Wikipedia

Gliese 581c

Gliese 581c/ˈɡliːzə/(Gl 581corGJ 581c) is anexoplanetorbiting within theGliese 581system. It is the second planet discovered in the system and the third in order from thestar.With a mass about 6.8 times that of theEarth,it is classified as asuper-Earth(a category of planets with masses greater than Earth's up to ten Earth masses).

Gliese 581c
Size comparison of Gliese 581c with Earth and Neptune
(based on selected hypotheticalmodeled compositions)[a]
Discovery[1]
Discovered byStéphane Udryet al.
Discovery siteLa Silla Observatory
Discovery date
  • 4 April 2007
  • 24 April 2007(announced)
Radial velocity
Orbital characteristics[2]
0.0718+0.0008
−0.0009AU
Eccentricity0.032+0.027
−0.021
12.9211+0.0008
−0.0007d
Inclination47°+15°
−13°
2454759.2 ± 0.1[3]
16°+61°
−89°
Semi-amplitude3.1±0.1m/s
StarGliese 581
Physical characteristics[2]
Mass6.81+0.21
−1.16M🜨

At the time of its discovery in 2007, Gliese 581c gained interest from astronomers because it was reported to be the first potentially Earth-like planet in thehabitable zoneof its star, with a temperature right for liquid water on its surface, and, by extension, potentially capable of supportingextremophileforms of Earth-like life. However, further research cast doubt upon the planet's habitability. Based on newer models of the habitable zone, the planet is likely too hot to be potentially habitable.[4][3]

In astronomical terms, the Gliese 581 system is relatively close to Earth, at 20.55light-years(194 trillion kilometres; 121 trillion miles) in the direction of theconstellationofLibra.This distance, along with thedeclinationandright ascensioncoordinates, give its exact location in theMilky Way.

Discovery

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The team released a paper of their findings dated 27 April 2007, published in the July 2007 journalAstronomy & Astrophysics.[1]At the time of discovery, it was reported to be the first potentially Earth-like planet in thehabitable zoneof its star[5][6]and the smallest-knownexoplanetaround amain-sequencestar, but on 21 April 2009, another planet orbiting Gliese 581,Gliese 581e,with an approximate mass of 1.9Earth masses,was announced. In the paper, they also announced the discovery of another planet in the system,Gliese 581d,with a minimum mass of 7.7 Earth masses and asemi-major axisof 0.25astronomical units.[7][3]

Physical characteristics

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Mass

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The existence of Gliese 581c and its mass have been measured by theradial velocity methodof detectingexoplanets.The mass of a planet is calculated by the small periodic movements around a common centre of mass between the host star Gliese 581 and its planets. When all planets are fitted with aKepleriansolution, theminimum massof the planet is determined to be 5.5 Earth masses.[3]The radial velocity method cannot by itself determine the true mass, but it cannot be very much larger than this or the system would be dynamically unstable.[1]Dynamical simulations of the Gliese 581 system which assume the orbits of the planets arecoplanarindicate that the planets cannot exceed approximately 1.6 to 2 times their minimum masses or the planetary system would be unstable (this is primarily due to the interaction between planets e and b). For Gliese 581c, the upper bound is 10.4 Earth masses.[7]

A 2024 study determined the inclination of the planet, allowing its true mass to be determined, which is about 30% greater than the minimum mass at about 6.8Earth masses.[2]

Radius

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Since Gliese 581c has not been detected in transit, there are no measurements of its radius. Furthermore, theradial velocity methodused to detect it only puts a lower limit on the planet's mass, which means theoretical models of planetary radius and structure can only be of limited use. However, assuming a random orientation of the planet's orbit, the true mass is likely to be close to the measured minimum mass.

Assuming that the true mass is the minimum mass, the radius may be calculated using various models. For example, if Gliese 581c is arockyplanet with a large iron core, it should have a radius approximately 50% larger than that of Earth, according to Udry's team.[1][8]Gravityon such a planet's surface would be approximately 2.24 times as strong as on Earth. However, if Gliese 581c is anicyand/orwateryplanet, its radius would be less than 2 times that of Earth, even with a very large outerhydrosphere,according to density models compiled by Diana Valencia and her team forGliese 876 d.[9]Gravity on the surface of such an icy and/or watery planet would be at least 1.25 times as strong as on Earth. They claim the real value of the radius may be anything between the two extremes calculated by density models outlined above.[10]

Other scientists' views differ.Sara Seagerat MIT has speculated that Gliese 581c and other five-Earth-mass planets could be:[11]

If the planet transits the star as seen from the direction of the Earth, the radius should be measurable, albeit with some uncertainty. Unfortunately, measurements made with the Canadian-builtMOSTspace telescope indicate that transits do not occur.[12]

The new research suggests that the rocky centres of super-Earths are unlikely to evolve into terrestrial rocky planets like the inner planets of the Solar System because they appear to hold onto their large atmospheres. Rather than evolving to a planet composed mainly of rock with a thin atmosphere, the small rocky core remains engulfed by its large hydrogen-rich envelope.[13][14]

Orbit

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The orbits of theGliese 581system, as per the 2009 four-planet model. In the picture, Gliese 581c is the third planet from the star.

Gliese 581c has anorbital period( "year" ) of 13 Earth days[15]and its orbital radius is only about 7% that of the Earth, about 11 million km,[16]while the Earth is 150 million km from theSun.[17]Since the host star is smaller and colder than the Sun—and thus less luminous—this distance places the planet on the "warm" edge of the habitable zone around the star according to Udry's team.[1][8]Note that in astrophysics, the "habitable zone" is defined as the range of distances from the star at which a planet could support liquid water on its surface: it should not be taken to mean that the planet's environment would be suitable for humans, a situation which requires a more restrictive range of parameters. In any case, based on newer models of the habitable zone, the planet is likely too hot to be potentially habitable.[4][3]

A typical radius for an M0 star of Gliese 581's age andmetallicityis 0.00128 AU,[18]against the Sun's 0.00465 AU. This proximity means that the primary star should appear 3.75 times wider and 14 times larger in area for an observer on the planet's surface looking at the sky than the Sun appears to be from Earth's surface.

Tidal lock

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Because of its small separation from Gliese 581, the planet has been generally considered to always have onehemispherefacing the star (only day), and the other always facing away (only night), or in other words beingtidally locked.[19][20]The most recent orbital fit to the system, taking stellar activity into account indicates a nearly circular orbit,[2]but older fits used an eccentricity between 0.10 and 0.22. If the orbit of the planet were eccentric, it would undergo violent tidal flexing.[21]Because tidal forces are stronger when the planet is close to the star, eccentric planets are expected to have a rotation period that is shorter than its orbital period, also called pseudo-synchronization.[22]An example of this effect is seen inMercury,which is tidally locked in a 3:2 resonance, completing three rotations every two orbits. In any case, even in the case of 1:1 tidal lock, the planet would undergolibrationand the terminator would be alternatively lit and darkened during libration.[23]

Models of the evolution of the planet's orbit over time suggest thatheatingresulting from this tidal locking may play a major role in the planet's geology. Models proposed by scientists predict that tidal heating could yield a surface heat flux about three times greater than that ofJupiter's moonIo,which could result in major geological activity such as volcanoes and plate tectonics.[24]

Habitability and climate

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The study of Gliese 581c by the von Bloh et al. team is quoted as concluding "The super-Earth Gl 581c is clearly outside the habitable zone, since it is too close to the star."[4]The study by Selsis et al. states that "a planet in the habitable zone is not necessarily habitable" itself, and this planet "is outside what can be considered the conservative habitable zone" of the parent star, and further that if there was any water there then it was lost when the red dwarf was a strong X-ray and EUV emitter, it could have surface temperatures ranging from 700 to 1,000K(427 to 727°C), likeVenustoday.[25]Temperature speculations by other scientists were based on the temperature of (and heat from) the parent starGliese 581and have been calculated without factoring in the margin of error (96 °C/K) for the star's temperature of 3,432 K to 3,528 K, which leads to a largeirradiancerange for the planet, even before eccentricity is considered.[26]

Effective temperatures

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Using the measured stellar luminosity of Gliese 581 of 0.013 times that of the Sun, it is possible to calculate Gliese 581c'seffective temperature,a.k.a.black bodytemperature, which probably differs from itssurface temperature.[citation needed]According to Udry's team, the effective temperature for Gliese 581c, assuming analbedo(reflectivity) such as that of Venus (0.64), would be −3 °C (27°F), and assuming an Earth-like albedo (0.296), it would be 40 °C (104 °F),[1][15]a range of temperatures that overlap with therange at which water would be liquid at a pressure of 1 atmosphere.However, the effective temperature and actual surface temperature can be very different due to the greenhouse properties of the planetary atmosphere. For example, Venus has an effective temperature of −41 °C (−42 °F), but a surface temperature of 464 °C (867 °F) (mainly due to a 96.5%carbon dioxide atmosphere), a difference of about 505 °C (910 °F).[27]

Studies of habitability (i.e. liquid water forextremophileforms of life)[28]conclude that Gliese 581c is likely to suffer from arunaway greenhouse effectsimilar to that found onVenusand, as such, is highly unlikely to be habitable. Nevertheless, this runaway greenhouse effect could be prevented by the presence of sufficient reflective cloud cover on the planet's day side.[29]Alternatively, if the surface were covered in ice, it would have a highalbedo(reflectivity), and thus could reflect enough of the incident sunlight back into space to render the planet too cold for habitability, although this situation is expected to be very unstable except for very high albedos greater than about 0.95 (i.e. ice): release ofcarbon dioxideby volcanic activity or of water vapor due to heating at the substellar point would trigger a runaway greenhouse effect.[30]

Liquid water

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Gliese 581c is likely to lie outside thehabitable zone.[4][31]No direct evidence has been found forwaterto be present, and it is probably not present in the liquid state. Techniques like the one used to measure the extrasolar planetHD 209458 bmay in the future be used to determine the presence of water in the form of vapor in the planet'satmosphere,but only in the rare case of a planet with an orbit aligned so as totransitits star, which Gliese 581c is not known to do.[12]

Tidally locked models

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Theoretical models predict that volatilecompoundssuch aswaterandcarbon dioxide,if present, might evaporate in the scorching heat of the sunward side, migrate to the cooler night side, and condense to formice caps.Over time, the entire atmosphere might freeze into ice caps on the night side of the planet. However, it remains unknown if water and/or carbon dioxide are even present on the surface of Gliese 581c. Alternatively, an atmosphere large enough to be stable would circulate the heat more evenly, allowing for a wider habitable area on the surface.[32]For example, althoughVenushas a small axial inclination, very little sunlight reaches the surface at the poles. A slow rotation rate approximately 117 times slower than Earth's produces prolonged days and nights. Despite the uneven distribution of sunlight cast on Venus at any given time, polar areas and the night side of Venus are kept almost as hot as on the day side by globally circulating winds.[33]

A Message from Earth

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A Message from Earth (AMFE) is a high-powered digital radio signal that was sent on 9 October 2008 towards Gliese 581c. The signal is a digitaltime capsulecontaining 501 messages that were selected through a competition on thesocial networking siteBebo.The message was sent using theRT-70radar telescopeofUkraine'sState Space Agency.The signal will reach the planet Gliese 581c in early 2029.[34]More than half a million people including celebrities and politicians participated in the AMFE project, which was the world's first digital time capsule where the content was selected by the public.[35][36]

As of 22 January 2015, the message has traveled 59.48 trillion km of the total 192 trillion km, which is 31.0% of the distance to the Gliese 581 system.[37]

See also

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Notes

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  1. ^The estimated range of sizes in this image from 2009 is based on the planet'sminimum mass.Since the true mass is now known to be somewhat greater, the sizes are likely underestimated, especially if the planet has a hydrogen atmosphere.

References

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  1. ^abcdefUdry, Stéphane; Bonfils, Xavier; Delfosse, Xavier; Forveille, Thierry; Mayor, Michel; Perrier, Christian; Bouchy, François; Lovis, Christophe; Pepe, Francesco; Queloz, Didier; Bertaux, Jean-Loup (2007)."The HARPS search for southern extra-solar planets XI. Super-Earths (5 and 8ME) in a 3-planet system "(PDF).Astronomy & Astrophysics.469(3): L43–L47.arXiv:0704.3841.Bibcode:2007A&A...469L..43U.doi:10.1051/0004-6361:20077612.S2CID119144195.Archived fromthe original(PDF)on 8 October 2010.
  2. ^abcdvon Stauffenberg, A.; Trifonov, T.; Quirrenbach, A.; et al. (5 June 2024). "The CARMENES search for exoplanets around M dwarfs. Revisiting the GJ 581 multi-planetary system with new Doppler measurements from CARMENES, HARPS, and HIRES".Astronomy & Astrophysics.688.arXiv:2407.11520.Bibcode:2024A&A...688A.112V.doi:10.1051/0004-6361/202449375.ISSN0004-6361.
  3. ^abcdeRobertson, Paul;Mahadevan, Suvrath;Endl, Michael; Roy, Arpita (3 July 2014). "Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581".Science.345(6195): 440–444.arXiv:1407.1049.Bibcode:2014Sci...345..440R.doi:10.1126/science.1253253.PMID24993348.S2CID206556796.
  4. ^abcdvon Bloh; et al. (2007). "The Habitability of Super-Earths in Gliese 581".Astronomy and Astrophysics.476(3): 1365–1371.arXiv:0705.3758.Bibcode:2007A&A...476.1365V.doi:10.1051/0004-6361:20077939.S2CID14475537.
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  6. ^Than, Ker (24 February 2007)."Planet Hunters Edge Closer to Their Holy Grail".space.com.Archivedfrom the original on 13 December 2010.Retrieved29 April2007.
  7. ^abMayor, Michel; Bonfils, Xavier; Forveille, Thierry; et al. (2009)."The HARPS search for southern extra-solar planets, XVIII. An Earth-mass planet in the GJ 581 planetary system"(PDF).Astronomy and Astrophysics.507(1): 487–494.arXiv:0906.2780.Bibcode:2009A&A...507..487M.doi:10.1051/0004-6361/200912172.S2CID2983930.Archived fromthe original(PDF)on 21 May 2009.
  8. ^ab"Astronomers Find First Earth-like Planet in Habitable Zone".ESO. Archived fromthe originalon 28 August 2008.Retrieved10 May2007.
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  12. ^ab"Boring Star May Mean Livelier Planet".Spaceref.com. 10 June 2007. Archived fromthe originalon 26 May 2012.Retrieved15 September2008.
  13. ^Black, Charles."Super-Earths are more like mini-Neptunes".Archivedfrom the original on 14 March 2013.Retrieved14 March2013.
  14. ^Lammer, Helmut (2013)."Probing the blow-off criteria of hydrogen-rich 'super-Earths'".Monthly Notices of the Royal Astronomical Society.430(2). Royal Astronomical Society: 1247–1256.arXiv:1210.0793.Bibcode:2013MNRAS.430.1247L.doi:10.1093/mnras/sts705.S2CID55890198.
  15. ^ab"New 'super-Earth' found in space".BBC News.25 April 2007.Archivedfrom the original on 10 November 2012.Retrieved25 April2007.
  16. ^Overbye, Dennis (25 April 2007)."20 light years away, the most Earthlike planet yet".International Herald Tribune.Archivedfrom the original on 27 April 2007.Retrieved10 May2007.
  17. ^"Earth Fact Sheet".NASA.Archivedfrom the original on 8 May 2013.Retrieved21 December2015.
  18. ^Girardi L.; Bressan A.; Bertelli G.; Chiosi C. (2000). "Evolutionary tracks and isochrones for low- and intermediate-mass stars: From 0.15 to 7M☉,and from Z=0.0004 to 0.03 ".Astron. Astrophys. Suppl. Ser.141(3): 371–383.arXiv:astro-ph/9910164.Bibcode:2000A&AS..141..371G.doi:10.1051/aas:2000126.S2CID14566232.
  19. ^Vergano, Dan (25 April 2007)."Out of our world: Earthlike planet".USA Today.Archivedfrom the original on 23 May 2011.Retrieved10 May2007.
  20. ^Selsis 2.4.1"becomes tidally locked in less than 1 Gyr."
  21. ^Beust, H.; et al. (2008). "Dynamical evolution of the Gliese 581 planetary system".Astronomy and Astrophysics.479(1): 277–282.arXiv:0712.1907.Bibcode:2008A&A...479..277B.doi:10.1051/0004-6361:20078794.S2CID119152085.
  22. ^Hut, P. (1981). "Tidal Evolution in Close Binary Systems".Astronomy and Astrophysics.99(1): 126–140.Bibcode:1981A&A....99..126H.
  23. ^Perlman, David (24 April 2007)."New planet found: It might hold life".San Francisco Chronicle.Archivedfrom the original on 27 April 2007.Retrieved24 April2007.
  24. ^Jackson, Brian; Richard Greenberg; Rory Barnes (2008). "Tidal Heating of Extra-Solar Planets".Astrophysical Journal.681(2): 1631–1638.arXiv:0803.0026.Bibcode:2008ApJ...681.1631J.doi:10.1086/587641.S2CID42315630.
  25. ^Selsis, Franck; Kasting, James F.; Levrard, Benjamin; Paillet, Jimmy; Ribas, Ignasi; Delfosse, Xavier (2007)."Habitable planets around the star Gl 581?".Astronomy and Astrophysics.476(3): 1373–1387.arXiv:0710.5294.Bibcode:2007A&A...476.1373S.doi:10.1051/0004-6361:20078091.S2CID11492499.Archivedfrom the original on 4 November 2018.Retrieved4 November2018.
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  27. ^"Venus Fact Sheet".NASA.Archivedfrom the original on 8 March 2016.Retrieved20 September2008.
  28. ^Selsis 5."Gl 581c is very unlikely to be habitable"
  29. ^Selsis 3.1"would be habitable only if clouds with the highest reflectivity covered most of the daytime hemisphere."
  30. ^Selsis 3.1.2
  31. ^SelsisAbstract, 3. Figure 4.
  32. ^Alpert, Mark (7 November 2005)."Red Star Rising".Scientific American.293(5): 28.Bibcode:2005SciAm.293e..28A.doi:10.1038/scientificamerican1105-28.PMID16318021.Archived fromthe originalon 12 October 2007.Retrieved25 April2007.
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  34. ^Moore, Matthew (9 October 2008)."Messages from Earth sent to distant planet by Bebo"..telegraph.co.uk.Archivedfrom the original on 11 October 2008.Retrieved9 October2008.
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