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Have included a brief synthesis of problems and the conclusions of recent research into the problem. It is no longer ambiguous whether red dwarfs are likely to host life or not.
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Determining the '''habitability of red dwarf systems''' could help reveal the likelihood of [[extraterrestrial life]], as [[red dwarfs]] make up most stars in the [[Milky Way Galaxy]]. While the relatively little energy output, small [[habitable zone]]s, probability of [[tidally locked]] planets, and high [[stellar variation]] are postulated impediments to [[planetary habitability|habitability]], the [[Wiktionary:ubiquity#Etymology|ubiquity]] and longevity of red dwarfs are possible positive factors.
Determining the '''habitability of red dwarf systems''' could help reveal the likelihood of [[extraterrestrial life]], as [[red dwarfs]] make up most stars in the [[Milky Way Galaxy]]. While the relatively little energy output, small [[habitable zone]]s, probability of [[tidally locked]] planets, and high [[stellar variation]] are postulated impediments to [[planetary habitability|habitability]], the [[Wiktionary:ubiquity#Etymology|ubiquity]] and longevity of red dwarfs are possible positive factors.


As of 2012, many factors appear to indicate that red dwarfs are a very low probability for having indigenous life. Planets in the habitable zone of red dwarfs would experience such a strong tidal heating that the hydrogen necessary for water and all known life would be 'baked out' of the planets before a stable orbit could be achieved, creating so called 'Tidal Venuses'. Combined with the other problems created by tidal locking, the variable radiation of the star, lack of planetary axial tilts,<ref>http://io9.com/5876644/life-might-not-be-possible-around-red-dwarf-stars</ref> small habitable zones due to low energy output. etc., this would make the probability of red dwarf stars hosting life as we know it very low compared to other star types. <ref>http://www.universetoday.com/94353/tidal-venuses-may-have-been-wrung-out-to-dry/</ref> However, this does not mean that the planets around red dwarfs are 'uninhabitable' (could not be inhabited by future humans or other non-indigenous life).
As of 2012, many factors appear to indicate that red dwarfs have a very low probability for hosting indigenous life. Planets in the habitable zone of red dwarfs would experience such a strong tidal heating that the hydrogen necessary for water and all known life would be 'baked out' of the planets before a stable orbit could be achieved, creating so called 'Tidal Venuses'.<ref>http://www.universetoday.com/94353/tidal-venuses-may-have-been-wrung-out-to-dry/</ref> Combined with other problems, such as those created by tidal locking, the variable radiation of red dwarfs, lack of planetary axial tilts,<ref>http://io9.com/5876644/life-might-not-be-possible-around-red-dwarf-stars</ref> small habitable zones due to low energy output, etc., this would indicate that the probability of red dwarf stars hosting life as we know it is very low compared to other star types. <ref>http://www.universetoday.com/94353/tidal-venuses-may-have-been-wrung-out-to-dry/</ref> However, this does not mean that the planets around red dwarfs are 'uninhabitable' (could not be inhabited by future humans or other non-indigenous life).


Habitability around [[brown dwarf]]s, which are likely more numerous than red dwarfs, is considered unlikely, since what little heat they emit is quickly dispersed, and there is no steady source of heat being produced by means of [[nuclear fusion]].
Habitability around [[brown dwarf]]s, which are likely more numerous than red dwarfs, is considered unlikely, since what little heat they emit is quickly dispersed, and there is no steady source of heat being produced by means of [[nuclear fusion]].

Revision as of 05:25, 9 April 2012

An artist's impression of a planet in orbit around a red dwarf.
This artist's concept illustrates a young, red dwarf star surrounded by three planets.

Determining the habitability of red dwarf systems could help reveal the likelihood of extraterrestrial life, as red dwarfs make up most stars in the Milky Way Galaxy. While the relatively little energy output, small habitable zones, probability of tidally locked planets, and high stellar variation are postulated impediments to habitability, the ubiquity and longevity of red dwarfs are possible positive factors.

As of 2012, many factors appear to indicate that red dwarfs have a very low probability for hosting indigenous life. Planets in the habitable zone of red dwarfs would experience such a strong tidal heating that the hydrogen necessary for water and all known life would be 'baked out' of the planets before a stable orbit could be achieved, creating so called 'Tidal Venuses'.[1] Combined with other problems, such as those created by tidal locking, the variable radiation of red dwarfs, lack of planetary axial tilts,[2] small habitable zones due to low energy output, etc., this would indicate that the probability of red dwarf stars hosting life as we know it is very low compared to other star types. [3] However, this does not mean that the planets around red dwarfs are 'uninhabitable' (could not be inhabited by future humans or other non-indigenous life).

Habitability around brown dwarfs, which are likely more numerous than red dwarfs, is considered unlikely, since what little heat they emit is quickly dispersed, and there is no steady source of heat being produced by means of nuclear fusion.

Red dwarf characteristics

Red dwarfs[4] are the smallest, coolest, and most common type of star. Estimates of their abundance range from 70% to more than 90% of all stars in the galaxy,[5][6] an often quoted median figure being 73%.[7] Red dwarfs are either late K or M spectral type.[8] Given their low energy output, red dwarfs are never visible by the unaided eye from Earth; neither the closest red dwarf star to the Sun when viewed individually, Proxima Centauri (which is also the closest star to the Sun), nor the closest solitary red dwarf, Barnard's star, is anywhere near visual magnitude.

Research

Light emission and tidal lock

Relative star sizes and photospheric temperatures. Any planet around a red dwarf, such as the one shown here, would have to huddle close to achieve Earth-like temperatures, likely inducing tidal lock. See Aurelia.

For many years, astronomers 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 reactions proceed exceptionally slowly and they emit very little light (from 3% of that produced by the Sun to as little as 0.01%). Any planet in orbit around a red dwarf would have to orbit very close to its parent star to attain Earth-like surface temperatures; from 0.3 AU (just inside the orbit of Mercury) for a star like Lacaille 8760, to as little as 0.032 AU for a star like Proxima Centauri[9] (such a world would have a year lasting just 6.3 days).

Planets that are close enough to red dwarfs to receive a sufficient amount of radiation for liquid water are likely to have long been tidally locked to their respective stars so that the planet rotates only once for every time it completes an orbit; this means that one face always points at the star (creating perpetual day) and one face always points away (creating perpetual night). Potential life could be limited to a ring-like region, known as the terminator, where the sun would always appear on the horizon.

Should the planet have a moon massive enough to maintain an Earthlike atmosphere, however, the moon could be tidally locked with the more massive planet instead of with the star, and as such it could have a day-and-night cycle, increasing chances of habitability on the moon. Tidal forces between the two bodies would also keep the centers of both the planet and its moon liquid, which should result in a strong enough magnetic field to protect the planet and its moon from outbursts coming from the parent star.

The tidally locked planet would likely need an atmosphere thick enough to transfer some of the star's heat from the day side to the night side; this would prevent the colder night side's atmospheric temperature from dropping below condensation point, causing a drop in atmospheric pressure that would draw more of the atmosphere towards the night side until all of the atmosphere gets frozen on the night side. It was long assumed that an atmosphere would need to be so thick as to impede photosynthesis from any plants on the day side surface. However, more recent research has suggested otherwise. A 2010 study concluded that Earth-like aquaplanets tidally locked to their stars would still have temperatures above -33 Celsius on the night side.[10] Studies by Robert Haberle and Manoj Joshi of NASA's Ames Research Center in California have shown that a planet's atmosphere (assuming it included greenhouse gases CO2 and H2O) need only be 100 mb, or 10% of Earth's atmosphere, for the star's heat to be effectively carried to the night side.[11] This is well within the levels required for photosynthesis on the day side, though some of their models still had water frozen on the dark side. Martin Heath of Greenwich Community College, has shown that seawater, too, could effectively circulate without freezing solid if the ocean basins were deep enough to allow free flow beneath the night side's ice cap. Geothermal heat might also help keep the lower parts of any ocean liquid.[citation needed] Further research—including a consideration of the amount of photosynthetically active radiation—has suggested that tidally locked planets in red dwarf systems might at least be habitable for higher plants.[12]

Compared to Sun-like stars, red dwarfs produce more wavelengths of light that are absorbed by water ice, which would increase the temperature of icy bodies, thereby extending the previously thought HZ around Red dwarfs.[13]

However, a 2012 study at University of Washington revealed that planets in the habitable zone of red dwarfs would experience such a strong geological tidal heating that the hydrogen necessary for water and all known life would be 'baked out' of the planets before a stable orbit could be achieved, creating so called 'Tidal Venuses'. Combined with the other problems of tidal locking, variable radiation, axial tilts,[14] etc., this would make the probability of red dwarf stars hosting life as we know it very low compared to other star types. [15]

Photosynthesis

Size and brightness are not the only factors in making red dwarfs potentially unsuitable for life. If the planet is tidally locked, 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 infrared, and on the Earth the process depends on visible light. Photosynthesis on a red dwarf planet would require additional photons to achieve excitation potentials comparable to those needed in Earth photosynthesis for electron transfers, due to the lower average energy level of near-infrared photons compared to visible.[16] 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.[16] Because water is strongly absorbing red light, less energy is availabe for possible ocean life.

Weather conditions and habitability

Due to differential heating, a tidally locked planet would experience fierce winds blowing continually towards the night side with permanent torrential rain at the point directly opposite the local star, the solar pole. In the opinion of one author this makes complex life improbable.[17] But the scientists who worked on the Aurelia and Blue Moon series disagree.

Plant life would have to adapt to this constant gale, e.g. by anchoring securely into the soil and sprouting long flexible leaves that do not snap. Plants would probably be less productive in the dim red sunlight, so consequently there would be less oxygen in the atmosphere and animal life would be constrained in size. Animals would likely rely on infrared vision as signaling by calls or scents would be difficult over the din of the planet-wide gale. Underwater life would, however, be protected from fierce winds and flares, and vast blooms of black photosynthetic plankton and algae could support the sea life.[18]

There may not even be enough water for habitable planets around many red dwarf stars. [19]

Variability

Red dwarfs are far more variable and violent than their more stable, larger cousins. Often they are covered in starspots 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 albedo would be higher causing light from the red dwarf to be reflected. This could cause conditions similar to Snowball Earth so cold could last millions of years. [citation needed]

At other times, red dwarf stars emit gigantic flares that can double their brightness in a matter of minutes.[20] Indeed, as more and more red dwarfs have been scrutinized for variability, more of them have been classified as flare stars to some degree or other. Such variation in brightness could be very damaging for life. Flares might also produce torrents of charged particles that could strip off sizable portions of the planet's atmosphere. 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.[21]

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.[22]

Life could initially protect itself from radiation by remaining underwater until the star had passed through its early flare stage, assuming the planet could retain enough of an atmosphere to produce liquid oceans. The scientists who wrote Aurelia believed that life could survive on land despite a red dwarf star flaring. Once life 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 never need to produce oxygen.[16]

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 moment. Active red dwarfs that emit coronal mass ejections would bow back the magnetosphere until it contacted the planetary atmosphere. As a result, the atmosphere would undergo strong erosion, possibly leaving the planet uninhabitable.[23]

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.[24] 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 unknown, the total amount of habitable zone around all red dwarfs combined is equal to the total amount around sun-like stars given their ubiquity.[25] The first super-Earth with a mass of a 3 to 4 times that of the Earth's found in the potentially habitable zone of its star is Gliese 581 g, and its star, Gliese 581, is indeed a red dwarf. Although tidally locked, it is thought possible that at its terminator liquid water may well exist.[26] The planet is thought to have existed for approximately 7 billion years and has a large enough mass to support an atmosphere.

In fiction

In Olaf Stapledon's 1937 science fiction novel Star Maker, one of the many alien civilizations in our galaxy he describes is one in the terminator zone of a tidally locked planet of a red dwarf system. This planet is inhabited by intelligent plants that look like carrots with arms, legs, and a head that "sleep" part of the time by inserting themselves in soil on plots of land and absorbing sunlight by photosynthesis, and that are awake part of the time, emerging from their plots of soil as locomoting beings who participate in all the complex activities of a modern industrial civilization. Stapledon also describes how life evolved on this planet.[27]

In Larry Niven's "Draco Tavern" stories, the highly advanced Chirpsithra aliens evolved on a tide-locked Oxygen world around a Red Dwarf. However, no detail is given beyond that it was about 1 terrestrial mass, a little colder, and used Red Dwarf sunlight.

See also

Learning materials from Wikiversity:

References

  1. ^ http://www.universetoday.com/94353/tidal-venuses-may-have-been-wrung-out-to-dry/
  2. ^ http://io9.com/5876644/life-might-not-be-possible-around-red-dwarf-stars
  3. ^ http://www.universetoday.com/94353/tidal-venuses-may-have-been-wrung-out-to-dry/
  4. ^ The term dwarf applies to all stars in the main sequence, including the Sun.
  5. ^ van Dokkum, Pieter G. & Conroy, Charlie. "A substantial population of low-mess stars in luminous elliptical galaxies. Nature. 2010 468(7326):940
  6. ^ "Discovery Triples Number of Stars in Universe" Yale University. Published December 1, 2010. Retrieved December 17, 2010, from http://www.sciencedaily.com/releases/2010/12/101201134158.htm
  7. ^ Dole, Stephen H. Habitable Planets for Man 1965 Rand Corporation report--A figure of 73% is given for the percentage of red dwarfs in the Milky Way Galaxy.
  8. ^ the term is sometimes used as coterminus with M class. K class stars tend toward an orange color.
  9. ^ "Habitable zones of stars". NASA Specialized Center of Research and Training in Exobiology. University of Southern California, San Diego. Retrieved 2007-05-11.
  10. ^ Merlis, T. M. and T. Schneider Atmospheric dynamics of Earth-like tidally locked aquaplanets J. Adv. Model. Earth Syst. (2010)
  11. ^ Joshi, M. M. (1997). "Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability" (PDF). Icarus. 129 (2): 450–465. Bibcode:1997Icar..129..450J. doi:10.1006/icar.1997.5793. Retrieved 2007-08-11. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  12. ^ Heath, Martin J. (1999). "Habitability of Planets Around Red Dwarf Stars" (PDF). Origins of Life and Evolution of the Biosphere. 29 (4): 405–424. doi:10.1023/A:1006596718708. PMID 10472629. Retrieved 2007-08-11. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  13. ^ Suppression of the water ice and snow albedo feedback on planets orbiting red dwarf stars and the subsequent widening of the habitable zone, M. Joshi, R. Haberle, 2011
  14. ^ http://io9.com/5876644/life-might-not-be-possible-around-red-dwarf-stars
  15. ^ http://www.universetoday.com/94353/tidal-venuses-may-have-been-wrung-out-to-dry/
  16. ^ a b c Nancy Y. Kiang (April, 2008). "The color of plants on other worlds". Scientific American. Retrieved 2008-06-27. {{cite journal}}: Check date values in: |date= (help)
  17. ^ http://astroprofspage.com/archives/996
  18. ^ Lewis Dartnell (April, 2010). "Meet the Alien Neighbours: Red Dwarf World (p.45)". Focus. Retrieved 2010-03-29. {{cite journal}}: Check date values in: |date= (help) [dead link‍]
  19. ^ Jack J. Lissauer Planets formed in habitable zones of M dwarf stars probably are deficient in volatiles The Astrophysical Journal 660.2 (2007): 149–152
  20. ^ Croswell, Ken (27 January 2001). "Red, willing and able" (Full reprint). New Scientist. Retrieved 2007-08-05. {{cite web}}: External link in |format= (help)
  21. ^ http://www.sciam.com/article.cfm?id=red-star-rising
  22. ^ Cain, Fraser; and Gay, Pamela (2007). "AstronomyCast episode 40: American Astronomical Society Meeting, May 2007". Universe Today. Retrieved 2007-06-17.{{cite web}}: CS1 maint: multiple names: authors list (link)
  23. ^ Khodachenko, Maxim L. (2007). "Coronal Mass Ejection (CME) Activity of Low Mass M Stars as An Important Factor for The Habitability of Terrestrial Exoplanets. I. CME Impact on Expected Magnetospheres of Earth-Like Exoplanets in Close-In Habitable Zones". Astrobiology. 7 (1): 167–184. Bibcode:2007AsBio...7..167K. doi:10.1089/ast.2006.0127. PMID 17407406. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  24. ^ "'The end of the world' has already begun, UW scientists say" (Press release). Science Daily. January 30, 2003. Retrieved 2011-07-05.
  25. ^ "M Dwarfs: The Search for Life is On, Interview with Todd Henry". Astrobiology Magazine. August 29, 2005. Retrieved 2007-08-05..
  26. ^ Steven S. Vogt, R. Paul Butler, E. J. Rivera, N. Haghighipour, Gregory W. Henry, and Michael H. Williamson. The Lick-Carnegie Exoplanet Survey: A 3.1 M⊕ Planet in the Habitable Zone of the Nearby M3V Star Gliese 581. The Astrophysical Journal, 2010;
  27. ^ Stapledon, Olaf Star Maker 1937 Chapter 7 "More Worlds" Part 3 "Plant Men and Others"
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