Determining the
habitability of red dwarf systems could help reveal how likely
extraterrestrial lifeExtraterrestrial life is defined as life which does not originate from planet Earth. The existence of life outside the planet is theoretical and all assertions of such life remain disputed....
is to exist, as red dwarfs make up a majority of all the stars in the galaxy. Critical factors assumed to be impediments to
habitabilityPlanetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...
include relatively little energy output and thus reduced
habitable zoneThe habitable zone in astronomy is a region of space where an Earth-like planet can maintain liquid water on its surface and Earth-like life. The habitable zone is the intersection of two regions that must both be favorable to life; one within a planetary system, and the other within a galaxy...
s, the probability of tidally locked planets, and high stellar variation. Positive factors include the ubiquity and longevity of red dwarfs.
Brown dwarfBrown dwarfs are sub-stellar objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores, as do stars on the main sequence, but which have fully convective surfaces and interiors, with no chemical differentiation by depth...
s are probably more numerous than red dwarfs.
Determining the
habitability of red dwarf systems could help reveal how likely
extraterrestrial lifeExtraterrestrial life is defined as life which does not originate from planet Earth. The existence of life outside the planet is theoretical and all assertions of such life remain disputed....
is to exist, as red dwarfs make up a majority of all the stars in the galaxy. Critical factors assumed to be impediments to
habitabilityPlanetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...
include relatively little energy output and thus reduced
habitable zoneThe habitable zone in astronomy is a region of space where an Earth-like planet can maintain liquid water on its surface and Earth-like life. The habitable zone is the intersection of two regions that must both be favorable to life; one within a planetary system, and the other within a galaxy...
s, the probability of tidally locked planets, and high stellar variation. Positive factors include the ubiquity and longevity of red dwarfs.
Brown dwarfBrown dwarfs are sub-stellar objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores, as do stars on the main sequence, but which have fully convective surfaces and interiors, with no chemical differentiation by depth...
s are probably more numerous than red dwarfs. However, they are not generally classified as stars, and the possibility of life as we know it evolving are considered remote, since what little heat they emit quickly disappears.
Red dwarf characteristics
Red dwarfs are the smallest and coolest type of star and by far the most common. Estimates of their abundance range from 70% to more than 90% of all stars. (The term "dwarf" technically applies to all stars on the
main sequenceThe main sequence is a continuous and distinctive band of stars that appear on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung-Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
, including the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
.) Red dwarfs are either late K or M spectral type (the term is sometimes used as coterminus with M class—K class stars tend toward an orange colour). Given their small size, red dwarfs are never visible by the unaided eye from Earth; both the closest red dwarf star to the sun when viewed individually,
Proxima CentauriProxima Centauri is a red dwarf star approximately 4.2 light-years distant in the constellation of Centaurus. It was discovered in 1915 by Robert Innes, the Director of the Union Observatory in South Africa...
, and the closest solitary red dwarf,
Barnard's starBarnard's Star, also known occasionally as Barnard's "Runaway" Star, is a very low-mass red dwarf star approximately 6 light-years away from Earth in the constellation of Ophiuchus . In 1916, American astronomer E. E...
, are nowhere near visual magnitude.
Light emission and tidal lock
Astronomers for many years ruled out red dwarf systems as potential abodes for life. Their small size (from 0.1 to 0.6 solar masses) means that their
nuclear reactionIn nuclear physics and nuclear chemistry, a nuclear reaction is the process in which two nuclei or nuclear particles collide to produce products different from the initial particles...
s proceed exceptionally slowly, and they emit very little light (from 3% of that produced by the Sun to as little as 0.01%). Whether or not any planet is habitable may depend partly on whether the planet's atmosphere causes a
Greenhouse effectThe greenhouse effect is the heating of the surface of a planet or moon due to the presence of an atmosphere containing gases that absorb and emit infrared radiation. Thus, greenhouse gases trap heat within the surface-troposphere system...
and how strong any greenhouse effect is. Any planet in orbit around a red dwarf would have to huddle very close to its parent star to attain Earth-like surface temperatures; from 0.3 AU (just inside the orbit of
MercuryFor the liquid metallic element, see Mercury .Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three...
) for a star like
Lacaille 8760Lacaille 8760 is a red dwarf star in the constellation Microscopium, the microscope. Although it is slightly too faint to be seen without a telescope, this star is one of the nearest to our Sun at about 12.9 light years distance. This star was originally listed in a 1763 catalog that was published...
, to as little as 0.032
AUAn astronomical unit is a unit of length roughly equal to the mean distance between the Earth and the Sun. It is approximately 150 million kilometres ....
for a star like
Proxima CentauriProxima Centauri is a red dwarf star approximately 4.2 light-years distant in the constellation of Centaurus. It was discovered in 1915 by Robert Innes, the Director of the Union Observatory in South Africa...
(such a world would have a year lasting just 6.3 days). At those distances, the star's gravity would likely cause tidal lock. The daylight side of the planet would eternally face the star, while the night-time side would always face away from it. Because of that, potential life could be limited to a ring-like region between a hot area and a deep freeze. However the notion that all planets in the zone were water can exist would be tidal locked is disputed. The reason for this being is that every star is different.
The planet would need an atmosphere thick enough to transfer the star's heat from the day side to the night side. It was long assumed that such a thick atmosphere would prevent sunlight from reaching the surface in the first place, preventing
photosynthesisPhotosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of Bacteria, but not in Archaea...
, but now this pessimism has been tempered by research. Studies by Robert Haberle and Manoj Joshi of
NASAThe National Aeronautics and Space Administration is an agency of the United States government, responsible for the nation's public space program. NASA was established by the National Aeronautics and Space Act on July 29, 1958, replacing its predecessor, the National Advisory Committee for...
's Ames Research Center in California have shown that a planet's atmosphere (assuming it included greenhouse gases
CO2Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
and H
2O) need only be 100 mbs, or 10% of Earth's atmosphere, for the star's heat to be effectively carried to the night side. This is well within the levels required for photosynthesis, though water would still remain frozen on the dark side in some of their models. Martin Heath of
Greenwich Community CollegeGreenwich Community College is a post-secondary educational institution located in Greenwich, South East of Greater London, England.The college has an extremely strong cricket team; the players include a several of the college's present staff and also a number of former employees.Greenwich...
, has shown that seawater, too, could be effectively circulated without freezing solid if the ocean basins were deep enough to allow free flow beneath the night side's ice cap. Geothermal heat might help keep the lower parts of any ocean liquid. Further research—including a consideration of the amount photosynthetically active radiation—suggested that tidally locked planets in red dwarf systems might
at least be habitable for higher plants.
A moon of an tidally locked gas planet may have a day and night cycle of a few hours or days.
Photosynthesis
Size is not the only factor in making red dwarfs potentially unsuitable for life, however. On a red dwarf planet, photosynthesis on the night side would be impossible, since it would never see the sun. On the day side, because the sun does not rise or set, areas in the shadows of mountains would remain so forever. Photosynthesis as we understand it would be complicated by the fact that a red dwarf produces most of its radiation in the
infraredInfrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light , but shorter than that of terahertz radiation and microwaves...
, and on the Earth the process depends on visible light. Photosynthesis on a red dwarf planet would require four photons to split the water molecule for every two used on Earth, due to their lower average energy level. Having to adapt to a far wider spectrum to gain the maximum amount of energy, foliage on a habitable red dwarf planet would probably appear black if viewed in visible light.
Variability
Red dwarfs are far more variable and violent than their more stable, larger cousins. Often they are covered in
starspotStarspots are equivalent to sunspots but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness...
s that can dim their emitted light by up to 40% for months at a time. On earth life has adapted in many ways to the similarly reduced temperatures of the winter. Life may survive by hibernating and/or by diving into deep water where temperatures could be more constant. More serious is that the oceans could perhaps freeze over during cold periods. After the cold has ended the planet’s
albedoThe albedo of an object is the extent to which it diffusely reflects light from light sources such as the Sun. It is therefore a more specific form of the term reflectivity. Albedo is defined as the ratio of diffusely reflected to incident electromagnetic radiation. It is a unitless measure...
would be higher causing light from the red dwarf to be reflected. This could cause conditions similar to
Snowball EarthSnowball Earth refers to hypotheses regarding paleoclimatic global-scale glaciation, claiming that the Earth's surface was nearly or entirely frozen at some points in its past. The occurrence of Snowball Earths remains controversial...
so cold could last millions of years. At other times red dwarf stars emit gigantic flares that can double their brightness in a matter of minutes. Such variation could be very damaging for life. Flares might also blow off sizable portions of the planet's atmosphere.
- "No one found any showstoppers to habitability," says Gibor Basri of the University of California
The University of California is a public university system in the state of California. Under the California Master Plan for Higher Education, the University of California is a part of the state's three-tier public higher education system, which also includes the California State University system...
, Berkeley. One concern was that because M dwarfs frequently produce flares, the resulting torrents of charged particles could strip the atmosphere off any nearby planet. If the planet had a magnetic field, though, it would deflect the particles from the atmosphere. And even the slow rotation of a tidally locked M-dwarf planet—it spins once for every time it orbits its star—would be enough to generate a magnetic field as long as part of the planet's interior remained molten. http://www.sciam.com/article.cfm?id=red-star-rising
However, the violent flaring period of a red dwarf's lifecyle is estimated to only last roughly the first 1.2 billion years of its existence. If a planet forms far away from a red dwarf so as to avoid tidelock, and then migrates into the star's habitable zone after this turbulent initial period, it is possible that life may have a chance to develop.
Life could initially protect itself from radiation by remaining underwater until the star had passed through its early flare stage. Once it reached onto land, the low amount of UV produced by a quiescent red dwarf means that life could thrive without an ozone layer, and thus need never produce oxygen.
Other scientists disagree that red dwarf stars could sustain life. See Rare Earth hypothesis. Tidal-locking would likely result in a relatively low planetary
magnetic momentThe magnetic moment of a system is a measure of the strength and the direction of its magnetism. More technically , the term magnetic moment of a system usually refers to its magnetic dipole moment, and quantifies the contribution...
. Active red dwarfs that emit
coronal mass ejectionA coronal mass ejection is an ejection of material from the solar corona, usually observed with a white-light coronagraph.The ejected material is a plasma consisting primarily of electrons and protons , plus the entraining coronal magnetic field.-Impact of a CME:When the ejection reaches the...
s would bow back the
magnetosphereA magnetosphere is a highly magnetized region around and possessed by an astronomical object. Earth is surrounded by a magnetosphere, as are the magnetized planets Mercury, Jupiter, Saturn, Uranus, and Neptune. Jupiter's moon Ganymede is magnetized, but too weak to trap solar wind plasma. Mars has...
until it contacted the planetary atmosphere. As a result, the atmosphere would undergo strong erosion, possibly leaving the planet uninhabitable.
Abundance
There is, however, one major advantage that red dwarfs have over other stars as abodes for life: they live a long time. It took 4.5 billion years before humanity appeared on Earth, and life as we know it will see suitable conditions for as little as half a billion years more. Red dwarfs, by contrast, could live for trillions of years, because their nuclear reactions are far slower than those of larger stars, meaning that life both would have longer to evolve and longer to survive. Further, while the odds of finding a planet in the habitable zone around any specific red dwarf are slim, the total amount of habitable zone around all red dwarfs combined is equal to the total amount around sun-like stars given their ubiquity. The first possibly rocky extrasolar planet with the mass of a
Super-EarthA super-Earth is an extrasolar planet more massive than the Earth, but theoretically less massive than a gas giant. The term super-Earth refers only to the mass of the planet and does not imply anything about the surface conditions or habitability: in particular it does not imply that the planet...
that was found at the "warm" edge of the
habitable zoneThe habitable zone in astronomy is a region of space where an Earth-like planet can maintain liquid water on its surface and Earth-like life. The habitable zone is the intersection of two regions that must both be favorable to life; one within a planetary system, and the other within a galaxy...
of its star is
Gliese 581 cGliese 581 c or Gl 581 c is an extrasolar planet orbiting the red dwarf star Gliese 581. With a mass at least 5.36 times that of the Earth, it is classified as a super-Earth, a category which incorporates planets exceeding the mass of Earth but smaller than 10 Earth masses...
, and its star,
Gliese 581Gliese 581 is a red dwarf star with spectral type M3V, located 20.3 light years away from Earth. Its mass is estimated to be approximately a third of that of the Sun, and it is the 87th closest known star system to the Sun...
, is indeed a red dwarf.