Gliese 581c

Gliese 581c
Exoplanet List of exoplanets

Size comparison of Gliese 581c with Earth and Neptune.
(Based on selected hypothetical modeled compositions)
Parent star
Star Gliese 581[1]
Constellation Libra[2]
Right ascension (α) 15h 19m 26s[3]
Declination (δ) −07° 43′ 20″[3]
Distance20.37 ly
(6.26 pc)
Spectral type M3V[4]
Mass (m) 0.31[5] M☉
Radius (r) 0.29[4] R☉
Temperature (T) 3480 ± 48[6] K
Metallicity [Fe/H] -0.33 ± 0.12[6]
Age 7 – 11[5] Gyr
Orbital elements
Semi-major axis(a) 0.0721 ± 0.0003[7] AU
Eccentricity (e) 0.00 ± 0.06[7]
Orbital period(P) 12.914 ± 0.002[7] d
Time of periastron (T0) 2454759.2 ± 0.1[7] JD
Semi-amplitude (K) 3.3 ± 0.2[7] m/s
Physical characteristics
Minimum mass(m sin i)5.5 ± 0.3[7] M⊕
Stellar flux(F⊙)2.5 ⊕
Discovery information
Discovery date April 4, 2007
April 24, 2007 (announced)
Discoverer(s) Stéphane Udry et al.
Discovery method Radial velocity
Discovery site La Silla Observatory
Discovery status Published
Database references
Extrasolar Planets
Encyclopaedia		data
SIMBADdata
Exoplanet Archivedata
Open Exoplanet Cataloguedata

Gliese 581c /ˈɡliːzə/ or Gl 581c is a planet orbiting within the Gliese 581 planetary system. It is the second planet discovered in the system and the third in order from the star. With a mass at least 5.5 times that of the Earth, it is classified as a super-Earth (a category of planets from 5 to 10 Earth masses).

Gliese 581c gained interest from astronomers because it was reported to be the first potentially Earth-like planet in the habitable zone of its star, with a temperature right for liquid water on its surface, and by extension, potentially capable of supporting extremophile forms of Earth-like life. However, further research casts doubt upon the planet's habitability. It is tidally locked (always faces the parent star with the same face) so if life had a chance to emerge, the best hope of survival would be "the Goldilocks zone."

In astronomical terms, the Gliese 581 system is relatively close to Earth, at 20.37 light-years (192 trillion km or 119 trillion miles) in the direction of the constellation of Libra. This distance, along with the declination and right ascension coordinates, give its exact location in our galaxy.

Discovery

The team released a paper of their findings dated April 27, 2007, published in the July 2007 journal Astronomy and Astrophysics.[8] At the time of discovery, it was reported to be the first potentially Earth-like planet in the habitable zone of its star[1][9] and the smallest known extrasolar planet around a main-sequence star but on April 21, 2009, another planet orbiting Gliese 581, Gliese 581e, with an approximate mass of 1.9 Earth 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 a semi-major axis of 0.25 astronomical units.[7]

Physical characteristics

Mass

The existence of Gliese 581c and its mass have been measured by the radial velocity method of detecting extrasolar planets. 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 six planets are fitted with a Keplerian solution, the minimum mass of the planet is determined to be 5.5 Earth masses.[7] 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.[8] Dynamical simulations of the Gliese 581 system which assume the orbits of the planets are coplanar indicate 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.[10]

Radius

Since Gliese 581c has not been detected directly, there are no measurements of its radius. Furthermore, the radial velocity method used 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 a rocky planet with a large iron core, it should have a radius approximately 50% larger than that of Earth, according to Udry's team.[8][11] Gravity on such a planet's surface would be approximately 2.24 times as strong as on Earth. However, if Gliese 581c is an icy and/or watery planet, its radius would be less than 2 times that of Earth, even with a very large outer hydrosphere, according to density models compiled by Diana Valencia and her team for Gliese 876 d.[12] 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.[13]

Other scientists' views differ. Sara Seager at MIT has speculated that Gliese 581c and other five-Earth-mass planets could be:[14]

If the planet transits the star as seen from our direction, the radius should be measurable, albeit with some uncertainty. Unfortunately, measurements made with the Canadian-built MOST space telescope indicate that transits do not occur.[15]

The new research suggests that the rocky centres of super-Earths are unlikely to evolve into terrestrial rocky planets like the inner planets of our Solar System because they appear to hold on to 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.[16][17]

Orbit

The orbits of the Gliese 581 planetary system, as per the 2009 analysis excluding planets g and f. In the picture, Gliese 581c is the third planet from the star.

Gliese 581c has an orbital period ("year") of 13 Earth days[2] and its orbital radius is only about 7% that of the Earth, about 11 million km,[18] while the Earth is 150 million kilometres from the Sun.[19] 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.[8][11] 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. A typical radius for an M0 star of Gliese 581's age and metallicity is 0.00128 AU,[20] 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

Because of its small separation from Gliese 581, the planet has been generally considered to always have one hemisphere facing the star (only day), and the other always facing away (only night), or in other words being tidally locked.[21][22] The most recent orbital fit to the system, taking stellar activity into account indicates a circular orbit,[7] but older fits use an eccentricity between 0.10 and 0.22. If the orbit of the planet were eccentric, it would undergo violent tidal flexing.[23] Because tidal forces are stronger when the planet is close to the star, eccentric planets are expected to have a rotation period which is shorter than its orbital period, also called pseudo-synchronization.[24] An example of this effect is seen in Mercury, 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 undergo libration and the terminator would be alternatively lit and darkened during libration.[25]

Models of the evolution of the planet's orbit over time suggest that heating resulting 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 the Jupiter's moon Io's, which could result in major geological activity such as volcanoes and plate tectonics.[26]

Habitability and climate

The study of Gliese 581c by the von Bloh et al. team has been quoted as concluding "The super-Earth Gl 581c is clearly outside the habitable zone, since it is too close to the star."[27] The study by Selsis et al. claims even "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 K to 1000 K (430 to 730 °C), like Venus today.[28] Temperature speculations by other scientists were based on the temperature of (and heat from) the parent star Gliese 581 and have been calculated without factoring in the margin of error (96 °C/K) for the star's temperature of 3432 K to 3528 K, which leads to a large irradiance range for the planet, even before eccentricity is considered.[29]

Effective temperatures

Using the measured stellar luminosity of Gliese 581 is of 0.013 times that of our Sun, it is possible to calculate Gliese 581c's effective temperature a.k.a. black body temperature. (note: this probably differs from its surface temperature). According to Udry's team, the effective temperature for Gliese 581c, assuming an albedo (reflectivity) such as Venus' (0.64), would be −3 °C (27 °F), and assuming an Earth-like albedo (0.296), then it would be 40 °C (104 °F),[2][8] a range of temperatures which overlaps with the range that 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 34.25 °C (307.40 K; 93.65 °F), but a surface temperature of 463.85 °C (737.00 K; 866.93 °F) (mainly due to a 96.5% carbon dioxide atmosphere), a difference of about 430 °C (770 °F).[30] Studies of the habitability (i.e. liquid water for extremophile forms of life)[31] conclude that Gliese 581c is likely to suffer from a runaway greenhouse effect similar to that found on Venus, 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.[32] Alternatively, if the surface were covered in ice, it would have a high albedo (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 of carbon dioxide by volcanic activity or of water vapor due to heating at the substellar point would trigger a runaway greenhouse effect.[33]

Liquid water

Gliese 581c is likely to lie outside the habitable zone.[27][34] No direct evidence has been found for water to be present, and it is probably not present in the liquid state. Techniques like the one used to measure the extrasolar planet HD 209458 b may in the future be used to determine the presence of water in the form of vapor in the planet's atmosphere, but only in the rare case of a planet with an orbit aligned so as to transit its star, which Gliese 581c is not known to do.[15]

Tidally-locked models

Theoretical models predict that volatile compounds such as water and carbon dioxide, if present, might evaporate in the scorching heat of the sunward side, migrate to the cooler night side, and condense to form ice 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.[35] For example, although Venus has 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.[36]

A Message from Earth

A Message from Earth (AMFE) is a high-powered digital radio signal that was sent on 9 October 2008 towards Gliese 581c, a large terrestrial extrasolar planet orbiting the red dwarf star Gliese 581. The signal is a digital time capsule containing 501 messages that were selected through a competition on the social networking site Bebo. The message was sent using the RT-70 radar telescope of Ukraine's State Space Agency. The signal will reach the planet Gliese 581c in early 2029.[37] 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.[38][39]

As of 22 January 2015, the message has traveled 59.48 trillion kilometers of the total 192 trillion kilometers, which is 31.0% of the distance to the Gliese 581 system.[40]

On 13 February 2015, scientists (including David Grinspoon, Seth Shostak, and David Brin) at an annual meeting of the American Association for the Advancement of Science, discussed Active SETI and whether transmitting a message to possible intelligent extraterrestrials in the Cosmos was a good idea;[41][42] That same week, a statement was released, signed by many in the SETI community, that a "worldwide scientific, political and humanitarian discussion must occur before any message is sent".[43] However neither Frank Drake, nor Seth Shostak signed this appeal. On 28 March 2015, a related essay with some different point of view was written by Seth Shostak and published in The New York Times.[44]

See also

References

  1. 1 2 Than, Ker (2007-04-24). "Major Discovery: New Planet Could Harbor Water and Life". space.com. Retrieved 2007-04-29.
  2. 1 2 3 "New 'super-Earth' found in space". BBC News. April 25, 2007. Retrieved 2007-04-25.
  3. 1 2 "GJ 581". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2008-08-21.
  4. 1 2 Bonfils, X.; et al. (2005). "The HARPS search for southern extra-solar planets VI: A Neptune-mass planet around the nearby M dwarf Gl 581". Astronomy and Astrophysics Letters. 443 (3): L15–L18. Bibcode:2005A&A...443L..15B. arXiv:astro-ph/0509211 Freely accessible. doi:10.1051/0004-6361:200500193.
  5. 1 2 "Star: Gliese 581". Extrasolar Planets Encyclopaedia. Mass 0.31 MSun, Age 8+3
    −1     Gyr
  6. 1 2 Bean, J. L; Benedict, G. F.; Endl, M. (2006). "Metallicities of M Dwarf Planet Hosts from Spectral Synthesis". Astrophysical Journal Letters. 653 (1): L65–L68. Bibcode:2006ApJ...653L..65B. arXiv:astro-ph/0611060 Freely accessible. doi:10.1086/510527.
  7. 1 2 3 4 5 6 7 8 9 Robertson, 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: 440–444. Bibcode:2014Sci...345..440R. arXiv:1407.1049 Freely accessible. doi:10.1126/science.1253253.
  8. 1 2 3 4 5 Udry; et al. (2007). "The HARPS search for southern extra-solar planets, XI. Super-Earths (5 and 8 M⊕) in a 3-planet system". Astronomy and Astrophysics. 469 (3): L43–L47. Bibcode:2007A&A...469L..43U. arXiv:0704.3841 Freely accessible. doi:10.1051/0004-6361:20077612.
  9. ↑ Than, Ker (2007-02-24). "Planet Hunters Edge Closer to Their Holy Grail". space.com. Retrieved 2007-04-29.
  10. ↑ Mayor, M.; Bonfils, X.; Forveille, T.; Delfosse, X.; Udry, S.; Bertaux, J. -L.; Beust, H.; Bouchy, F.; Lovis, C.; Pepe, F.; Perrier, C.; Queloz, D.; Santos, N. C. (2009). "The HARPS search for southern extra-solar planets". Astronomy and Astrophysics. 507 (1): 487–494. Bibcode:2009A&A...507..487M. arXiv:0906.2780 Freely accessible. doi:10.1051/0004-6361/200912172.
  11. 1 2 "Astronomers Find First Earth-like Planet in Habitable Zone". ESO. Retrieved 2007-05-10.
  12. ↑ Valencia; Sasselov, Dimitar D.; O'Connell, Richard J. (2006). "Radius and Structure Models of the First Super-Earth Planet". The Astrophysical Journal. 656 (1): 545–551. Bibcode:2007ApJ...656..545V. arXiv:astro-ph/0610122 Freely accessible. doi:10.1086/509800.
  13. ↑ Valencia, D.; Sasselov, Dimitar D.; O’Connell, Richard J. (2007). "Detailed Models of Super-Earths: How Well Can We Infer Bulk Properties?". The Astrophysical Journal. 665 (2): 1413–1420. Bibcode:2007ApJ...665.1413V. arXiv:0704.3454 Freely accessible. doi:10.1086/519554.
  14. ↑ Seager (2008). "Alien Earths from A to Z". Sky & Telescope. ISSN. 0037-6604 (January): 22–25.
  15. 1 2 "Boring Star May Mean Livelier Planet". Spaceref.com. Retrieved 2008-09-15.
  16. ↑ Black, Charles. "Super-Earths are more like mini-Neptunes".
  17. ↑ Lammer, Helmut. "Probing the blow-off criteria of hydrogen-rich ‘super-Earths’". Monthly Notices of the Royal Astronomical Society. Royal Astronomical Society. 430: 1247–1256. doi:10.1093/mnras/sts705.
  18. ↑ Overbye, Dennis (2007-04-25). "20 light years away, the most Earthlike planet yet". International Herald Tribune. Retrieved 2007-05-10.
  19. ↑ "Earth Fact Sheet". NASA. Retrieved 2015-12-21.
  20. ↑ Girardi L.; Bressan A.; Bertelli G.; Chiosi C. (2000). "Evolutionary tracks and isochrones for low- and intermediate-mass stars: From 0.15 to 7 M☉, and from Z=0.0004 to 0.03". Astron. Astrophys. Suppl. Ser. 141 (3): 371–383. Bibcode:2000A&AS..141..371G. arXiv:astro-ph/9910164 Freely accessible. doi:10.1051/aas:2000126.
  21. ↑ Vergano, Dan (2007-04-25). "Out of our world: Earthlike planet". USA Today. Retrieved 2007-05-10.
  22. ↑ Selsis 2.4.1 "becomes tidally locked in less than 1 Gyr. "
  23. ↑ Beust, H.; et al. (2008). "Dynamical evolution of the Gliese 581 planetary system". Astronomy and Astrophysics. 479 (1): 277–282. Bibcode:2008A&A...479..277B. arXiv:0712.1907 Freely accessible. doi:10.1051/0004-6361:20078794.
  24. ↑ Hut, P. (1981). "Tidal Evolution in Close Binary Systems". Astronomy and Astrophysics. 99 (1): 126–140. Bibcode:1981A&A....99..126H.
  25. ↑ Perlman, David (2007-04-24). "New planet found: It might hold life". San Francisco Chronicle. Retrieved 2007-04-24.
  26. ↑ Jackson, Brian; Richard Greenberg; Rory Barnes (2008). "Tidal Heating of Extra-Solar Planets". ApJ. 681 (2): 1631–1638. Bibcode:2008ApJ...681.1631J. arXiv:0803.0026 Freely accessible. doi:10.1086/587641.
  27. 1 2 von Bloh; et al. (2007). "The Habitability of Super-Earths in Gliese 581". Astronomy and Astrophysics. 476 (3): 1365–1371. Bibcode:2007A&A...476.1365V. arXiv:0705.3758 Freely accessible. doi:10.1051/0004-6361:20077939.
  28. ↑ Selsis; Kasting, J. F.; Levrard, B.; Paillet, J.; Ribas, I.; Delfosse, X. (2007). "Habitable planets around the star Gl 581?". Astronomy and Astrophysics. 476 (3): 1373–1387. Bibcode:2007A&A...476.1373S. arXiv:0710.5294 Freely accessible. doi:10.1051/0004-6361:20078091.
  29. ↑ Bean, J. L.; Benedict, G. F.; Endl, M. (2006). "Metallicities of M Dwarf Planet Hosts from Spectral Synthesis". The Astrophysical Journal. 653 (1): L65–L68. Bibcode:2006ApJ...653L..65B. arXiv:astro-ph/0611060 Freely accessible. doi:10.1086/510527.
  30. ↑ "Venus Fact Sheet". NASA. Retrieved 2008-09-20.
  31. ↑ Selsis 5. "Gl 581c is very unlikely to be habitable"
  32. ↑ Selsis 3.1 "would be habitable only if clouds with the highest reflectivity covered most of the daytime hemisphere. "
  33. ↑ Selsis 3.1.2
  34. ↑ Selsis Abstract, 3. Figure 4.
  35. ↑ Alpert, Mark (2005-11-07). "Red Star Rising". Scientific American. Retrieved 2007-04-25.
  36. ↑ Ralph D Lorenz; Jonathan I Lunine; Paul G Withers; Christopher P. McKay (2001). "Titan, Mars and Earth: Entropy Production by Latitudinal Heat Transport" (PDF). Ames Research Center, University of Arizona Lunar and Planetary Laboratory. Retrieved 2007-08-21.
  37. ↑ Moore, Matthew (9 October 2008). "Messages from Earth sent to distant planet by Bebo". .telegraph.co.uk. Archived from the original on 11 October 2008. Retrieved 9 October 2008.
  38. ↑ "Stars' Faces Beamed Into Space". Sky News. 10 October 2008. Retrieved 5 November 2008.
  39. ↑ Sarah Gavin (29 July 2008). "One Giant Leap For The Bebo Community". Bebo. Retrieved 15 November 2008.
  40. ↑ http://www.bebo.com/amessagefromearth
  41. ↑ Borenstein, Seth (of AP News) (13 February 2015). "Should We Call the Cosmos Seeking ET? Or Is That Risky?". New York Times. Retrieved 14 February 2015.
  42. ↑ Ghosh, Pallab (12 February 2015). "Scientist: 'Try to contact aliens'". BBC News. Retrieved 12 February 2015.
  43. ↑ Various (13 February 2015). "Statement - Regarding Messaging To Extraterrestrial Intelligence (METI) / Active Searches For Extraterrestrial Intelligence (Active SETI)". University of California, Berkeley. Retrieved 14 February 2015.
  44. ↑ Shostak, Seth (28 March 2015). "Should We Keep a Low Profile in Space?". New York Times. Retrieved 29 March 2015.

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Coordinates: 15h 19m 26s, −07° 43′ 20″

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