The habitability of
natural satellites is a measure of their potential to sustain life in
favorable circumstances.
Habitable environments do not necessarily harbor life. Natural satellite habitability is a new area that is significant to
astrobiology
Astrobiology, and the related field of exobiology, is an interdisciplinary scientific field that studies the origins, early evolution, distribution, and future of life in the universe. Astrobiology is the multidisciplinary field that investig ...
for
various reasons, the most important of which being that
natural satellites are expected to
outnumber planets by a large margin, and it is projected that habitability parameters will be comparable to those of planets. There are, nevertheless, significant
environmental variables that affect
moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width ...
s as prospective
alien life
Extraterrestrial life, colloquially referred to as alien life, is life that may occur outside Earth and which did not originate on Earth. No extraterrestrial life has yet been conclusively detected, although efforts are underway. Such life might ...
locations.
The strongest candidates for natural satellite habitability are currently
icy satellites
such as those of
Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth t ...
and
Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
—
Europa and
Enceladus
Enceladus is the sixth-largest moon of Saturn (19th largest in the Solar System). It is about in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most refle ...
respectively, although if life exists in either place, it would probably be confined to subsurface habitats. Historically, life on Earth was thought to be strictly a surface phenomenon, but recent studies have shown that up to half of Earth's
biomass
Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...
could live below the surface. Europa and
Enceladus
Enceladus is the sixth-largest moon of Saturn (19th largest in the Solar System). It is about in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most refle ...
exist outside the
circumstellar habitable zone
In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.J. F. Ka ...
which has historically defined the limits of life within the
Solar System
The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
as the zone in
which water can exist as liquid at the surface. In the Solar System's habitable zone, there are only three natural satellites—the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width ...
, and Mars's moons
Phobos and
Deimos (although some estimates show Mars and its moons to be slightly outside the habitable zone) —none of which sustain an atmosphere or water in liquid form.
Tidal forces
The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...
are likely to play as significant a role providing heat as
stellar radiation in the ''potential habitability'' of natural satellites.
Exomoon
An exomoon or extrasolar moon is a natural satellite that orbits an exoplanet or other non-stellar extrasolar body.
Exomoons are difficult to detect and confirm using current techniques, and to date there have been no confirmed exomoon detecti ...
s are not yet confirmed to exist. Detecting them is extremely difficult, because current methods are limited to transit timing.
It is possible that some of their attributes could be determined by similar methods as those of transiting planets.
Despite this, some scientists estimate that there are as many habitable exomoons as habitable exoplanets.
Given the general planet-to-satellite(s) mass ratio of 10,000,
large Saturn or
Jupiter sized gas planet
A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas gia ...
s in the
habitable zone
In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.J. F. Kas ...
are thought to be the best candidates to harbour
Earth-like moons.
Presumed conditions
The conditions of habitability for natural satellites are similar to those of
planetary habitability
Planetary habitability is the measure of a planet's or a natural satellite's potential to develop and maintain environments hospitable to life. Life may be generated directly on a planet or satellite endogenously or be transferred to it from a ...
. However, there are several factors which differentiate natural satellite habitability and additionally extend their habitability outside the planetary habitable zone.
Liquid water
Liquid water is thought by most astrobiologists to be an essential prerequisite for extraterrestrial life. There is growing evidence of subsurface liquid water on several moons in the Solar System orbiting the
gas giant
A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...
s
Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth t ...
,
Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
,
Uranus
Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares ( Mars), grandfather of Zeus ( Jupiter) and father of ...
, and
Neptune
Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 time ...
. However, none of these subsurface bodies of water has been confirmed to date.
Orbital stability
For a stable orbit the ratio between the moon's
orbital period
The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets, ...
''P''
s around its primary and that of the primary around its star ''P''
p must be < , e.g. if a planet takes 90 days to orbit its star, the maximum stable orbit for a moon of that planet is less than 10 days.
Simulations suggest that a moon with an orbital period less than about 45 to 60 days will remain safely bound to a massive giant planet or
brown dwarf
Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most m ...
that orbits 1
AU from a Sun-like star.
Atmosphere
An atmosphere is considered by astrobiologists to be important in developing
prebiotic chemistry, sustaining life and for surface water to exist. Most natural satellites in the Solar System lack significant atmospheres, the sole exception being Saturn's moon
Titan.
Sputtering
In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas. It occurs naturally in outer space, and ...
, a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles, presents a significant problem for natural satellites. All the gas giants in the Solar System, and likely those orbiting other stars, have
magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dyna ...
s with radiation belts potent enough to completely erode an atmosphere of an Earth-like moon in just a few hundred million years. Strong stellar winds can also strip gas atoms from the top of an atmosphere causing them to be lost to space.
To support an Earth-like atmosphere for about 4.6 billion years (Earth's current age), a moon with a Mars-like density is estimated to need at least 7% of Earth's mass. One way to decrease loss from sputtering is for the moon to have a strong
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
of its own that can deflect
stellar wind
A stellar wind is a flow of gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.
...
and radiation belts. NASA's
Galileo's measurements suggest that large moons can have magnetic fields; it found
Ganymede has its own magnetosphere, even though its mass is only 2.5% of Earth's.
Alternatively, the moon's atmosphere may be constantly replenished by gases from subsurface sources, as thought by some scientists to be the case with Titan.
Tidal effects
While the effects of
tidal acceleration
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (e.g. the Moon) and the primary planet that it orbits (e.g. Earth). The acceleration causes a gradual recession of a satellite in a prograde orbit away from ...
are relatively modest on planets, it can be a significant source of energy for natural satellites and an alternative energy source for sustaining life.
Moons orbiting
gas giant
A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...
s or
brown dwarf
Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most m ...
s are likely to be
tidally locked
Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked ...
to their primary: that is, their days are as long as their orbits. While tidal locking may adversely affect planets within habitable zones by interfering with the distribution of stellar radiation, it may work in favour of satellite habitability by allowing
tidal heating. Scientists at the NASA
Ames Research Center modelled the temperature on tide-locked exoplanets in the
habitability zone of
red dwarf stars. They found that an atmosphere with a carbon dioxide () pressure of only not only allows habitable temperatures, but allows liquid water on the dark side of the satellite. The temperature range of a moon that is tidally locked to a gas giant could be less extreme than with a planet locked to a star. Even though no studies have been done on the subject, modest amounts of are speculated to make the temperature habitable.
Tidal effects could also allow a moon to sustain
plate tectonics
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
, which would cause volcanic activity to regulate the moon's temperature
and create a
geodynamo effect which would give the satellite a strong
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
.
Axial tilt and climate
Provided gravitational interaction of a moon with other satellites can be neglected, moons tend to be tidally locked with their planets. In addition to the rotational locking mentioned above, there will also be a process termed 'tilt erosion', which has originally been coined for the tidal erosion of planetary obliquity against a planet's orbit around its host star.
The final spin state of a moon then consists of a rotational period equal to its orbital period around the planet and a rotational axis that is perpendicular to the orbital plane.
If the moon's mass is not too low compared to the planet, it may in turn stabilize the planet's
axial tilt
In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orb ...
, i.e. its obliquity against the orbit around the star. On Earth, the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width ...
has played an important role in stabilizing the axial tilt of the Earth, thereby reducing the impact of gravitational perturbations from the other planets and ensuring only moderate climate variations throughout the planet.
On
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin atm ...
, however, a planet without significant tidal effects from its relatively low-mass moons
Phobos and
Deimos, axial tilt can undergo extreme changes from 13° to 40° on timescales of to .
Being tidally locked to a giant planet or
sub-brown dwarf
A sub-brown dwarf or planetary-mass brown dwarf is an astronomical object that formed in the same manner as stars and brown dwarfs (i.e. through the collapse of a gas cloud) but that has a planetary mass, therefore by definition below the lim ...
would allow for more moderate climates on a moon than there would be if the moon were a similar-sized planet orbiting in locked rotation in the habitable zone of the star.
This is especially true of
red dwarf systems, where comparatively high gravitational forces and low luminosities leave the habitable zone in an area where tidal locking would occur. If tidally locked, one rotation about the axis may take a long time relative to a planet (for example, ignoring the slight axial tilt of Earth's moon and topographical shadowing, any given point on it has two weeks – in Earth time – of sunshine and two weeks of night in its lunar day) but these long periods of light and darkness are not as challenging for habitability as the eternal days and eternal nights on a planet tidally locked to its star.
In the Solar System
The following is a list of natural satellites and environments in the Solar System with a possibility of hosting habitable environments:
Extrasolar
A total of 9 exomoon candidates have been detected, but none of them have been confirmed.
Given the general planet-to-satellite(s) mass ratio of 10,000, Large Saturn or Jupiter sized gas planets in the habitable zone are believed to be the best candidates to harbour Earth-like moons with more than 120 such planets by 2018.
Massive exoplanets known to be located within a habitable zone (such as
Gliese 876 b,
55 Cancri f,
Upsilon Andromedae d,
47 Ursae Majoris b,
HD 28185 b and
HD 37124 c) are of particular interest as they may potentially possess natural satellites with liquid water on the surface.
Habitability of extrasolar moons will depend on stellar and planetary illumination on moons as well as the effect of eclipses on their orbit-averaged surface illumination.
Beyond that, tidal heating might play a role for a moon's habitability. In 2012, scientists introduced a concept to define the habitable orbits of moons;
they define an inner border of an habitable moon around a certain planet and call it the circumplanetary "habitable edge". Moons closer to their planet than the habitable edge are uninhabitable. When effects of eclipses as well as constraints from a satellite's orbital stability are used to model the runaway greenhouse limit of hypothetical moons, it is estimated that — depending on a moon's orbital eccentricity — there is a minimum mass of roughly 0.20 solar masses for stars to host habitable moons within the stellar habitable zone.
The magnetic environment of exomoons, which is critically triggered by the intrinsic magnetic field of the host planet, has been identified as another factor of exomoon habitability.
Most notably, it was found that moons at distances between about 5 and 20 planetary radii from a giant planet could be habitable from an illumination and tidal heating point of view,
but still the planetary magnetosphere would critically influence their habitability.
In popular culture
Natural satellites that host life are common in science fiction. Notable examples in film include: Earth's moon in ''
A Trip to the Moon
''A Trip to the Moon'' (french: Le Voyage dans la Lune) is a 1902 French adventure short film directed by Georges Méliès. Inspired by a wide variety of sources, including Jules Verne's 1865 novel ''From the Earth to the Moon'' and its 1870 ...
'' (1903);
Yavin 4 from ''
Star Wars'' (1977);
Endor Endor or Ein Dor may refer to:
Places
* Endor (village), from the Hebrew Bible, a Canaanite village where the Witch of Endor lived
* Indur, a Palestinian village depopulated during the 1948 Arab-Israeli war
* Ein Dor, a Kibbutz in modern Israel
...
in ''
Return of the Jedi
''Return of the Jedi'' (also known as ''Star Wars: Episode VI – Return of the Jedi'' is a 1983 American Epic film, epic space opera film directed by Richard Marquand. The screenplay is by Lawrence Kasdan and George Lucas from a story by Luc ...
'' (1983); LV-426 in ''
Alien'' (1979) and ''
Aliens'' (1986); Pandora in ''
Avatar
Avatar (, ; ), is a concept within Hinduism that in Sanskrit literally means "descent". It signifies the material appearance or incarnation of a powerful deity, goddess or spirit on Earth. The relative verb to "alight, to make one's appearanc ...
'' (2009);
LV-223 in ''
Prometheus'' (2012);
Europa in ''
Europa Report'' (2013) and
Watchmen (TV series)
''Watchmen'' is an American superhero drama limited series based on the 1986 DC Comics series of the same title created by Alan Moore and Dave Gibbons. The TV series was created for HBO by Damon Lindelof, who also served as an executive ...
(2019); and, K23 in
The Midnight Sky
''The Midnight Sky'' is a 2020 American science fiction film directed by George Clooney based on the 2016 novel ''Good Morning, Midnight'' by Lily Brooks-Dalton. The script was written by Mark L. Smith. Clooney plays a leading role in his fi ...
(2020).
In the video game
Kerbal Space Program
''Kerbal Space Program'' (''KSP'') is a space flight simulation video game developed by Mexican developer Squad for Microsoft Windows, macOS, Linux, PlayStation 4, and Xbox One. In the game, players direct a nascent space program, staffed and ...
and its upcoming
sequel
A sequel is a work of literature, film, theatre, television, music or video game that continues the story of, or expands upon, some earlier work. In the common context of a narrative work of fiction, a sequel portrays events set in the sa ...
, there is a habitable satellite named Laythe.
See also
*
Earth analog
An Earth analog, also called an Earth analogue, Earth twin, or second Earth, is a planet or moon with environmental conditions similar to those found on Earth. The term Earth-like planet is also used, but this term may refer to any terrestrial p ...
*
Kepler-1625b I
Kepler 1625b I, a possible moon of exoplanet Kepler-1625b, may be the first exomoon ever discovered (pending confirmation), and was first indicated after preliminary observations by the Kepler Space Telescope. A more thorough observing campaign ...
, possible
exomoon
An exomoon or extrasolar moon is a natural satellite that orbits an exoplanet or other non-stellar extrasolar body.
Exomoons are difficult to detect and confirm using current techniques, and to date there have been no confirmed exomoon detecti ...
of
exoplanet
An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
Kepler-1625b
*
Habitability of neutron star systems
References
{{Portal bar, Astronomy, Stars, Spaceflight, Outer space, Solar System
Natural satellites
Moons