Planetary habitability

Planetary habitability is a way scientists measure how likely a planet or a natural satellite is to support life. It helps us understand if a world could have the right conditions for life to develop and stay there. Scientists look at many things, like how far the planet is from its star and what kind of atmosphere it has, to figure this out.

The Planetary Habitability Laboratory keeps a list of exoplanets—planets that orbit stars other than our Sun—that might be able to support life. This helps researchers around the world learn more about where life might exist beyond Earth.

Understanding planetary habitability is important because it helps us know where to look when searching for life in space. It makes us think about what kinds of places could be like our own Earth, where life thrives.

Background

Scientists study how planets and moons might be able to support life. They look for things like liquid water, the right chemicals, and energy sources. This idea comes from what we know about Earth and our Sun.

We do not yet know if life exists elsewhere, but we can make guesses by comparing other places to Earth. In the late 1900s, we learned a lot by looking at other planets in our solar system with robots and by finding planets around other stars. These discoveries show that many places in space might be able to support life, even if they look very different from Earth.

Stellar characteristics

The chance that a planet can support life depends a lot on its star. The "habitable zone" (HZ) is the area around a star where a planet could have liquid water on its surface. Life could also exist deeper inside a planet where water can still be liquid, even if the planet is far from its star.

Scientists have made a list of stars that might support habitable planets. They picked out 17,000 stars that seem good candidates. Very large galaxies may be better for forming habitable planets than smaller ones like our Milky Way.

What makes a planet habitable is more complex than just being in the right place to have liquid water. Things like the planet’s geology, radiation from the star, and the star’s environment all play a role. Liquid water is important but not the only factor for life as we know it.

The type of star matters a lot. Stars that are too hot or too cool might not support life. The Sun is a good example because it has the right temperature and lasts long enough for life to develop. Some smaller, cooler stars might also support life, but there are still many questions about this.

Planetary characteristics

Habitability indicators and biosignatures must be interpreted within a planetary and environmental context. Whether a planet will emerge as habitable depends on the sequence of events that led to its formation, which could include the production of organic molecules in molecular clouds and protoplanetary disks, delivery of materials during and after planetary accretion, and the orbital location in the planetary system. The chief assumption about habitable planets is that they are terrestrial. Such planets, roughly within one order of magnitude of Earth mass, are primarily composed of silicate rocks, and have not accreted the gaseous outer layers of hydrogen and helium found on gas giants. The possibility that life could evolve in the cloud tops of giant planets has not been decisively ruled out, though it is considered unlikely, as they have no surface and their gravity is enormous. The natural satellites of giant planets, meanwhile, remain valid candidates for hosting life.

In February 2011 the Kepler Space Observatory Mission team released a list of 1235 extrasolar planet candidates, including 54 that may be in the habitable zone. Six of the candidates in this zone are smaller than twice the size of Earth. A more recent study found that one of these candidates (KOI 326.01) is much larger and hotter than first reported. Based on the findings, the Kepler team estimated there to be "at least 50 billion planets in the Milky Way" of which "at least 500 million" are in the habitable zone.

In analyzing which environments are likely to support life, a distinction is usually made between simple, unicellular organisms such as bacteria and archaea and complex metazoans (animals). Unicellularity necessarily precedes multicellularity in any hypothetical tree of life, and where single-celled organisms do emerge there is no assurance that greater complexity will then develop. The planetary characteristics listed below are considered crucial for life generally, but in every case, multicellular organisms are more picky than unicellular life.

In August 2021, a new class of habitable planets, named ocean planets, which involves "hot, ocean-covered planets with hydrogen-rich atmospheres", has been reported. Hycean planets may soon be studied for biosignatures by terrestrial telescopes as well as space telescopes, such as the James Webb Space Telescope (JWST), which was launched on 25 December 2021.

Mass and size

Low-mass planets are poor candidates for life for two reasons. First, their lesser gravity makes atmosphere retention difficult. Constituent molecules are more likely to reach escape velocity and be lost to space when buffeted by solar wind or stirred by collision. Planets without a thick atmosphere lack the matter necessary for primal biochemistry, have little insulation and poor heat transfer across their surfaces, and provide less protection against meteoroids and high-frequency radiation. Further, where an atmosphere is less dense than 0.006 Earth atmospheres, water cannot exist in liquid form as the required atmospheric pressure does not occur. In addition, a lessened pressure reduces the range of temperatures at which water is liquid.

Secondly, smaller planets have smaller diameters and thus higher surface-to-volume ratios than their larger cousins. Such bodies tend to lose the energy left over from their formation quickly and end up geologically dead, lacking the volcanoes, earthquakes and tectonic activity which supply the surface with life-sustaining material and the atmosphere with temperature moderators like carbon dioxide. Plate tectonics appear particularly crucial, at least on Earth: not only does the process recycle important chemicals and minerals, but it also fosters bio-diversity through continent creation and increased environmental complexity and helps create the convective cells necessary to generate Earth's magnetic field. Although geologically active planets with volcanism but no plate tectonics, called Ignan Earths, could also be habitable.

"Low mass" is partly a relative label: the Earth is low mass when compared to the Solar System's gas giants, but it is the largest, by diameter and mass, and the densest of all terrestrial bodies. It is large enough to retain an atmosphere through gravity alone and large enough that its molten core remains a heat engine, driving the diverse geology of the surface. Mars, by contrast, is nearly (or perhaps totally) geologically dead and has lost much of its atmosphere. Thus it would be fair to infer that the lower mass limit for habitability lies somewhere between that of Mars and that of Earth or Venus. Venus, which has 85% of Earth's mass, shows no signs of tectonic activity. Conversely, "super-Earths", terrestrial planets with higher masses than Earth, would have higher levels of plate tectonics and thus be firmly placed in the habitable range.

Exceptional circumstances do offer exceptional cases: Jupiter's moon Io (which is smaller than any of the terrestrial planets) is volcanically dynamic because of the gravitational stresses induced by its orbit, and its neighbor Europa may have a liquid ocean or icy slush underneath a frozen shell also due to power generated from orbiting a gas giant.

Saturn's Titan, meanwhile, has an outside chance of harboring life, as it has retained a thick atmosphere and has liquid methane seas on its surface. Organic-chemical reactions that only require minimum energy are possible in these seas, but whether any living system can be based on such minimal reactions is unclear, and would seem unlikely. These satellites are exceptions, but they prove that mass, as a criterion for habitability, cannot necessarily be considered definitive at this stage of our understanding.

A larger planet is likely to have a more massive atmosphere. A combination of higher escape velocity to retain lighter atoms, and extensive outgassing from enhanced plate tectonics may greatly increase the atmospheric pressure and temperature at the surface compared to Earth. The enhanced greenhouse effect of such a heavy atmosphere would tend to suggest that the habitable zone should be further out from the central star for such massive planets.

Finally, a larger planet is likely to have a large iron core. This allows for a magnetic field to protect the planet from stellar wind and cosmic radiation, which otherwise would tend to strip away planetary atmosphere and bombard living things with ionized particles. Mass is not the only criterion for producing a magnetic field—as the planet must also rotate fast enough to produce a dynamo effect within its core—but it is a significant component of the process.

The mass of a potentially habitable exoplanet is between 0.1 and 5.0 Earth masses. However, it is possible for a habitable world to have a mass as low as 0.0268 Earth Masses. The radius of a potentially habitable exoplanet would range between 0.5 and 1.5 Earth radii.

Orbit and rotation

As with other criteria, stability is the critical consideration in evaluating the effect of orbital and rotational characteristics on planetary habitability. Orbital eccentricity is the difference between a planet's farthest and closest approach to its parent star divided by the sum of said distances. It is a ratio describing the shape of the elliptical orbit. The greater the eccentricity the greater the temperature fluctuation on a planet's surface. Although they are adaptive, living organisms can stand only so much variation, particularly if the fluctuations overlap both the freezing point and boiling point of the planet's main biotic solvent (e.g., water on Earth). If, for example, Earth's oceans were alternately boiling and freezing solid, it is difficult to imagine life as we know it having evolved. The more complex the organism, the greater the temperature sensitivity. The Earth's orbit is almost perfectly circular, with an eccentricity of less than 0.02; other planets in the Solar System (with the exception of Mercury and Mars) have eccentricities that are similarly benign.

Habitability is also influenced by the architecture of the planetary system around a star. The evolution and stability of these systems are determined by gravitational dynamics, which drive the orbital evolution of terrestrial planets. Data collected on the orbital eccentricities of extrasolar planets has surprised most researchers: 90% have an orbital eccentricity greater than that found within the Solar System, and the average is fully 0.25. This means that the vast majority of planets have highly eccentric orbits and of these, even if their average distance from their star is deemed to be within the HZ, they nonetheless would be spending only a small portion of their time within the zone.

A planet's movement around its rotational axis must also meet certain criteria if life is to have the opportunity to evolve. A first assumption is that the planet should have moderate seasons. If there is little or no axial tilt (or obliquity) relative to the perpendicular of the ecliptic, seasons will not occur and a main stimulant to biospheric dynamism will disappear. The planet would also be colder than it would be with a significant tilt: when the greatest intensity of radiation is always within a few degrees of the equator, warm weather cannot move poleward and a planet's climate becomes dominated by colder polar weather systems.

If a planet is radically tilted, seasons will be extreme and make it more difficult for a biosphere to achieve homeostasis. The axial tilt of the Earth is higher now (in the Quaternary) than it has been in the past, coinciding with reduced polar ice, warmer temperatures, and less seasonal variation. Scientists do not know whether this trend will continue indefinitely with further increases in axial tilt.

The exact effects of these changes can only be computer modelled at present, and studies have shown that even extreme tilts of up to 85 degrees do not absolutely preclude life "provided it does not occupy continental surfaces plagued seasonally by the highest temperature." Not only the mean axial tilt, but also its variation over time must be considered. The Earth's tilt varies between 21.5 and 24.5 degrees over 41,000 years. A more drastic variation, or a much shorter periodicity, would induce climatic effects such as variations in seasonal severity.

Other orbital considerations include:

• The planet should rotate relatively quickly so that the day-night cycle is not overlong. If a day takes years, the temperature differential between the day and night side will be pronounced, and problems similar to those noted with extreme orbital eccentricity will come to the fore.
• The planet also should rotate quickly enough so that a magnetic dynamo may be started in its iron core to produce a magnetic field.
• Change in the direction of the axis rotation (precession) should not be pronounced. In itself, precession need not affect habitability as it changes the direction of the tilt, not its degree. However, precession tends to accentuate variations caused by other orbital deviations.

The Earth's Moon appears to play a crucial role in moderating the Earth's climate by stabilising the axial tilt. It has been suggested that a chaotic tilt may be a "deal-breaker" in terms of habitability—i.e. a satellite the size of the Moon is not only helpful but required to produce stability. This position remains controversial.

In the case of the Earth, the sole Moon is sufficiently massive and orbits so as to significantly contribute to ocean tides, which in turn aids the dynamic churning of Earth's large liquid water oceans. These lunar forces not only help ensure that the oceans do not stagnate, but also play a critical role in Earth's dynamic climate.

Geology

Concentrations of radionuclides in rocky planet mantles may be critical for the habitability of Earth-like planets. Such planets with higher abundances likely lack a persistent dynamo for a significant fraction of their lifetimes, and those with lower concentrations may often be geologically inert. Planetary dynamos create strong magnetic fields which may often be necessary for life to develop or persist as they shield planets from solar winds and cosmic radiation. The electromagnetic emission spectra of stars could be used to identify those which are more likely to host habitable Earth-like planets. As of 2020, radionuclides are thought to be produced by rare stellar processes such as neutron star mergers.

Additional geological characteristics may be essential or major factors in the habitability of natural celestial bodies – including some that may shape the body's heat and magnetic field. Some of these are unknown or not well understood and being investigated by planetary scientists, geochemists and others.

Geochemistry

Further information: Geochemistry

It is generally assumed that any extraterrestrial life that might exist will be based on the same fundamental biochemistry as found on Earth, as the four elements most vital for life, carbon, hydrogen, oxygen, and nitrogen, are also the most common chemically reactive elements in the universe. Indeed, simple biogenic compounds, such as very simple amino acids such as glycine, have been found in meteorites and in the interstellar medium. These four elements together comprise over 96% of Earth's collective biomass. Carbon has an unparalleled ability to bond with itself and to form a massive array of intricate and varied structures, making it an ideal material for the complex mechanisms that form living cells. Hydrogen and oxygen, in the form of water, compose the solvent in which biological processes take place and in which the first reactions occurred that led to life's emergence. The energy released in the formation of powerful covalent bonds between carbon and oxygen, available by oxidizing organic compounds, is the fuel of all complex life-forms. These four elements together make up amino acids, which in turn are the building blocks of proteins, the substance of living tissue. In addition, neither sulfur (required for the building of proteins) nor phosphorus (needed for the formation of DNA, RNA, and the adenosine phosphates essential to metabolism) are rare.

Relative abundance in space does not always mirror differentiated abundance within planets; of the four life elements, for instance, only oxygen is present in any abundance in the Earth's crust. This can be partly explained by the fact that many of these elements, such as hydrogen and nitrogen, along with their simplest and most common compounds, such as carbon dioxide, carbon monoxide, methane, ammonia, and water, are gaseous at warm temperatures. In the hot region close to the Sun, these volatile compounds could not have played a significant role in the planets' geological formation. Instead, they were trapped as gases underneath the newly formed crusts, which were largely made of rocky, involatile compounds such as silica (a compound of silicon and oxygen, accounting for oxygen's relative abundance). Outgassing of volatile compounds through the first volcanoes would have contributed to the formation of the planets' atmospheres. The Miller–Urey experiment showed that, with the application of energy, simple inorganic compounds exposed to a primordial atmosphere can react to synthesize amino acids.

Even so, volcanic outgassing could not have accounted for the amount of water in Earth's oceans. The vast majority of the water—and arguably carbon—necessary for life must have come from the outer Solar System, away from the Sun's heat, where it could remain solid. Comets impacting with the Earth in the Solar System's early years would have deposited vast amounts of water, along with the other volatile compounds life requires, onto the early Earth, providing a kick-start to the origin of life.

Thus, while there is reason to suspect that the four "life elements" ought to be readily available elsewhere, a habitable system probably also requires a supply of long-term orbiting bodies to seed inner planets. Without comets there is a possibility that life as we know it would not exist on Earth.

Microenvironments and extremophiles

One important qualification to habitability criteria is that only a tiny portion of a planet is required to support life, a so-called Goldilocks Edge or Great Prebiotic Spot. Astrobiologists often concern themselves with "micro-environments", noting that "we lack a fundamental understanding of how evolutionary forces, such as mutation, selection, and genetic drift, operate in micro-organisms that act on and respond to changing micro-environments." Extremophiles are Earth organisms that live in niche environments under severe conditions generally considered inimical to life. Usually (although not always) unicellular, extremophiles include acutely alkaliphilic and acidophilic organisms and others that can survive water temperatures above 100 °C in hydrothermal vents.

Several complex multicellular life forms (or eukaryotes) have been identified with the potential to survive conditions that might exist outside the conservative habitable zone. Geothermal energy sustains ancient circumvent ecosystems, supporting large complex life forms such as Riftia pachyptila. Numerous microorganisms have been tested in simulated conditions and in low Earth orbit, including eukaryotes. An animal example is the Milnesium tardigradum, which can withstand extreme temperatures well above the boiling point of water and the cold vacuum of outer space. A desert moss, Syntrichia caninervis is one of few plants believed capable of surviving on Mars. In addition, the lichens Rhizocarpon geographicum and Rusavskia elegans have been found to survive in an environment where the atmospheric pressure is far too low for surface liquid water and where the radiant energy is also much lower than that which most plants require to photosynthesize. The fungi Cryomyces antarcticus and Cryomyces minteri are also able to survive and reproduce in Mars-like conditions.

The discovery of life in extreme conditions has complicated definitions of habitability, but also generated much excitement amongst researchers in greatly broadening the known range of conditions under which life can persist. For example, a planet that might otherwise be unable to support an atmosphere given the solar conditions in its vicinity, might be able to do so within a deep shadowed rift or volcanic cave. Similarly, craterous terrain might offer a refuge for primitive life. The Lawn Hill crater has been studied as an astrobiological analog, with researchers suggesting rapid sediment infill created a protected microenvironment for microbial organisms; similar conditions may have occurred over the geological history of Mars.

Earth environments that cannot support life are still instructive to astrobiologists in defining the limits of what organisms can endure. The heart of the Atacama Desert, generally considered the driest place on Earth, appears unable to support life, and it has been subject to study by NASA and ESA for that reason: it provides a Mars analog and the moisture gradients along its edges are ideal for studying the boundary between sterility and habitability. The Atacama was the subject of study in 2003 that partly replicated experiments from the Viking landings on Mars in the 1970s; no DNA could be recovered from two soil samples, and incubation experiments were also negative for biosignatures.

Ecological factors

The two current ecological approaches for predicting the potential habitability use 19 or 20 environmental factors, with emphasis on water availability, temperature, presence of nutrients, an energy source, and protection from solar ultraviolet and galactic cosmic radiation.

Classification terminology

The Habitable Exoplanets Catalog uses estimated surface temperature range to classify exoplanets:

• hypopsychroplanets – very cold ( 100 °C)

Mesoplanets would be ideal for complex life, whereas hypopsychroplanets and hyperthermoplanets might only support extremophilic life.

The HEC uses the following terms to classify exoplanets in terms of mass, from least to greatest: asteroidan, mercurian, subterran, terran, superterran, neptunian, and jovian.

Alternative star systems

Astronomers once looked only at stars like our Sun for places where life might exist. But since systems like ours seem rare, they now think life could develop around very different kinds of stars.

We think stars called F, G, K, and M could have planets where life might survive. About half of Sun-like stars might have rocky planets that could hold liquid water, based on data from NASA's Kepler Space Telescope.

Binary systems

Main article: Habitability of binary star systems

Most stars might come in pairs, called binary systems. In these systems, the stars can be very close together or far apart. When stars are far apart, planets around one star might still be able to support life. But when stars are very close, it might be hard for planets to stay in stable orbits. Some studies suggest that even in close binary systems like Alpha Centauri, planets could still have areas where life might survive.

Red dwarf systems

Main article: Habitability of red dwarf systems

Stars called red dwarfs, which make up most stars in the galaxy, might also have planets where life could exist. However, these stars can have big flares that might make life harder on nearby planets. Even though red dwarfs are small and dim, planets very close to them might still hold liquid water. But these planets might always face the same way to their star, with one side always hot and the other always cold. Special conditions would be needed for life to survive there.

Massive stars

Some very large stars might have planets for a short time, but these stars live only a short time themselves. Very big stars might help create conditions for life around smaller stars after they explode, but it’s unclear if planets could form there.

Neutron stars

Main article: Habitability of neutron star systems

Post-main sequence stars

Main article: Red giant § Prospects for habitability

Four classes of habitable planets based on water

Scientists have grouped planets that might support life into four types, based on how water behaves on them.

• Class I planets have conditions that let liquid water exist on the surface, along with sunlight, which could allow complex life to develop.

• Class II planets start with Earth-like conditions but later lose their ability to keep water on the surface. Mars and possibly Venus are examples where complex life may not develop.

• Class III planets have oceans of liquid water below their surfaces, warmed by heat from inside the planet. Examples include Europa and Enceladus. Life here would not have sunlight.

• Class IV planets have layers of liquid water trapped between ice layers. Ganymede and Callisto might be examples, where life would struggle to develop.

The galactic neighborhood

The place where a planet is in its galaxy can affect whether life might develop there. Scientists think some parts of galaxies are better for life than others. Our Solar System is in a good spot called the Orion Arm on the edge of the Milky Way galaxy.

This area is good for life because it is not too crowded with stars. In very crowded places, like globular clusters, there is too much harmful radiation and stars moving close together. Our Solar System is also far from dangerous sources of radiation, like active gamma ray sources or the busy center of the galaxy. The center has many stars and powerful objects like magnetars, supernovae, and supermassive black holes. The Sun's path around the galaxy also keeps it away from areas with intense radiation.

Being in a quiet, not-too-crowded place helps protect any life that might exist. Our "suburban" location in the galaxy is just right for supporting life.

Other considerations

Main article: Hypothetical types of biochemistry

Scientists think about life that might not need the same things as life on Earth. They wonder if life could use different materials or live in strange places. For example, some think life might not use carbon but something like silicon. Others think life could live in liquids that are not water, like ammonia or hydrocarbons. Some even imagine tiny life on a neutron star or floating in the clouds of a giant planet like Jupiter.

Big planets like Jupiter can help smaller planets stay safe. They can keep the orbits of smaller planets steady and protect them from harmful space rocks. But sometimes, big planets can also make it hard for smaller planets to stay in the right place for life.

Life itself can also change a planet to make it more friendly for living things. For example, simple plants on Earth changed the air long ago, which later helped animals live. Some scientists think that a planet and its life work together to keep things just right for living.

Chance also matters. Even with the right conditions, things might still go wrong, and a planet could become too hot or too cold for life.