existence was a matter of
speculation.
The concept of extraterrestrial life, particularly extraterrestrial intelligence, has had a major cultural impact, especially extraterrestrials in fiction. Science fiction
has communicated scientific ideas, imagined a range of possibilities,
and influenced public interest in and perspectives on extraterrestrial
life. One shared space is the debate over the wisdom of attempting
communication with extraterrestrial intelligence. Some encourage
aggressive methods to try to contact intelligent extraterrestrial life.
Others – citing the tendency of technologically advanced human societies
to enslave or destroy less advanced societies – argue it may be dangerous to actively draw attention to Earth.
Context
Initially, after the Big Bang,
the universe was too hot to allow life. It is estimated that the
temperature of the universe was around 10 billion K at the one second
mark. 15 million years later,
it cooled to temperate levels, but the elements that make up living
things did not exist yet. The only freely available elements at that
point were hydrogen and helium. Carbon and oxygen (and later, water)
would not appear until 50 million years later, created through stellar
fusion. At that point, the difficulty for life to appear was not the
temperature, but the scarcity of free heavy elements. Planetary systems emerged, and the first organic compounds may have formed in the protoplanetary disk of dust grains
that would eventually create rocky planets like Earth. Although Earth
was in a molten state after its birth and may have burned any organics
that fell in it, it would have been more receptive once it cooled down. Once the right conditions on Earth were met, life started by a chemical process known as abiogenesis. Alternatively, life may have formed less frequently, then spread – by meteoroids, for example – between habitable planets in a process called panspermia.
During most of its stellar evolution
stars combine hydrogen nuclei to make helium nuclei by stellar fusion,
and the comparatively lighter weight of helium allows the star to
release the extra energy. The process continues until the star uses all
of its available fuel, with the speed of consumption being related to
the size of the star. During its last stages, stars start combining
helium nuclei to form carbon nuclei. The higher-sized stars can further
combine carbon nuclei to create oxygen and silicon, oxygen into neon and
sulfur, and so on until iron. In the end, the star blows much of its
content back into the stellar medium, where it would join clouds that
would eventually become new generations of stars and planets. Many of
those materials are the raw components of life on Earth. As this process
takes place in all the universe, said materials are ubiquitous in the
cosmos and not a rarity from the Solar System.
Earth is a planet in the Solar System, a planetary system formed by a star at the center, the Sun, and the objects that orbit it: other planets, moons, asteroids, and comets. The sun is part of the Milky Way, a galaxy. The Milky Way is part of the Local Group, a galaxy group that is in turn part of the Laniakea Supercluster. The universe is composed of all similar structures in existence. The immense distances between celestial objects is a difficulty for the
study of extraterrestrial life. So far, humans have only set foot on
the Moon
and sent robotic probes to other planets and moons in the Solar System.
Although probes can withstand conditions that may be lethal to humans,
the distances cause time delays: the New Horizons took nine years after launch to reach Pluto. No probe has ever reached extrasolar planetary systems. The Voyager 2 has left the Solar System at a speed of 50,000 kilometers per hour, if it headed towards the Alpha Centauri
system, the closest one to Earth at 4.4 light years, it would reach it
in 100,000 years. Under current technology, such systems can only be
studied by telescopes, which have limitations. It is estimated that dark matter
has a larger amount of combined matter than stars and gas clouds, but
as it plays no role on the stellar evolution of stars and planets, it is
usually not taken into account by astrobiology.
There is an area around a star, the circumstellar habitable zone
or "Goldilocks zone", where water may be at the right temperature to
exist in liquid form at a planetary surface. This area is neither too
close to the star, where water would become steam, nor too far away,
where water would be frozen as ice. However, although useful as an
approximation, planetary habitability
is complex and defined by several factors. Being in the habitable zone
is not enough for a planet to be habitable, not even to actually have
such liquid water. Venus is located in the habitable zone of the Solar
System but does not have liquid water because of the conditions of its
atmosphere. Jovian planets or gas giants are not considered habitable even if they orbit close enough to their stars as hot Jupiters, due to crushing atmospheric pressures. The actual distances for the habitable zones vary according to the type of star, and even the solar activity
of each specific star influences the local habitability. The type of
star also defines the time the habitable zone will exist, as its
presence and limits will change along with the star's stellar evolution.
The Big Bang took place 13.8 billion years ago, the Solar System
was formed 4.6 billion years ago, and the first hominids appeared 6
million years ago. Life on other planets may have started, evolved,
given birth to extraterrestrial intelligences, and perhaps even faced a
planetary extinction event millions or even billions of years ago. The
brief times of existence of Earth's species, when considered from a
cosmic perspective, may suggest that extraterrestrial life may be
equally fleeting under such a scale.
During a period of about 7 million years, from about 10 to 17
million after the Big Bang, the background temperature was between 373
and 273 K (100 and 0 °C; 212 and 32 °F), allowing the possibility of liquid water if any planets existed.
Avi Loeb (2014) speculated that primitive life might in principle have appeared during this window, which he called "the Habitable Epoch of the Early Universe".
Life on Earth is quite ubiquitous across the planet and has adapted over time to almost all the available environments in it, extremophiles and the deep biosphere
thrive at even the most hostile ones. As a result, it is inferred that
life in other celestial bodies may be equally adaptive. However, the
origin of life is unrelated to its ease of adaptation and may have
stricter requirements. A celestial body may not have any life on it,
even if it were habitable.
It is unclear if life, and more importantly, intelligent life in the
cosmos is ubiquitous or rare. The hypothesis of ubiquitous
extraterrestrial life relies on three main ideas. The first one, the size of the universe allows for plenty of planets to have a similar habitability to Earth, and the age of the universe gives enough time for a long process analog to the history of Earth
to happen there. The second is that the chemical elements that make up
life, such as carbon and water, are ubiquitous in the universe. The
third is that the physical laws
are universal, which means that the forces that would facilitate or
prevent the existence of life would be the same ones as on Earth. According to this argument, made by scientists such as Carl Sagan and Stephen Hawking, it would be improbable for life not to exist somewhere else other than Earth. This argument is embodied in the Copernican principle, which states that Earth does not occupy a unique position in the Universe, and the mediocrity principle, which states that there is nothing special about life on Earth.
Other authors consider instead that life in the cosmos, or at least multicellular life, may be actually rare. The Rare Earth hypothesis
maintains that life on Earth is possible because of a series of factors
that range from the location in the galaxy and the configuration of the
Solar System
to local characteristics of the planet, and that it is unlikely that
all such requirements are simultaneously met by another planet. The
proponents of this hypothesis consider that very little evidence
suggests the existence of extraterrestrial life and that at this point
it is just a desired result and not a reasonable scientific explanation
for any gathered data.
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life
fl = the fraction of planets that actually support life
fi = the fraction of planets with life that evolves to become intelligent life (civilisations)
fc = the fraction of civilizations that develop a technology to broadcast detectable signs of their existence into space
L = the length of time over which such civilizations broadcast detectable signals into space
Drake's proposed estimates are as follows, but numbers on the right
side of the equation are agreed as speculative and open to substitution:
The Drake equation has proved controversial since, although it is
written as a math equation, none of its values were known at the time.
Although some values may eventually be measured, others are based on social sciences and are not knowable by their very nature. This does not allow one to make noteworthy conclusions from the equation.
Based on observations from the Hubble Space Telescope, there are nearly 2 trillion galaxies in the observable universe. It is estimated that at least ten percent of all Sun-like stars have a system of planets, i.e. there are 6.25×1018
stars with planets orbiting them in the observable universe. Even if it
is assumed that only one out of a billion of these stars has planets
supporting life, there would be some 6.25 billion life-supporting
planetary systems in the observable universe. A 2013 study based on
results from the Kepler
spacecraft estimated that the Milky Way contains at least as many
planets as it does stars, resulting in 100–400 billion exoplanets. The Nebular hypothesis
that explains the formation of the Solar System and other planetary
systems would suggest that those can have several configurations, and
not all of them may have rocky planets within the habitable zone.
The apparent contradiction between high estimates of the
probability of the existence of extraterrestrial civilisations and the
lack of evidence for such civilisations is known as the Fermi paradox. Dennis W. Sciama claimed that life's existence in the universe depends on various fundamental constants. Zhi-Wei Wang and Samuel L. Braunstein
suggest that a random universe capable of supporting life is likely to
be just barely able to do so, giving a potential explanation to the
Fermi paradox.
If extraterrestrial life exists, it could range from simple microorganisms and multicellular organisms similar to animals or plants, to complex alien intelligences akin to humans.
When scientists talk about extraterrestrial life, they consider all
those types. Although it is possible that extraterrestrial life may have
other configurations, scientists use the hierarchy of lifeforms from
Earth for simplicity, as it is the only one known to exist.
The first basic requirement for life is an environment with non-equilibrium thermodynamics, which means that the thermodynamic equilibrium
must be broken by a source of energy. The traditional sources of energy
in the cosmos are the stars, such as for life on Earth, which depends
on the energy of the sun. However, there are other alternative energy
sources, such as volcanoes, plate tectonics, and hydrothermal vents. There are ecosystems on Earth in deep areas of the ocean that do not receive sunlight, and take energy from black smokers instead. Magnetic fields and radioactivity have also been proposed as sources of energy, although they would be less efficient ones.
Life on Earth requires water in a liquid state as a solvent in which biochemical reactions take place. It is highly unlikely that an abiogenesis
process can start within a gaseous or solid medium: the atom speeds,
either too fast or too slow, make it difficult for specific ones to meet
and start chemical reactions. A liquid medium also allows the transport
of nutrients and substances required for metabolism. Sufficient quantities of carbon and other elements, along with water, might enable the formation of living organisms on terrestrial planets with a chemical make-up and temperature range similar to that of Earth. Life based on ammonia
rather than water has been suggested as an alternative, though this
solvent appears less suitable than water. It is also conceivable that
there are forms of life whose solvent is a liquid hydrocarbon, such as methane, ethane or propane.
Another unknown aspect of potential extraterrestrial life would be the chemical elements that would compose it. Life on Earth is largely composed of carbon, but there could be other hypothetical types of biochemistry.
A replacement for carbon would need to be able to create complex
molecules, store information required for evolution, and be freely
available in the medium. To create DNA, RNA,
or a close analog, such an element should be able to bind its atoms
with many others, creating complex and stable molecules. It should be
able to create at least three covalent bonds: two for making long
strings and at least a third to add new links and allow for diverse
information. Only nine elements meet this requirement: boron, nitrogen, phosphorus, arsenic, antimony (three bonds), carbon, silicon, germanium and tin
(four bonds). As for abundance, carbon, nitrogen, and silicon are the
most abundant ones in the universe, far more than the others. On Earth's crust the most abundant of those elements is silicon, in the Hydrosphere
it is carbon and in the atmosphere, it is carbon and nitrogen. Silicon,
however, has disadvantages over carbon. The molecules formed with
silicon atoms are less stable, and more vulnerable to acids, oxygen, and
light. An ecosystem of silicon-based lifeforms would require very low
temperatures, high atmospheric pressure,
an atmosphere devoid of oxygen, and a solvent other than water. The low
temperatures required would add an extra problem, the difficulty to
kickstart a process of abiogenesis to create life in the first place. Norman Horowitz,
head of the Jet Propulsion Laboratory bioscience section for the
Mariner and Viking missions from 1965 to 1976 considered 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 of life on other planets. However, he also considered that the conditions found on Mars were incompatible with carbon based life.
Even if extraterrestrial life is based on carbon and uses water
as a solvent, like Earth life, it may still have a radically different biochemistry. Life is generally considered to be a product of natural selection. It has been proposed that to undergo natural selection a living entity must have the capacity to replicate
itself, the capacity to avoid damage/decay, and the capacity to acquire
and process resources in support of the first two capacities. Life on Earth may have started with an RNA world and later evolved to its current form, where some of the RNA tasks were transferred to DNA and proteins.
Extraterrestrial life may still be stuck using RNA, or evolve into
other configurations. It is unclear if our biochemistry is the most
efficient one that could be generated, or which elements would follow a
similar pattern. However, it is likely that, even if cells had a different composition to those from Earth, they would still have a cell membrane. Life on Earth jumped from prokaryotes to eukaryotes and from unicellular organisms to multicellular organisms through evolution.
So far no alternative process to achieve such a result has been
conceived, even if hypothetical. Evolution requires life to be divided
into individual organisms, and no alternative organisation has been
satisfactorily proposed either. At the basic level, membranes define the
limit of a cell, between it and its environment, while remaining
partially open to exchange energy and resources with it.
The evolution from simple cells to eukaryotes, and from them to multicellular lifeforms, is not guaranteed. The Cambrian explosion
took place thousands of millions of years after the origin of life, and
its causes are not fully known yet. On the other hand, the jump to
multicellularity took place several times, which suggests that it could
be a case of convergent evolution, and so likely to take place on other planets as well. Palaeontologist Simon Conway Morris
considers that convergent evolution would lead to kingdoms similar to
our plants and animals, and that many features are likely to develop in
alien animals as well, such as bilateral symmetry, limbs, digestive systems and heads with sensory organs. Scientists from the University of Oxford analysed it from the perspective of evolutionary theory and wrote in a study in the International Journal of Astrobiology that aliens may be similar to humans. The planetary context would also have an influence: a planet with higher gravity would have smaller animals, and other types of stars can lead to non-green photosynthesizers. The amount of energy available would also affect biodiversity,
as an ecosystem sustained by black smokers or hydrothermal vents would
have less energy available than those sustained by a star's light and
heat, and so its lifeforms would not grow beyond a certain complexity. There is also research in assessing the capacity of life for developing
intelligence. It has been suggested that this capacity arises with the
number of potential niches
a planet contains, and that the complexity of life itself is reflected
in the information density of planetary environments, which in turn can
be computed from its niches.
Harsh environmental conditions on Earth harboring life
It
is common knowledge that the conditions on other planets in the solar
system, in addition to the many galaxies outside of the Milky Way galaxy, are very harsh and seem to be too extreme to harbor any life. The environmental conditions on these planets can have intense UV radiation paired with extreme temperatures, lack of water, and much more that can lead to conditions that don't seem to favor the
creation or maintenance of extraterrestrial life. However, there has
been much historical evidence that some of the earliest and most basic
forms of life on Earth originated in some extreme environments that seem unlikely to have harbored life at least at one point in
Earth's history. Fossil evidence as well as many historical theories
backed up by years of research and studies have marked environments like
hydrothermal vents or acidic hot springs as some of the first places that life could have originated on Earth. These environments can be considered extreme when compared to the
typical ecosystems that the majority of life on Earth now inhabit, as
hydrothermal vents are scorching hot due to the magma escaping from the Earth's mantle
and meeting the much colder oceanic water. Even in today's world, there
can be a diverse population of bacteria found inhabiting the area
surrounding these hydrothermal vents which can suggest that some form of life can be supported even in the
harshest of environments like the other planets in the solar system.
The aspects of these harsh environments that make them ideal for
the origin of life on Earth, as well as the possibility of creation of
life on other planets, is the chemical reactions forming spontaneously. For example, the hydrothermal vents found on the ocean floor are known to support many chemosynthetic processes which allow organisms to utilize energy through reduced chemical compounds that fix carbon. In return, these reactions will allow for organisms to live in
relatively low oxygenated environments while maintaining enough energy
to support themselves. The early Earth environment was reducing and therefore, these carbon fixing compounds were necessary for the survival and possible origin of life on Earth. With the little amount of information that scientists have found regarding the atmosphere on other planets in the Milky Way galaxy and beyond, the atmospheres are most likely reducing or with very low oxygen levels, especially when compared with Earth's atmosphere. If there were the
necessary elements and ions on these planets, the same carbon fixing,
reduced chemical compounds occurring around hydrothermal vents could
also occur on these planets' surfaces and possibly result in the origin
of extraterrestrial life.
Besides Earth, Mars, Europa and Enceladus are the most likely places in the Solar System to find life.
The Solar System has a wide variety of planets, dwarf planets, and
moons, and each one is studied for its potential to host life. Each one
has its own specific conditions that may benefit or harm life. So far,
the only lifeforms found are those from Earth. No extraterrestrial intelligence other than humans exists or has ever existed within the Solar System. Astrobiologist Mary Voytek points out that it would be unlikely to find
large ecosystems, as they would have already been detected by now.
The inner Solar System is likely devoid of life. However, Venus is still of interest to astrobiologists, as it is a terrestrial planet that was likely similar to Earth in its early stages and developed in a different way. There is a greenhouse effect,
the surface is the hottest in the Solar System, sulfuric acid clouds,
all surface liquid water is lost, and it has a thick carbon-dioxide
atmosphere with huge pressure. Comparing both helps to understand the precise differences that lead to
beneficial or harmful conditions for life. And despite the conditions
against life on Venus, there are suspicions that microbial life-forms may still survive in high-altitude clouds.
Mars is a cold and almost airless desert, inhospitable to life. However, recent studies revealed that water on Mars used to be quite abundant, forming rivers, lakes, and perhaps even oceans. Mars may have been habitable back then, and life on Mars
may have been possible. But when the planetary core ceased to generate a
magnetic field, solar winds removed the atmosphere and the planet
became vulnerable to solar radiation. Ancient life-forms may still have
left fossilised remains, and microbes may still survive deep
underground.
As mentioned, the gas giants and ice giants are unlikely to contain life. The most distant solar system bodies, found in the Kuiper Belt and outwards, are locked in permanent deep-freeze, but cannot be ruled out completely.
Although the giant planets themselves are highly unlikely to have
life, there is much hope to find it on moons orbiting these planets. Europa, from the Jovian system, has a subsurface ocean below a thick layer of ice. Ganymede and Callisto
also have subsurface oceans, but life is less likely in them because
water is sandwiched between layers of solid ice. Europa would have
contact between the ocean and the rocky surface, which helps the
chemical reactions. It may be difficult to dig so deep in order to study
those oceans, though. Enceladus, a tiny moon of Saturn with another subsurface ocean, may not need to be dug, as it releases water to space in eruption columns. The space probe Cassini
flew inside one of these, but could not make a full study because NASA
did not expect this phenomenon and did not equip the probe to study
ocean water. Still, Cassini detected complex organic molecules, salts, evidence of hydrothermal activity, hydrogen, and methane.
Titan
is the only celestial body in the Solar System besides Earth that has
liquid bodies on the surface. It has rivers, lakes, and rain of
hydrocarbons, methane, and ethane, and even a cycle similar to Earth's water cycle. This special context encourages speculations about lifeforms
with different biochemistry, but the cold temperatures would make such
chemistry take place at a very slow pace. Water is rock-solid on the
surface, but Titan does have a subsurface water ocean like several other
moons. However, it is of such a great depth that it would be very
difficult to access it for study.
The science that searches and studies life in the universe, both on Earth and elsewhere, is called astrobiology.
With the study of Earth's life, the only known form of life,
astrobiology seeks to study how life starts and evolves and the
requirements for its continuous existence. This helps to determine what
to look for when searching for life in other celestial bodies. This is a
complex area of study, and uses the combined perspectives of several
scientific disciplines, such as astronomy, biology, chemistry, geology, oceanography, and atmospheric sciences.
The scientific search for extraterrestrial life is being carried out both directly and indirectly. As of September 2017, 3,667 exoplanets in 2,747 systems have been identified, and other planets and moons in the Solar System hold the potential for hosting primitive life such as microorganisms. As of 8 February 2021, an updated status of studies considering the possible detection of lifeforms on Venus (via phosphine) and Mars (via methane) was reported.
Search for basic life
Lifeforms produce a variety of biosignatures that may be detectable by telescopes.
Scientists search for biosignatures within the Solar System by studying planetary surfaces and examining meteorites. Some claim to have identified evidence that microbial life has existed on Mars.In 1996, a controversial report stated that structures resembling nanobacteria were discovered in a meteorite, ALH84001, formed of rock ejected from Mars. Although all the unusual properties of the meteorite were eventually
explained as the result of inorganic processes, the controversy over its
discovery laid the groundwork for the development of astrobiology.
An experiment on the two Viking
Mars landers reported gas emissions from heated Martian soil samples
that some scientists argue are consistent with the presence of living
microorganisms. Lack of corroborating evidence from other experiments on the same
samples suggests that a non-biological reaction is a more likely
hypothesis.
In February 2005 NASA scientists reported they may have found some evidence of extraterrestrial life on Mars. The two scientists, Carol Stoker and Larry Lemke of NASA's Ames Research Center,
based their claim on methane signatures found in Mars's atmosphere
resembling the methane production of some forms of primitive life on
Earth, as well as on their own study of primitive life near the Rio Tinto river
in Spain. NASA officials soon distanced NASA from the scientists'
claims, and Stoker herself backed off from her initial assertions.
In November 2011, NASA launched the Mars Science Laboratory that landed the Curiosity
rover on Mars. It is designed to assess the past and present
habitability on Mars using a variety of scientific instruments. The
rover landed on Mars at Gale Crater in August 2012.
A group of scientists at Cornell University started a catalog of
microorganisms, with the way each one reacts to sunlight. The goal is to
help with the search for similar organisms in exoplanets, as the
starlight reflected by planets rich in such organisms would have a
specific spectrum, unlike that of starlight reflected from lifeless
planets. If Earth was studied from afar with this system, it would
reveal a shade of green, as a result of the abundance of plants with
photosynthesis.
In August 2011, NASA studied meteorites found on Antarctica, finding adenine, guanine, hypoxanthine and xanthine.
Adenine and guanine are components of DNA, and the others are used in
other biological processes. The studies ruled out pollution of the
meteorites on Earth, as those components would not be freely available
the way they were found in the samples. This discovery suggests that
several organic molecules that serve as building blocks of life may be generated within asteroids and comets. In October 2011, scientists reported that cosmic dust contains complex organic compounds ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars. It is still unclear if those compounds played a role in the creation of
life on Earth, but Sun Kwok, of the University of Hong Kong, thinks so.
"If this is the case, life on Earth may have had an easier time getting
started as these organics can serve as basic ingredients for life."
In August 2012, and in a world first, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde, in a distant star system. The molecule was found around the protostellar binary IRAS 16293-2422, which is located 400 light years from Earth. Glycolaldehyde is needed to form ribonucleic acid,
or RNA, which is similar in function to DNA. This finding suggests that
complex organic molecules may form in stellar systems prior to the
formation of planets, eventually arriving on young planets early in
their formation.
In December 2023, astronomers reported the first time discovery, in the plumes of Enceladus, moon of the planet Saturn, of hydrogen cyanide, a possible chemical essential for life as we know it, as well as other organic molecules,
some of which are yet to be better identified and understood. According
to the researchers, "these [newly discovered] compounds could
potentially support extant microbial communities or drive complex organic synthesis leading to the origin of life."
Although most searches are focused on the biology of extraterrestrial
life, an extraterrestrial intelligence capable enough to develop a civilization may be detectable by other means as well. Technology may generate technosignatures,
effects on the native planet that may not be caused by natural causes.
There are three main types of techno-signatures considered: interstellar communications, effects on the atmosphere, and planetary-sized structures such as Dyson spheres.
Organizations such as the SETI Institute search the cosmos for potential forms of communication. They started with radio waves, and now search for laser pulses
as well. The challenge for this search is that there are natural
sources of such signals as well, such as gamma-ray bursts and
supernovae, and the difference between a natural signal and an
artificial one would be in its specific patterns. Astronomers intend to
use artificial intelligence for this, as it can manage large amounts of data and is devoid of biases and preconceptions. Besides, even if there is an advanced extraterrestrial civilization,
there is no guarantee that it is transmitting radio communications in
the direction of Earth. The length of time required for a signal to
travel across space means that a potential answer may arrive decades or
centuries after the initial message.
The atmosphere of Earth is rich in nitrogen dioxide as a result of air pollution,
which can be detectable. The natural abundance of carbon, which is also
relatively reactive, makes it likely to be a basic component of the
development of a potential extraterrestrial technological civilization,
as it is on Earth. Fossil fuels may likely be generated and used on such worlds as well. The abundance of chlorofluorocarbons in the atmosphere can also be a clear technosignature, considering their role in ozone depletion. Light pollution
may be another technosignature, as multiple lights on the night side of
a rocky planet can be a sign of advanced technological development.
However, modern telescopes are not strong enough to study exoplanets
with the required level of detail to perceive it.
The Kardashev scale
proposes that a civilization may eventually start consuming energy
directly from its local star. This would require giant structures built
next to it, called Dyson spheres. Those speculative structures would
cause an excess infrared radiation, that telescopes may notice. The
infrared radiation is typical of young stars, surrounded by dusty protoplanetary disks
that will eventually form planets. An older star such as the Sun would
have no natural reason to have excess infrared radiation. The presence of heavy elements in a star's light-spectrum is another potential biosignature; such elements would (in theory) be found if the star were being used as an incinerator/repository for nuclear waste products.
Some astronomers search for extrasolar planets that may be conducive to life, narrowing the search to terrestrial planets within the habitable zones of their stars.Since 1992, over four thousand exoplanets have been discovered (6,032 planets in 4,530 planetary systems including 989 multiple planetary systems as of 29 July 2025).
The extrasolar planets so far discovered range in size from that
of terrestrial planets similar to Earth's size to that of gas giants
larger than Jupiter. The number of observed exoplanets is expected to increase greatly in the coming years. The Kepler space telescope has also detected a few thousandcandidate planets, of which about 11% may be false positives.
There is at least one planet on average per star. About 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone, with the nearest expected to be within 12 light-years distance from Earth. Assuming 200 billion stars in the Milky Way, that would be 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if red dwarfs are included. The rogue planets in the Milky Way possibly number in the trillions.
One sign that a planet probably already contains life is the presence of an atmosphere with significant amounts of oxygen,
since that gas is highly reactive and generally would not last long
without constant replenishment. This replenishment occurs on Earth
through photosynthetic organisms. One way to analyse the atmosphere of
an exoplanet is through spectrography when it transits its star, though this might only be feasible with dim stars like white dwarfs.
The Greek Epicurus proposed that other worlds may have their own animals and plants.
The modern concept of extraterrestrial life is based on assumptions that were not commonplace during the early days of astronomy. The first explanations for the celestial objects seen in the night sky were based on mythology. Scholars from Ancient Greece
were the first to consider that the universe is inherently
understandable and rejected explanations based on supernatural
incomprehensible forces, such as the myth of the Sun being pulled across
the sky in the chariot of Apollo. They had not developed the scientific method
yet and based their ideas on pure thought and speculation, but they
developed precursor ideas to it, such as that explanations had to be
discarded if they contradict observable facts. The discussions of those
Greek scholars established many of the pillars that would eventually
lead to the idea of extraterrestrial life, such as Earth being round and
not flat. The cosmos was first structured in a geocentric model
that considered that the sun and all other celestial bodies revolve
around Earth. However, they did not consider them as worlds. In Greek
understanding, the world was composed by both Earth and the celestial
objects with noticeable movements. Anaximander thought that the cosmos was made from apeiron, a substance that created the world, and that the world would eventually return to the cosmos.
Eventually two groups emerged, the atomists that thought that matter at both Earth and the cosmos was equally made of small atoms of the classical elements (earth, water, fire and air), and the Aristotelians who thought that those elements were exclusive of Earth and that the cosmos was made of a fifth one, the aether. Atomist Epicurus
thought that the processes that created the world, its animals and
plants should have created other worlds elsewhere, along with their own
animals and plants. Aristotle thought instead that all the earth element
naturally fell towards the center of the universe, and that would make
it impossible for other planets to exist elsewhere. Under that
reasoning, Earth was not only in the center, it was also the only planet
in the universe.
Cosmic pluralism, the plurality of worlds, or simply pluralism,
describes the philosophical belief in numerous "worlds" in addition to
Earth, which might harbor extraterrestrial life. The earliest recorded
assertion of extraterrestrial human life is found in ancient scriptures
of Jainism. There are multiple "worlds" mentioned in Jain scriptures that support human life. These include, among others, Bharat Kshetra, Mahavideh Kshetra, Airavat Kshetra, and Hari kshetra. Medieval Muslim writers like Fakhr al-Din al-Razi and Muhammad al-Baqir supported cosmic pluralism on the basis of the Qur'an. Chaucer's poem The House of Fame engaged in medieval thought experiments that postulated the plurality of worlds. However, those ideas about other worlds were different from the current
knowledge about the structure of the universe, and did not postulate
the existence of planetary systems other than the Solar System. When
those authors talk about other worlds, they talk about places located at
the center of their own systems, and with their own stellar vaults and
cosmos surrounding them.
The Greek ideas and the disputes between atomists and Aristotelians outlived the fall of the Greek empire. The Great Library of Alexandria
compiled information about it, part of which was translated by Islamic
scholars and thus survived the end of the Library. Baghdad combined the
knowledge of the Greeks, the Indians, the Chinese and its own scholars,
and the knowledge expanded through the Byzantine Empire. From there it eventually returned to Europe by the time of the Middle Ages. However, as the Greek atomist doctrine held that the world was created by random movements of atoms, with no need for a creator deity, it became associated with atheism, and the dispute intertwined with religious ones. Still, the Church did not react to those topics in a homogeneous way,
and there were stricter and more permissive views within the church
itself.
The first known mention of the term 'panspermia' was in the writings of the 5th-century BC Greek philosopher Anaxagoras. He proposed the idea that life exists everywhere.
By the time of the late Middle Ages there were many known inaccuracies in the geocentric model, but it was kept in use because naked eye observations provided limited data. Nicolaus Copernicus started the Copernican Revolution
by proposing that the planets revolve around the sun rather than Earth.
His proposal had little acceptance at first because, as he kept the
assumption that orbits were perfect circles, his model led to as many
inaccuracies as the geocentric one. Tycho Brahe improved the available data with naked-eye observatories, which worked with highly complex sextants and quadrants. Tycho could not make sense of his observations, but Johannes Kepler
did: orbits were not perfect circles, but ellipses. This knowledge
benefited the Copernican model, which worked now almost perfectly. The
invention of the telescope a short time later, perfected by Galileo Galilei, clarified the final doubts, and the paradigm shift was completed. Under this new understanding, the notion of extraterrestrial life
became feasible: if Earth is but just a planet orbiting around a star,
there may be planets similar to Earth elsewhere. The astronomical study
of distant bodies also proved that physical laws are the same elsewhere
in the universe as on Earth, with nothing making the planet truly
special.
The new ideas were met with resistance from the Catholic church. Galileo was tried for the heliocentric model, which was considered heretical, and forced to recant it. The best-known early-modern proponent of ideas of extraterrestrial life was the Italian philosopher Giordano Bruno, who argued in the 16th century for an infinite universe in which every star is surrounded by its own planetary system.
Bruno wrote that other worlds "have no less virtue nor a nature
different to that of our earth" and, like Earth, "contain animals and
inhabitants". Bruno's belief in the plurality of worlds was one of the charges leveled against him by the Venetian Holy Inquisition, which tried and executed him.
The heliocentric model was further strengthened by the postulation of the theory of gravity by Sir Isaac Newton.
This theory provided the mathematics that explains the motions of all
things in the universe, including planetary orbits. By this point, the
geocentric model was definitely discarded. By this time, the use of the
scientific method had become a standard, and new discoveries were
expected to provide evidence and rigorous mathematical explanations.
Science also took a deeper interest in the mechanics of natural
phenomena, trying to explain not just the way nature works but also the
reasons for working that way.
There was very little actual discussion about extraterrestrial
life before this point, as the Aristotelian ideas remained influential
while geocentrism was still accepted. When it was finally proved wrong,
it not only meant that Earth was not the center of the universe, but
also that the lights seen in the sky were not just lights, but physical
objects. The notion that life may exist in them as well soon became an
ongoing topic of discussion, although one with no practical ways to
investigate.
The possibility of extraterrestrials remained a widespread speculation as scientific discovery accelerated. William Herschel, the discoverer of Uranus, was one of many 18th–19th-century astronomers who believed that the Solar System is populated by alien life. Other scholars of the period who championed "cosmic pluralism" included Immanuel Kant and Benjamin Franklin. At the height of the Enlightenment, even the Sun and Moon were considered candidates for extraterrestrial inhabitants.
19th century
Artificial Martian channels, depicted by Percival Lowell
Speculation about life on Mars increased in the late 19th century, following telescopic observation of apparent Martian canals – which soon, however, turned out to be optical illusions. Despite this, in 1895, American astronomer Percival Lowell published his book Mars, followed by Mars and its Canals in 1906, proposing that the canals were the work of a long-gone civilisation.
Spectroscopic analysis of Mars's atmosphere began in earnest in 1894, when U.S. astronomer William Wallace Campbell showed that neither water nor oxygen was present in the Martian atmosphere. By 1909 better telescopes and the best perihelic opposition of Mars since 1877 conclusively put an end to the canal hypothesis.
As a consequence of the belief in the spontaneous generation
there was little thought about the conditions of each celestial body:
it was simply assumed that life would thrive anywhere. This theory was
disproved by Louis Pasteur in the 19th century. Popular belief in thriving alien civilisations elsewhere in the solar system still remained strong until Mariner 4 and Mariner 9
provided close images of Mars, which debunked forever the idea of the
existence of Martians and decreased the previous expectations of finding
alien life in general. The end of the spontaneous generation belief forced investigation into the origin of life. Although abiogenesis
is the more accepted theory, a number of authors reclaimed the term
"panspermia" and proposed that life was brought to Earth from elsewhere. Some of those authors are Jöns Jacob Berzelius (1834), Kelvin (1871), Hermann von Helmholtz (1879) and, somewhat later, by Svante Arrhenius (1903).
The science fiction genre, although not so named during the time,
developed during the late 19th century. The expansion of the genre of extraterrestrials in fiction
influenced the popular perception over the real-life topic, making
people eager to jump to conclusions about the discovery of aliens.
Science marched at a slower pace, some discoveries fueled expectations
and others dashed excessive hopes. For example, with the advent of
telescopes, most structures seen on the Moon or Mars were immediately
attributed to Selenites or Martians, and later ones (such as more
powerful telescopes) revealed that all such discoveries were natural
features. A famous case is the Cydonia region of Mars, first imaged by the Viking 1
orbiter. The low-resolution photos showed a rock formation that
resembled a human face, but later spacecraft took photos in higher
detail that showed that there was nothing special about the site.
The search and study of extraterrestrial life became a science of its own, astrobiology. Also known as exobiology, this discipline is studied by the NASA, the ESA, the INAF, and others. Astrobiology studies life from Earth as well, but with a cosmic perspective. For example, abiogenesis
is of interest to astrobiology, not because of the origin of life on
Earth, but for the chances of a similar process taking place in other
celestial bodies. Many aspects of life, from its definition to its
chemistry, are analyzed as either likely to be similar in all forms of
life across the cosmos or only native to Earth. Astrobiology, however, remains constrained by the current lack of
extraterrestrial life-forms to study, as all life on Earth comes from
the same ancestor, and it is hard to infer general characteristics from a
group with a single example to analyse.
The 20th century came with great technological advances, speculations about future hypothetical technologies, and an increased basic knowledge of science by the general population thanks to science divulgation
through the mass media. The public interest in extraterrestrial life
and the lack of discoveries by mainstream science led to the emergence
of pseudosciences that provided affirmative, if questionable, answers to the existence of aliens. Ufology claims that many unidentified flying objects (UFOs) would be spaceships from alien species, and ancient astronauts
hypothesis claim that aliens would have visited Earth in antiquity and
prehistoric times but people would have failed to understand it by then. Most UFOs or UFO sightings can be readily explained as sightings of Earth-based aircraft (including top-secret aircraft), known astronomical objects or weather phenomenons, or as hoaxes.
Looking beyond the pseudosciences, Lewis White Beck
strove to elevate the level of public discourse on the topic of
extraterrestrial life by tracing the evolution of philosophical thought
over the centuries from ancient times into the modern era. His review of
the contributions made by Lucretius, Plutarch, Aristotle, Copernicus, Immanuel Kant, John Wilkins, Charles Darwin and Karl Marx
demonstrated that even in modern times, humanity could be profoundly
influenced in its search for extraterrestrial life by subtle and
comforting archetypal ideas which are largely derived from firmly held
religious, philosophical and existential belief systems. On a positive
note, however, Beck further argued that even if the search for
extraterrestrial life proves to be unsuccessful, the endeavor itself
could have beneficial consequences by assisting humanity in its attempt
to actualize superior ways of living here on Earth.
By the 21st century, it was accepted that multicellular life in
the Solar System can only exist on Earth, but the interest in
extraterrestrial life increased regardless. This is a result of the
advances in several sciences. The knowledge of planetary habitability
allows to consider on scientific terms the likelihood of finding life at
each specific celestial body, as it is known which features are
beneficial and harmful for life. Astronomy and telescopes also improved
to the point exoplanets can be confirmed and even studied, increasing
the number of search places. Life may still exist elsewhere in the Solar
System in unicellular form, but the advances in spacecraft allow to
send robots to study samples in situ, with tools of growing complexity
and reliability. Although no extraterrestrial life has been found and
life may still be just a rarity from Earth, there are scientific reasons
to suspect that it can exist elsewhere, and technological advances that
may detect it if it does.
Many scientists are optimistic about the chances of finding alien
life. In the words of SETI's Frank Drake, "All we know for sure is that
the sky is not littered with powerful microwave transmitters". Drake noted that it is entirely possible that advanced technology
results in communication being carried out in some way other than
conventional radio transmission. At the same time, the data returned by
space probes, and giant strides in detection methods, have allowed
science to begin delineating habitability criteria on other worlds, and to confirm that at least other planets are plentiful, though aliens remain a question mark. The Wow! signal, detected in 1977 by a SETI project, remains a subject of speculative debate.
On the other hand, other scientists are pessimistic. Jacques Monod
wrote that "Man knows at last that he is alone in the indifferent
immensity of the universe, whence which he has emerged by chance". In 2000, geologist and paleontologistPeter Ward and astrobiologistDonald Brownlee published a book entitled Rare Earth: Why Complex Life is Uncommon in the Universe. In it, they discussed the Rare Earth hypothesis, in which they claim that Earth-like life is rare in the universe, whereas microbial
life is common. Ward and Brownlee are open to the idea of evolution on
other planets that is not based on essential Earth-like characteristics
such as DNA and carbon.
As for the possible risks, theoretical physicist Stephen Hawking
warned in 2010 that humans should not try to contact alien life forms.
He warned that aliens might pillage Earth for resources. "If aliens
visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans", he said. Jared Diamond had earlier expressed similar concerns. On 20 July 2015, Hawking and Russian billionaire Yuri Milner, along with the SETI Institute, announced a well-funded effort, called the Breakthrough Initiatives, to expand efforts to search for extraterrestrial life. The group contracted the services of the 100-meter Robert C. ByrdGreen Bank Telescope in West Virginia in the United States and the 64-meter Parkes Telescope in New South Wales, Australia. On 13 February 2015, scientists (including Geoffrey Marcy, Seth Shostak, Frank Drake and David Brin) at a convention of the American Association for the Advancement of Science, discussed Active SETI and whether transmitting a message to possible intelligent extraterrestrials in the Cosmos was a good idea; one result was a statement, signed by many, that a "worldwide
scientific, political and humanitarian discussion must occur before any
message is sent".
The 1967 Outer Space Treaty and the 1979 Moon Agreement define rules of planetary protection against potentially hazardous extraterrestrial life. COSPAR also provides guidelines for planetary protection. A committee of the United Nations Office for Outer Space Affairs
had in 1977 discussed for a year strategies for interacting with
extraterrestrial life or intelligence. The discussion ended without any
conclusions. As of 2010, the UN lacks response mechanisms for the case
of an extraterrestrial contact.
One of the NASA divisions is the Office of Safety and Mission
Assurance (OSMA), also known as the Planetary Protection Office. A part
of its mission is to "rigorously preclude backward contamination of
Earth by extraterrestrial life."
In 2016, the Chinese Government released a white paper detailing its space program. According to the document, one of the research objectives of the program is the search for extraterrestrial life. It is also one of the objectives of the Chinese Five-hundred-meter Aperture Spherical Telescope (FAST) program.
In 2020, Dmitry Rogozin, the head of the Russian space agency, said the search for extraterrestrial life is one of the main goals of deep space research.
He also acknowledged the possibility of existence of primitive life on other planets of the Solar System.
The French space agency has an office for the study of "non-identified aero spatial phenomena". The agency is maintaining a publicly accessible database of such
phenomena, with over 1600 detailed entries. According to the head of the
office, the vast majority of entries have a mundane explanation; but
for 25% of entries, their extraterrestrial origin can neither be
confirmed nor denied.
In 2020, chairman of the Israel Space AgencyIsaac Ben-Israel stated that the probability of detecting life in outer space is "quite large". But he disagrees with his former colleague Haim Eshed who stated that there are contacts between an advanced alien civilisation and some of Earth's governments.
Grey aliens are a common way to depict extraterrestrials in fiction.
Although the idea of extraterrestrial peoples became feasible once
astronomy developed enough to understand the nature of planets, they
were not thought of as being any different from humans. Having no
scientific explanation for the origin of mankind and its relation to other species, there was no reason to expect them to be any other way. This was changed by the 1859 book On the Origin of Species by Charles Darwin, which proposed the theory of evolution. Now with the notion that evolution on other planets may take other directions, science fiction
authors created bizarre aliens, clearly distinct from humans. A usual
way to do that was to add body features from other animals, such as
insects or octopuses. Costuming and special effects feasibility
alongside budget considerations forced films and TV series to tone down
the fantasy, but these limitations lessened since the 1990s with the
advent of computer-generated imagery (CGI), and later on as CGI became more effective and less expensive.
Real-life events sometimes captivate people's imagination and this influences the works of fiction. For example, during the Barney and Betty Hill incident, the first recorded claim of an alien abduction,
the couple reported that they were abducted and experimented on by
aliens with oversized heads, big eyes, pale grey skin, and small noses, a
description that eventually became the grey alien archetype once used in works of fiction.
