Biological immortality

From Wikipedia, the free encyclopedia

Biological immortality (sometimes referred to bio-indefinite mortality) is a state in which the rate of mortality from senescence is stable or decreasing, thus decoupling it from chronological age. Various unicellular and multicellular species, including some vertebrates, achieve this state either throughout their existence or after living long enough. A biologically immortal living being can still die from means other than senescence, such as through injury or disease.

This definition of immortality has been challenged in the Handbook of the Biology of Aging,^[1] because the increase in rate of mortality as a function of chronological age may be negligible at extremely old ages, an idea referred to as the late-life mortality plateau. The rate of mortality may cease to increase in old age, but in most cases that rate is typically very high.^[2] As a hypothetical example, there is only a 50% chance of a human surviving another year at age 110 or greater.

The term is also used by biologists to describe cells that are not subject to the Hayflick limit on how many times they can divide.

Cell lines

Biologists chose the word "immortal" to designate cells that are not subject to the Hayflick limit, the point at which cells can no longer divide due to DNA damage or shortened telomeres. Prior to Leonard Hayflick's theory, Alexis Carrel hypothesized that all normal somatic cells were immortal.^[3]

The term "immortalization" was first applied to cancer cells that expressed the telomere-lengthening enzyme telomerase, and thereby avoided apoptosis—i.e. cell death caused by intracellular mechanisms. Among the most commonly used cell lines are HeLa and Jurkat, both of which are immortalized cancer cell lines. HeLa cells originated from a sample of cervical cancer taken from Henrietta Lacks in 1951.^[4] These cells have been and still are widely used in biological research such as creation of the polio vaccine,^[5] sex hormone steroid research,^[6] and cell metabolism.^[7] Normal stem cells and germ cells can also be said to be immortal (when humans refer to the cell line).^{[citation needed]}

Immortal cell lines of cancer cells can be created by induction of oncogenes or loss of tumor suppressor genes. One way to induce immortality is through viral-mediated induction of the large T‑antigen,^[8] commonly introduced through simian virus 40 (SV-40).^[9]

Organisms

According to the Animal Aging and Longevity Database, the list of organisms with negligible aging (along with estimated longevity in the wild) includes:^[10]

• Rougheye rockfish (Sebastes aleutianus) – 205 years
• Olm (Proteus anguinus) – 102 years
• Painted turtle (Chrysemys picta) – 61 years
• Blanding's turtle (Emydoidea blandingii) – 77 years
• Eastern box turtle (Terrapene carolina) – 138 years
• Red sea urchin (Strongylocentrotus franciscanus) – 200 years
• Ocean quahog clam (Arctica islandica) – 507 years
• Great Basin bristlecone pine (Pinus longaeva) – 4,713 years
• Greenland Shark – 400 years^[11]

In 2018, scientists working for Calico, a company owned by Alphabet, published a paper in the journal eLife which presents possible evidence that Heterocephalus glaber (Naked mole rat) do not face increased mortality risk due to aging.^[12][13][14]

Bacteria and some yeast

Many unicellular organisms age: as time passes, they divide more slowly and ultimately die. Asymmetrically dividing bacteria and yeast also age. However, symmetrically dividing bacteria and yeast can be biologically immortal under ideal growing conditions.^[15] In these conditions, when a cell splits symmetrically to produce two daughter cells, the process of cell division can restore the cell to a youthful state. However, if the parent asymmetrically buds off a daughter only the daughter is reset to the youthful state—the parent isn't restored and will go on to age and die. In a similar manner stem cells and gametes can be regarded as "immortal".

Hydra

Hydra

Hydras are a genus of the Cnidaria phylum. All cnidarians can regenerate, allowing them to recover from injury and to reproduce asexually. Hydras are simple, freshwater animals possessing radial symmetry and no post-mitotic cells. All hydra cells continually divide.^{[citation needed]} It has been suggested that hydras do not undergo senescence, and, as such, are biologically immortal. In a four-year study, 3 cohorts of hydra did not show an increase in mortality with age. It is possible that these animals live much longer, considering that they reach maturity in 5 to 10 days.^[16] However, this does not explain how hydras are consequently able to maintain telomere lengths.

Jellyfish

Turritopsis dohrnii, or Turritopsis nutricula, is a small (5 millimeters (0.20 in)) species of jellyfish that uses transdifferentiation to replenish cells after sexual reproduction. This cycle can repeat indefinitely, potentially rendering it biologically immortal. This organism originated in the Caribbean sea, but has now spread around the world. Similar cases include hydrozoan Laodicea undulata^[17] and scyphozoan Aurelia sp.1.^[18]

Lobsters

Research suggests that lobsters may not slow down, weaken, or lose fertility with age, and that older lobsters may be more fertile than younger lobsters. This does not however make them immortal in the traditional sense, as they are significantly more likely to die at a shell moult the older they get (as detailed below).

Their longevity may be due to telomerase, an enzyme that repairs long repetitive sections of DNA sequences at the ends of chromosomes, referred to as telomeres. Telomerase is expressed by most vertebrates during embryonic stages but is generally absent from adult stages of life.^[19] However, unlike vertebrates, lobsters express telomerase as adults through most tissue, which has been suggested to be related to their longevity.^[20][21][22] Contrary to popular belief, lobsters are not immortal. Lobsters grow by moulting which requires a lot of energy, and the larger the shell the more energy is required.^[23] Eventually, the lobster will die from exhaustion during a moult. Older lobsters are also known to stop moulting, which means that the shell will eventually become damaged, infected, or fall apart and they die.^[24] The European lobster has an average life span of 31 years for males and 54 years for females.

Planarian flatworms

Polycelis felina, a freshwater planarian

Planarian flatworms have both sexually and asexually reproducing types. Studies on genus Schmidtea mediterranea suggest these planarians appear to regenerate (i.e. heal) indefinitely, and asexual individuals have an "apparently limitless [telomere] regenerative capacity fueled by a population of highly proliferative adult stem cells". "Both asexual and sexual animals display age-related decline in telomere length; however, asexual animals are able to maintain telomere lengths somatically (i.e. during reproduction by fission or when regeneration is induced by amputation), whereas sexual animals restore telomeres by extension during sexual reproduction or during embryogenesis like other sexual species. Homeostatic telomerase activity observed in both asexual and sexual animals is not sufficient to maintain telomere length, whereas the increased activity in regenerating asexuals is sufficient to renew telomere length... "^[25]

Lifespan: For sexually reproducing planaria: "the lifespan of individual planarian can be as long as 3 years, likely due to the ability of neoblasts to constantly replace aging cells". Whereas for asexually reproducing planaria: "individual animals in clonal lines of some planarian species replicating by fission have been maintained for over 15 years". They do not live forever.^[26]

Attempts to engineer biological immortality in humans

Although the premise that biological aging can be halted or reversed by foreseeable technology remains controversial,^[27] research into developing possible therapeutic interventions is underway.^[28] Among the principal drivers of international collaboration in such research is the SENS Research Foundation, a non-profit organization that advocates a number of what it claims are plausible research pathways that might lead to engineered negligible senescence in humans.^[29][30]

In 2015, Elizabeth Parrish, CEO of BioViva, treated herself using gene therapy, with the goal of not just halting, but reversing aging.^[31] She has since reported feeling more energetic, but long-term study of the treatment is ongoing.^{[citation needed]}

For several decades,^[32] researchers have also pursued various forms of suspended animation as a means by which to indefinitely extend mammalian lifespan. Some scientists have voiced support^[33] for the feasibility of the cryopreservation of humans, known as cryonics. Cryonics is predicated on the concept that some people considered clinically dead by today's medicolegal standards are not actually dead according to information-theoretic death and can, in principle, be resuscitated given sufficient technological advances.^[34] The goal of current cryonics procedures is tissue vitrification, a technique first used to reversibly cryopreserve a viable whole organ in 2005.^[35][36]

Similar proposals involving suspended animation include chemical brain preservation. The non-profit Brain Preservation Foundation offers a cash prize valued at over $100,000 for demonstrations of techniques that would allow for high-fidelity, long-term storage of a mammalian brain.^[37]

In 2016, scientists at the Buck Institute for Research on Aging and the Mayo Clinic employed genetic and pharmacological approaches to ablate pro-aging senescent cells, extending healthy lifespan of mice by over 25%. The startup Unity Biotechnology is further developing this strategy in human clinical trials.^[38]

In early 2017, Harvard scientists headed by biologist David Sinclair announced they have tested a compound called NAD+ on mice and have successfully reversed the cellular aging process and can protect the DNA from future damage.^[39] "The old mouse and young mouse cells are indistinguishable", David was quoted. Human trials are to begin shortly in what the team expect is 6 months at Brigham and Women's Hospital, in Boston.^{[citation needed]}

Immortalism and immortality as a movement

In 2012 in Russia, and then in the United States, Israel, and the Netherlands, pro-immortality transhumanist political parties were launched.^[40] They aim to provide political support to anti-aging and radical life extension research and technologies and want to ensure the fastest possible—and at the same time, the least disruptive—societal transition to radical life extension, life without aging, and ultimately, immortality. They aim to make it possible to provide access to such technologies to the majority of people alive today.^[41]

Other life extensionists

Biogerontologist Marios Kyriazis suggested that biological immortality in humans is an inevitable consequence of natural evolution. ^[42] His theory of extreme lifespans through perpetual-equalising interventions (ELPIs) proposes that^[43] the ability to attain indefinite lifespans is inherent in human biology, and that there will come a time when humans will continue to develop their intelligence by living indefinitely, rather than through evolution by natural selection.^[44][45]

Future medicine, life extension and "swallowing the doctor"

Future advances in nanomedicine could give rise to life extension through the repair of many processes thought to be responsible for aging. K. Eric Drexler, one of the founders of nanotechnology, postulated cell repair devices, including ones operating within cells and utilizing as yet hypothetical molecular machines, in his 1986 book Engines of Creation. Raymond Kurzweil, a futurist and transhumanist, stated in his book The Singularity Is Near that he believes that advanced medical nanorobotics could completely remedy the effects of aging by 2030.^[46] According to Richard Feynman, it was his former graduate student and collaborator Albert Hibbs who originally suggested to him (circa 1959) the idea of a medical use for Feynman's theoretical micromachines (see biological machine). Hibbs suggested that certain repair machines might one day be reduced in size to the point that it would, in theory, be possible to (as Feynman put it) "swallow the doctor". The idea was incorporated into Feynman's 1959 essay There's Plenty of Room at the Bottom.^[47]

Formamide-based prebiotic chemistry

From Wikipedia, the free encyclopedia

Formamide-based prebiotic chemistry refers to ongoing scientific efforts aimed at reconstructing the beginnings of life on our planet assuming that formamide could accumulate in sufficiently high amounts to serve as the building block and reaction medium for the synthesis of the first biogenic molecules.^[1]

Formamide (NH₂CHO), the simplest naturally occurring amide, contains all the elements (hydrogen, carbon, oxygen, and nitrogen), which are required for the synthesis of biomolecules, and is a ubiquitous molecule in the Universe.^[2] Formamide has been detected in galactic centers,^[3],[4] star-forming regions of dense molecular clouds,^[5] high-mass young stellar objects,^[6] the interstellar medium,^[7] comets,^[8],[9],[10] and satellites.^[11] In particular, dense clouds containing formamide, with sizes on the order of kiloparsecs, have been observed in the vicinity of the Solar System.^[5]

Formamide forms under a variety of conditions, corresponding to both terrestrial environments and interstellar media: e.g., on high-energy particle irradiation of binary mixtures of ammonia (NH₃) and carbon monoxide (CO),^[12] or from the reaction between formic acid (HCOOH) with NH₃.^[13] It has been suggested that in hydrothermal pores formamide may accumulate in sufficiently high concentrations to enable synthesis of biogenic molecules.^[14] Ab initio molecular dynamics simulations have revealed that formamide could be a key intermediate of the Miller-Urey experiment as well.^[15]

The combinatorial power of carbon is manifested in the composition of the molecular populations detected in circum- and interstellar media (see the Astrochemistry.net^[16] web site). The number and the complexity of carbon-containing molecules are significantly higher than those of inorganic compounds, presumably all over the Universe. One of the most abundant C-containing three-atoms molecule observed in space is hydrogen cyanide (HCN).^[17] The chemistry of HCN has thus attracted attention in Origin of Life studies since the earliest times, and the laboratory synthesis of adenine from HCN under presumptive prebiotic conditions was reported as early as 1961.^[18] The intrinsic limit of HCN stems from its high reactivity, which leads in turn, to instability and the difficulty associated with its concentration and accumulation in unreacted form.^[19] The “Warm Little Pond” in which life is supposed to have started, as imagined by Charles Darwin^[20],[21] and re-elaborated by Alexander Oparin,^[22] had most likely to reach sufficiently high concentrations to start creating the next levels of complexity. Hence the necessity of a derivative of HCN that is sufficiently stable to survive for time periods extended enough to allow its concentration in the actual physico-chemical settings, but that is sufficiently reactive to originate new compounds in prebiotically plausible environments.^[19] Ideally, this derivative should be able to undergo reactions in various directions, without prohibitively high energy barriers, thus allowing the production of different classes of potentially prebiotic compounds. Formamide fulfils all these requirements and, due to its significantly higher boiling point (210 °C), enables chemical synthesis in a much broader temperature range than water.^[1],[23]

Prebiotic chemistry

Current living forms on Earth are essentially composed of four types of molecular entities: (i) nucleic acids, (ii) proteins, (iii) carbohydrates, and (iv) lipids.^[24] Nucleic acids (DNA and RNA) embody and express the genetic information and, together, constitute the genome and the apparatus for its expression (the genotype). Proteins, carbohydrates, and lipids form the structures, which harness and handle energy from the environment for organizing matter according to the instructions specified by the genotype, aiming to its conservation and transmission. The ensemble of proteins, carbohydrates, lipids and nucleic acids constitute the phenotype. Life is thus made of the interaction of metabolism and genetics, of the genotype with the phenotype. Both are built around the chemistry of the most common elements of the Universe (hydrogen, oxygen, nitrogen, and carbon), important although ancillary roles being played by phosphorus and sulphur, and by other elements.^[25]

Given the overwhelming variety of the chemically conceivable molecules, the fact that in biological systems we observe only a small subset of organic molecules has raised questions how and which different reaction pathways could have plausibly lead to the synthesis of pre-biological molecules on the primordial Earth. These are the main objectives of prebiotic chemistry research.

Precursor of biogenic molecules

Figure 1. Relationship between formamide and other prebiotic feedstock molecules, such as HCN and ammonium formate(NH₄⁺HCOO⁻).^[1]

Figure 1 summarizes the basic chemistry of formamide and its chemical connection with HCN and ammonium formate (NH₄⁺HCOO⁻), considering selected examples of preparative and degradative reactions.^[1]

The synthesis of purine from formamide was first reported in 1980.^[26] A series of studies building on this observation was started 20 years later: the synthesis of a large panel of prebiotically relevant compounds (including purine, adenine, cytosine, and 4(3H)pyrimidinone) in good yields was reported in 2001.^[27] These products were obtained by heating formamide in the presence of simple catalysts such as calcium carbonate (CaCO₃), silica (SiO₂), or alumina (Al₂O₃).

In addition to nucleobases, sugars,^[28] carboxylic acids,^[29] amino acids,^[29] as well as heterogeneous compounds of various classes,^[29] (including urea and carbodiimide) were also synthesized. The catalysts studied include, in addition to those mentioned, titanium oxides,^[30] clays,^[31] cosmic dust analogues,^[32] phosphates,^[33] iron sulphide minerals,^[34] zirconium minerals,^[35] borate minerals,^[36] or numerous materials of meteoritic origin ^[28],[29] encompassing iron, stony-iron, chondrites, and achondrites meteorites.

Various energy sources, including thermal energy,^[27] UV-radiation,^[33] irradiation with high-energy (terawatt) laser pulses,^[37] or slow protons^[28] were tested. Mimics of different formamide-based prebiotic scenarios have been reconstructed and analyzed, including space-wise solar wind irradiation of meteorites,^[28] dynamic chemical gardens,^[38] and meteorites in aqueous environments.^[39] It has been suggested that the stepwise decrease of the temperature of the prebiotic environment could induce a sequence of strongly non-equilibrium chemical events that led to the emergence of more and more complex species from formamide on the early Earth.^[23],[40]

For each studied combination of catalyst/energy source/environment, formamide condensed into a variety of different prebiotically relevant compounds, each combination giving rise to a specific set of relatively complex molecules, usually encompassing several nucleobases, amino acids, and carboxylic acids.^[1] The highest level of complexity was attained for the formamide/meteorite system,^[29] using proton irradiation as the energy source, where the one-pot synthesis of four nucleosides (uridine, cytidine, adenosine, thymidine) was observed.^[28] So far, no other one-carbon atom compound has shown the versatility of products that can be formed from formamide under plausible prebiotic conditions in a one-pot chemistry (see Figure 2).^[41]
 

Figure 2. Main prebiotic building blocks that can be synthesized from formamide under plausible prebiotic conditions.^[1],[28]

In addition to its dual function of substrate and solvent in one-pot syntheses affording prebiotic compounds as complex as nucleosides and long aliphatic chains,^[39] it has been observed that formamide plays a role in the generation of molecules which are closer to the biological domain. In the presence of a phosphate source (e.g., phosphate minerals), formamide promotes the phosphorylation of nucleosides, leading to the formation of nucleotides,^[42],[43] and strongly stimulates the non-enzymatic polymerization of 3’,5’ cyclic nucleotides, leading to the abiotic synthesis of RNA oligomers.^[44] This is the reason why formamide is considered a plausible medium for prebiotic phosphorylation reactions also in the “discontinuous synthesis” scenario of the origin of life.^[45],[46]

Superhabitable planet

From Wikipedia, the free encyclopedia

 

Artist's impression of one possible appearance of a superhabitable planet. The reddish hue is vegetation.^[1]

A superhabitable planet is a hypothetical type of exoplanet or exomoon that may be better suited than Earth for the emergence and evolution of life. The concept was introduced in 2014 by René Heller and John Armstrong,^[2] who have criticized the language used in the search for habitable planets, so they propose clarifications because a circumstellar habitable zone (HZ) is not enough to define a planet's habitability.^[3] Heller and Armstrong state that it is not clear why Earth should offer the most suitable physicochemical parameters to living organisms, because "planets could be non-Earth-like, yet offer more suitable conditions for the emergence and evolution of life than Earth did or does." While still assuming that life requires water, they hypothesize that Earth may not represent the optimal planetary habitability conditions for maximum biodiversity; in other words, they define a superhabitable world as a terrestrial planet or moon that could support more diverse flora and fauna than there are on Earth, as it would empirically show that its environment is more hospitable to life.

Heller and Armstrong also point out that not all rocky planets in a habitable zone (HZ) may be habitable, and that tidal heating can render terrestrial or icy worlds habitable beyond the stellar HZ, such as in Europa's internal ocean.^{[4][n. 1]} The authors propose that in order to identify a habitable—or superhabitable—planet, a characterization concept is required that is biocentric rather than geo- or anthropocentric.^[2] Heller and Armstrong proposed to establish a profile for exoplanets according to stellar type, mass and location in their planetary system, among other features. According to these authors, such superhabitable worlds would likely be larger, warmer, and older than Earth, and orbiting K-type main-sequence stars.

General characteristics

Heller and Armstrong proposed that a series of basic characteristics are required to classify an exoplanet or exomoon as superhabitable;^{[7][2][8][9] [10]} for size, it is required to be about 2 Earth masses, and 1.3 Earth radii will provide an optimal size for plate tectonics.^[11] In addition, it would have a greater gravitational attraction that would increase retention of gases during the planet's formation.^[10] It is therefore likely that they have a denser atmosphere that will offer greater concentration of oxygen and greenhouse gases, which in turn raise the average temperature to optimum levels for plant life to about 25 °C (77 °F).^[12][13] A denser atmosphere may also influence the surface relief, making it more regular and decreasing the size of the ocean basins, which would improve diversity of marine life in shallow waters.^[14]

Other factors to consider are the type of star in the system. K-type stars are less massive than the Sun, and are stable on the main sequence for a very long time (15 to 30 billion years, compared to 10 billion for the Sun, a G-class star),^[15][16] giving more time for the emergence of life and evolution. A superhabitable world would also require to be located near the center of the habitable zone of its star system for a long time.^[17][18]

Surface, size and composition

Kepler-62e, second from the left has a radius of 1.6 R_⊕. Earth is on the far right; scaled.

An exoplanet with a larger volume than that of Earth, or with a more complex terrain, or with a larger surface covered with liquid water, could be more hospitable for life than Earth.^[19] Since the volume of a planet tends to be directly related to its mass, the more massive it is, the greater its gravitational pull, which can result in a denser atmosphere.^[20]

Some studies indicate that there is a natural limit, set at 1.6R_⊕, below which nearly all planets are terrestrial, composed primarily of rock-iron-water mixtures.^[21] Generally, objects with a mass below 6 M_⊕ are very likely to be of similar composition as Earth.^[22] Above this limit, the density of the planets decreases with increasing size, the planet will become a "water world" and finally a gas giant.^[23][24] In addition, most super-Earths' high mass may cause them to lack plate tectonics.^[11] Thus, it is expected that any exoplanet similar to Earth's density and with a radius under 1.6 R_⊕ may be suitable for life.^[13] However, other studies indicate that water worlds represent a transitional stage between mini-Neptunes and the terrestrial planets, especially if they belong to red dwarfs or K dwarfs.^[25][26] Although water planets may be habitable, the average depth of the water and the absence of land area would not make them superhabitable as defined by Heller and Armstrong.^[27] From a geological perspective, the optimal mass of a planet is about 2 M_⊕, so it must have a radius that keeps the density of the Earth among 1.2 and 1.3R_⊕.^[28]

The average depth of the oceans also affects the habitability of a planet. The shallow areas of the sea, given the amount of light and heat they receive, usually are more comfortable for aquatic species, so it is likely that exoplanets with a lower average depth are more suitable for life.^[27][29] More massive exoplanets would tend to have a regular surface gravity, which can mean shallower—and more hospitable—ocean basins.^[30]

Geology

Plate tectonics, in combination with the presence of large bodies of water on a planet, is able to maintain high levels of carbon dioxide (CO
₂) in its atmosphere.^[31][32] This process appears to be common in geologically active terrestrial planets with a significant rotation speed.^[33] The more massive a planetary body, the longer time it will generate internal heat, which is a major contributing factor to plate tectonics.^[11] However, excessive mass can also slow plate tectonics because of increased pressure and viscosity of the mantle, which hinders the sliding of the lithosphere.^[11] Research suggests that plate tectonics peaks in activity in bodies with a mass between 1 and 5M_⊕, with an optimum mass of approximately 2M_⊕.^[28]

If the geological activity is not strong enough to generate a sufficient amount of greenhouse gases to increase global temperatures above the freezing point of water, the planet could experience a permanent ice age, unless the process is offset by an intense internal heat source such as tidal heating or stellar irradiation.^[34]

Magnetosphere

Another feature favorable to life is a planet's potential to develop a strong magnetosphere to protect its surface and atmosphere from cosmic radiation and stellar winds, especially around red dwarf stars.^[35] Less massive bodies and those with a slow rotation, or those that are tidally locked, have a weak or no magnetic field, which over time can result in the loss of a significant portion of its atmosphere, especially hydrogen, by hydrodynamic escape.^[11]

The climate of a warmer and wetter terrestrial exoplanet may resemble that of the tropical regions of Earth. In the picture, mangrove in Cambodia.

Temperature and climate

The optimum temperature for Earth-like life in general is unknown, although it appears that on Earth organism diversity has been greater in warmer periods.^[36] It is therefore possible that exoplanets with slightly higher average temperatures than that of Earth are more suitable for life.^[37] The thermoregulatory effect of large oceans on exoplanets located in a habitable zone may maintain a moderate temperature range.^[38][37] In this case, deserts would be more limited in area and would likely support habitat-rich coastal environments.^[37]

However, studies suggest that Earth already lies near to the inner edge of the habitable zone of the Solar System,^[39] and that may harm its long-term livability as the luminosities of main-sequence stars steadily increase over time, pushing the habitable zone outwards.^[40][41] Therefore, superhabitable exoplanets must be warmer than Earth, yet orbit further out than Earth does and closer to the center of the system's habitable zone.^[42][17] This would be possible with a thicker atmosphere or with a higher concentration of greenhouse gases.^[43][44]

Star

Habitable zone (HZ) position of some of the most similar and average surface temperature exoplanets.^{[45][n. 2]}

The star's type largely determines the conditions present in a system.^[46][47] The most massive stars O, B, and A have a very short life cycle, quickly leaving the main sequence.^[48][49] In addition, O and B type stars produce a photoevaporation effect that prevents the accretion of planets around the star.^[50][51]

On the opposite side, the less massive M and K types are by far the most common and long-lived stars of the universe, but their potential for supporting life is still under study.^[46][51] Their low luminosity reduces the size of the habitable zone, which are exposed to ultraviolet radiation outbreaks that occur frequently, especially during their first billion year of existence.^[15] When a planet's orbit is too short, it can cause tidal locking of the planet, where it always presents the same hemisphere to the star, known as day hemisphere.^[52][51] Even if the existence of life were possible in a system of this type, it is unlikely that any exoplanet belonging to a red dwarf star would be considered superhabitable.^[46]

Dismissing both ends, systems with a K-type stars offer the best habitable zones for life.^[15][51] K-type stars allow the formation of planets around them, have a long life expectancy, and provide a stable habitable zone free of the effects of excessive proximity to its star.^[51] Furthermore, the radiation produced by a K-type star is high enough to allow complex life without the need for an atmospheric ozone layer.^[15][53][54] They are also the most stable and their habitable zone does not move very much during its lifetime, so a terrestrial analog located near a K-type star may be habitable for almost all of the main sequence.^[15]

Orbit and rotation

Artistic impression of a possible Earth analog, Kepler-186f. Some superhabitable planets could have a similar appearance and may not have important differences with Earth.

Experts have not reached a consensus about what is the optimal rotation speed for an exoplanet, but it should not be too fast nor too slow. The latter case can cause some problems similar to those observed in Venus, which completes one rotation every 243 Earth days and as a result, cannot generate an Earth-like magnetic field.^[55][56]

Ideally, the orbit of a superhabitable world must be at the midpoint of the habitable zone of its star system.^[57][43]

Atmosphere

There are no solid arguments to explain if Earth's atmosphere has the optimal composition to host life.^[43] On Earth, during the period when coal was first formed, atmospheric oxygen (O
₂) levels were up to 35%, and coincided with the periods of greatest biodiversity.^[58] So, assuming that the presence of a significant amount of oxygen in the atmosphere is essential for exoplanets to develop complex life forms,^[59][43] the percentage of oxygen relative to the total atmosphere appears to limit the maximum size of the planet for optimum superhabitability and ample biodiversity^{[clarification needed]}.

Also, the atmospheric density should be higher in more massive planets, which reinforces the hypothesis that super-Earths can provide superhabitable conditions.^[43]

Age

The first stars that formed in the universe were metal-free stars, which probably prevented planet formation.

In a biological context, older planets than Earth may have greater biodiversity, since native species have had more time to evolve, adapt and stabilize the environmental conditions to sustain a suitable environment for life that can benefit their descendants.^[16]

However, for many years it was thought that since older star systems have lower metallicity, they should display low planet formation, and thus such old planets may have been scant in the beginning,^[60] but the number of metallic items in the universe must have grown steadily since its inception.^[61] The first exoplanetary discoveries, mostly gas giants orbiting very close to their stars, known as Hot Jupiters, suggest that planets were rare in systems with low metallicity, which invited suspicion of a time limit on the appearance of the first objects landmass.^[62] Later, in 2012, the Kepler telescope's observations allowed experts to find out that this relationship is much more restrictive in systems with Hot Jupiters, and that terrestrial planets could form in stars of much lower metallicity, to some extent.^[61] It is now thought that the first Earth-mass objects should appear sometime between 7 and 12 billion years.^[61] Given the greater stability of the orange dwarfs (K-type) compared to the Sun (G-type) and longer life expectancy, it is possible that superhabitable exoplanets belonging to K-type stars, orbiting within its habitable zone, could provide a longer, steadier, and better environment for life than Earth.^[15]

Profile summary

A size comparison and artist's impression of Kepler-442b (1.34 R_⊕) to the Earth (right).

Despite the scarcity of information available, the hypotheses presented above on superhabitable planets can be summarized as a preliminary profile, even if there is no scientific consensus.^[10]

• Mass: approximately 2M_⊕.
• Radius: to maintain a similar Earth density, its radius should be between 1.2 and 1.3R_⊕.
• Oceans: percentage of surface area covered by oceans should be Earth-like but more distributed, without large continuous land masses. The oceans should be shallow; the light then will penetrate easier through the water and will reach the fauna and flora, stimulating an abundance of life down in the ocean.
• Distance: shorter distance from the center of the habitable zone of the system than Earth.
• Temperature: average surface temperature of about 25 °C (77 °F).^[12]
• Star and age: belonging to an intermediate K-type star with an older age than the Sun (4.5 billion years) but younger than 7 billion years.
• Atmosphere: somewhat denser than Earth's and with a higher concentration of oxygen. That will make life larger and more abundant.

There is no confirmed exoplanet that meets all these requirements. After updating the database of exoplanets on 23 July 2015, the one that comes closest is Kepler-442b, belonging to an orange dwarf star, with a radius of 1.34R_⊕ and a mass of 2.34M_⊕, but with an estimated surface temperature of −2.65 °C (27.23 °F).^[63][64]

Appearance

The appearance of a superhabitable planet should be, in general, very similar to Earth.^[18] The main differences, in compliance with the profile seen previously, would be derived from its mass. Its denser atmosphere probably prevent the formation of ice sheets as a result of lower thermal difference between different regions of the planet.^[43] Also, it has a higher concentration of clouds, and abundant rainfall.

Probably the vegetation is very different due to the increased air density, precipitation, temperature, and stellar flux. For the type of light emitted from the K-type stars, plants may take other colors than green.^[1][65] The vegetation would cover more regions than vegetation here on Earth, making this visible from space.^[18]

In general, the climate of a superhabitable planet would be warmer, moist, homogeneous and have stable land, allowing life to extend across the surface without presenting large population differences, in contrast to Earth that has inhospitable areas such as glaciers, deserts and tropical regions.^[37] If the atmosphere contains enough molecular oxygen, the conditions of these planets may be bearable to humans even without the protection of a space suit, provided that the atmosphere does not contain excessive toxic gases, but would require some adaptation to the increased gravity, such as an increase in muscles and in bone density, etc.^[18][26][66]

Abundance

Set and subsets of terrestrial worlds.^[67]

Heller and Armstrong speculate that the number of superhabitable planets can far exceed that of Earth analogs:^[67] less massive stars in the main sequence are more abundant than the larger and brighter stars, so there are more orange dwarfs than solar analogues.^[68] It is estimated that about 9% of stars in the Milky Way are K-type stars.^[69]

Another point favoring the predominance of superhabitable planets in regard to Earth analogs is that, unlike the latter, most of the requirements of a superhabitable world can occur spontaneously and jointly simply by having a higher mass.^[70] A planetary body close to 2 or 3M_⊕ should have longer-lasting plate tectonics and also will have a larger surface area in comparison to Earth.^[10] Similarly, it is likely that its oceans are shallower by the effect of gravity on the planet's crust, its gravitational field more intense and, a denser atmosphere.^[12]

By contrast, Earth-mass planets may have a wider range of conditions. For example, some may sustain active tectonics for a shorter time period and will therefore end up with lower air density than Earth, increasing the probability of developing global ice coverage, or even a permanent Snowball Earth scenario.^[43] Another negative effect of lower atmospheric density can be manifested in the form of thermal oscillations, which can lead to high variability in the global climate and increase the chance for catastrophic events. In addition, by having a weaker magnetosphere, such planets may lose their atmospheric hydrogen by hydrodynamic escape easier and become a desert planet.^[43] Any of these examples could prevent the emergence of life on a planet's surface.^[71] In any case, the multitude of scenarios that can turn an Earth-mass planet located in the habitable zone of a solar analogue into an inhospitable place are less likely on a planet that meets the basic features of a superhabitable world, so that the latter should be more common.^[67]