Planetary habitability


Planetary habitability is a measure used in astrobiology to characterize a planet's or a natural satellite's potential to develop and sustain an environment hospitable to life. The Planetary Habitability Laboratory maintains a catalog of potentially habitable exoplanets.

Background

Research suggests that life on a planetary body may develop through abiogenesis or be transferred from one body to another, through a hypothetical process known as panspermia. Environments do not need to contain life to be considered habitable nor are accepted habitable zones the only areas in which life might arise.
As the existence of life beyond Earth is not known, planetary habitability is largely an extrapolation of conditions on Earth and the characteristics of the Sun and Solar System which appear favorable to life's flourishing. Of particular interest are those factors that have sustained complex, multicellular organisms on Earth and not just simpler, unicellular creatures. Research and theory in this regard is a component of a number of natural sciences, such as astronomy, planetary science, and the emerging discipline of astrobiology.
An absolute requirement for life is an energy source, and the notion of planetary habitability implies that many other geophysical, geochemical, and astrophysical criteria must be met before an astronomical body can support life. In its astrobiology roadmap, NASA has defined the principal habitability criteria as "extended regions of liquid water, conditions favorable for the assembly of complex organic molecules, and energy sources to sustain metabolism". In August 2018, researchers reported that water worlds could support life.
Habitability indicators and biosignatures must be interpreted within a planetary and environmental context. In determining the habitability potential of a body, studies focus on its bulk composition, orbital properties, atmosphere, and potential chemical interactions. Stellar characteristics of importance include mass and luminosity, stable variability, and high metallicity. Rocky, wet terrestrial-type planets and moons with the potential for Earth-like chemistry is a primary focus of astrobiological research. However, more speculative habitability theories occasionally examine alternative biochemistries and other types of astronomical bodies.
The idea that planets beyond Earth might host life is an ancient one, though historically it was framed by philosophy as much as physical science. The late 20th century saw two breakthroughs in the field. The observation and robotic spacecraft exploration of other planets and moons within the Solar System has provided critical information on defining habitability criteria and allowed for substantial geophysical comparisons between the Earth and other bodies. The discovery of exoplanets, beginning in the early 1990s and accelerating thereafter, has provided further information for the study of possible extraterrestrial life. These findings confirm that the Sun is not unique among stars in hosting planets and expands the habitability research horizon beyond the Solar System.
While Earth is the only place in the Universe known to harbor life, estimates of habitable zones around other stars, along with the discovery of thousands of exoplanets and new insights into the extreme habitats on Earth where organisms known as extremophiles live, suggest that there may be many more habitable places in the Universe than considered possible until very recently. On 4 November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way. 11 billion of these estimated planets may be orbiting Sun-like stars. The nearest such planet may be 12 light-years away, according to the scientists. As of June 2021, a total of 59 potentially habitable exoplanets have been found.

Stellar characteristics

An understanding of planetary habitability begins with the host star. The classical habitable zone is defined for surface conditions only; but a metabolism that does not depend on the stellar light can still exist outside the HZ, thriving in the interior of the planet where liquid water is available.
Under the auspices of SETI's Project Phoenix, scientists Margaret Turnbull and Jill Tarter developed the "HabCat" in 2002. The catalog was formed by winnowing the nearly 120,000 stars of the larger Hipparcos Catalogue into a core group of 17,000 potentially habitable stars, and the selection criteria that were used provide a good starting point for understanding which astrophysical factors are necessary for habitable planets. According to research published in August 2015, very large galaxies may be more favorable to the formation and development of habitable planets than smaller galaxies, like the Milky Way galaxy.
However, what makes a planet habitable is a much more complex question than having a planet located at the right distance from its host star so that water can be liquid on its surface: various geophysical and geodynamical aspects, the radiation, and the host star's plasma environment can influence the evolution of planets and life if it originated. Liquid water is a necessary but not sufficient condition for life as we know it, as habitability is a function of a multitude of environmental parameters.

Spectral class

The spectral class of a star indicates its photospheric temperature, which correlates to overall mass. The appropriate spectral range for habitable stars is considered to be "late F" or "G", to "mid-K". This corresponds to temperatures of a little more than 7,000 K down to a little less than 4,000 K ; the Sun, a G2 star at 5,777 K, is well within these bounds. This spectral range probably accounts for between 5% and 10% of stars in the local Milky Way galaxy. "Middle-class" stars of this sort have a number of characteristics considered important to planetary habitability:
  • They live at least a few hundred million years, allowing life a chance to evolve. More luminous main-sequence stars of the "O" classes and many members of the "B" classes usually live less than 500 million years and in exceptional cases less than 10 million.
  • They emit enough high-frequency ultraviolet radiation to trigger important atmospheric dynamics such as ozone formation, but not so much that ionisation destroys incipient life.
  • They emit sufficient radiation at wavelengths conducive to photosynthesis.
  • Liquid water may exist on the surface of planets orbiting them at a distance that does not induce tidal locking.
K-type stars may be able to support life far longer than the Sun.
Whether fainter late K and M class red dwarf stars are also suitable hosts for habitable planets is perhaps the most important open question in the entire field of planetary habitability given their prevalence. Gliese 581 c, a "super-Earth", has been found orbiting in the "habitable zone" of a red dwarf and may possess liquid water. However, it is also possible that a greenhouse effect may render it too hot to support life, while its neighbor, Gliese 581 d, may be a more likely candidate for habitability. In September 2010, the discovery was announced of another planet, Gliese 581 g, in an orbit between these two planets. However, reviews of the discovery have placed the existence of this planet in doubt, and it is listed as "unconfirmed". In September 2012, the discovery of two planets orbiting Gliese 163 was announced. One of the planets, Gliese 163 c, about 6.9 times the mass of Earth and somewhat hotter, was considered to be within the habitable zone.
A recent study suggests that cooler stars that emit more light in the infrared and near-infrared may actually host warmer planets with less ice and an incidence of snowball states. These wavelengths are absorbed by their planets' ice and greenhouse gases and remain warmer.
A 2020 study found that about half of Sun-like stars could host rocky, potentially habitable planets. Specifically, they estimated that, on average, the nearest habitable zone planet around G and K-type stars is about 6 parsecs away, and there are about 4 rocky planets around G and K-type stars within 10 parsecs of the Sun.

Stable habitable zone

The habitable zone is a shell-shaped region of space surrounding a star in which a planet could maintain liquid water on its surface. The concept was first proposed by astrophysicist Su-Shu Huang in 1959, based on climatic constraints imposed by the host star. After an energy source, liquid water is widely considered the most important ingredient for life, considering how integral it is to all life systems on Earth. However, if life is discovered in the absence of water, the definition of an HZ may have to be greatly expanded.
The inner edge of the HZ is the distance where runaway greenhouse effect vaporizes the whole water reservoir and, as a second effect, induces the photodissociation of water vapor and the loss of hydrogen to space. The outer edge of the HZ is the distance from the star where a maximum greenhouse effect fails to keep the surface of the planet above the freezing point, and by condensation.
A "stable" HZ implies two factors. First, the range of a HZ should not vary greatly over time. All stars increase in luminosity as they age, and a given HZ thus migrates outwards, but if this happens too quickly planets may only have a brief window inside the HZ and a correspondingly smaller chance of developing life. Calculating an HZ range and its long-term movement is never straightforward, as negative feedback loops such as the CNO cycle will tend to offset the increases in luminosity. Assumptions made about atmospheric conditions and geology thus have as great an impact on a putative HZ range as does stellar evolution: the proposed parameters of the Sun's HZ, for example, have fluctuated greatly.
Second, no large-mass body such as a gas giant should be present in or relatively close to the HZ, thus disrupting the formation of Earth-size bodies. The matter in the asteroid belt, for example, appears to have been unable to accrete into a planet due to orbital resonances with Jupiter; if the giant had appeared in the region that is now between the orbits of Venus and Mars, Earth would almost certainly not have developed in its present form. However a gas giant inside the HZ might have habitable moons under the right conditions.