Gliese 581d
Size comparison of Gliese 581d with Earth and Neptune.
(Based on selected hypothetical modeled compositions)
Discovery
Discovered byUdry et al.[1]
Discovery siteLa Silla Observatory, Chile
Discovery date24 April 2007[1]
Radial velocity
Orbital characteristics
Epoch JD 2451409.762[2]
0.21847 ± 0.00028 AU (32,683,000 ± 42,000 km)[2]
Eccentricity0[2]
66.87 ± 0.13[2] d
0.183 y
1600 h
56 ± 27[2]
Semi-amplitude1.91 ± 0.22[2]
StarGliese 581[2]
Physical characteristics
Mass6.98[3] MEarth

    Gliese 581d /ˈɡliːzə/ (often shortened to Gl 581d or GJ 581d) was a candidate extrasolar planet orbiting within the Gliese 581 system, approximately 20.4 light-years away in the Libra constellation. It was the third planet claimed in the system and the fourth (in a 4-planet model) or fifth (in a 5- or 6-planet model) in order from the star. Multiple subsequent studies found that the planetary signal in fact originates from stellar activity, and thus the planet does not exist.[4][5]

    Though significantly more massive than Earth (at a minimum mass of 6.98 Earth masses), this Super-Earth was the first exoplanet of potentially terrestrial mass regarded as orbiting within the habitable zone of its parent star. Assuming its existence, computer climate simulations have confirmed the possibility of the existence of surface water and these factors combine to a relatively high measure of planetary habitability.

    History

    Discovery

    A team of astronomers led by Stéphane Udry of the Geneva Observatory used the HARPS instrument on the European Southern Observatory 3.6 meter telescope in La Silla, Chile, to discover the planet in 2007. Udry's team employed the radial velocity technique, in which the mass of a planet is determined based on the small perturbations it induces in its parent star's orbit via gravity.[6]

    Disputed existence

    In September 2012, Roman Baluev filtered out the "red noise" from the Keck data and concluded that this planet's existence is probable only to 2.2 standard deviations, and thus is uncertain.[7] Earlier that same year, however, S. S. Vogt (USNO), together with R. P. Butler and N. Haghighipour, published a study that supported the existence of the planet with a much higher probability;[8] they also pursued a dynamical analysis of the system. Additional work on Gliese 581 as a four-planet system (thus, including planet d), demonstrating its long-term orbital stability, was given by Makarov and coauthors.[9]

    A study in 2014 concluded that Gliese 581d is "an artifact of stellar activity which, when incompletely corrected, causes the false detection of planet g."[4][10][11] In 2015, a study by Guillem Anglada-Escudé and Mikko Tuomi questioned the 2014 work owing to a significant shortcoming in the adopted statistical method;[12][13][14] however, this study was published along with a rebuttal by the team that published the 2014 refutation.[15] Another 2015 study added support to the conclusion that the radial velocity signal originates from stellar activity,[16] and a 2016 study provided additional strong evidence for it.[17]

    In 2016, E. R. Newton and collaborators pointed out that for early M dwarfs, planets in their habitable zones may have orbital periods coinciding with the stellar rotation period (or in rare cases, such as Gliese 581d, half of it); this aspect seriously complicates the verification of any such planets.[18]

    Evidence based on a 2022 paper confirms previous studies suggesting that the announcement of Gliese 581d stems from a false detection due to stellar activity. This work uses an updated technique correlating stellar activity to RV signals.[19]

    If the planet had existed, Gliese 581d would have constituted the first low-mass planet (including Super-Earths) identified to be located in a stellar habitable zone outside of the Solar System,[20][21] based on studies in 2007.

    Orbital characteristics

    Gliese 581d was thought to orbit Gliese 581 at 0.21847 AU, approximately a fifth of the distance that the Earth orbits the Sun, though its orbital eccentricity has not been confirmed. There were two models for its orbit, a circular one like Earth's, and an eccentric one like Mercury's. These are based on the four planet and six planet model for the Gliese 581 system, respectively. Under the four planet model Gliese 581d would most probably be in a spin-orbit resonance of 2:1, rotating twice for each orbit of its parent star. Therefore, the day on Gliese 581d should approximately be 67 Earth days long.[9][22]

    The orbital distance places it at the outer limits of the habitable zone, the distance at which it is believed possible for water to exist on the surface of a planetary body. At the time of its discovery, the planet's orbit was originally thought to be farther out. However, in late April 2009 the original discovery team revised its original estimate of the planet's orbital parameters, finding that it orbits closer to its star than originally determined with an orbital period of 66.87 days. They concluded that the planet is within the habitable zone where liquid water could exist,[23][24] thus confirming previous studies. Moreover, the data[25] also suggested that the proposed exoplanet[26] could have at least one[27] or more large oceans.

    Physical characteristics

    The motion of the parent star indicates a minimum mass for Gliese 581d of 5.6 Earth masses (earlier analyses gave higher values).[2] Dynamical simulations of the Gliese 581 system assuming that the orbits of the three planets are coplanar show that the system becomes unstable if the masses of the planets exceed 1.6–2 times the minimum values. Using earlier minimum mass values for Gliese 581d, this implies an upper mass limit for Gliese 581d of 13.8 Earth masses.[23] The composition of the planet, however, is not known.

    Climate and habitability

    The habitable zone of Gliese 581 compared with our Solar System's habitable zone, showing Gliese 581d near the outer edge

    As the planet is not known to transit from Earth and atmospheric conditions are not observable with current technology, no atmosphere for the planet has been confirmed to date. As such, all climate predictions for the planet are based on predicted orbits and computer modelling of theoretical atmospheric conditions.

    Because Gliese 581d was believed to orbit outside the habitable zone of its star it was originally thought to be too cold for liquid water to be present. With the 2009 revised orbit, climate simulations conducted by researchers in France in 2011 indicated possible temperatures suitable for surface water at sufficient atmospheric pressure.[28] According to Stéphane Udry, "It could be covered by a 'large and deep ocean'; it is the first serious ocean planet candidate."[29]

    On average, the light that Gliese 581d receives from its star has about 30% of the intensity of light the Earth receives from the Sun. By comparison, sunlight on Mars has about 40% of the intensity of that on Earth.[28] That might seem to suggest that Gliese 581d is too cold to support liquid water and hence is inhospitable to life. However, an atmospheric greenhouse effect can significantly raise planetary temperatures. For example, Earth's own mean temperature would be about −18 °C[30] without any greenhouse gases, ranging from around 100 °C on the day side to −150 °C at night, much like that found on the Moon. If the atmosphere of Gliese 581d produces a sufficiently large greenhouse effect, and the planet's geophysics stabilize the CO2 levels (as Earth's does via plate tectonics), then the surface temperature might permit a liquid water cycle, conceivably allowing the planet to support life.[31][32][33][34] Calculations by Barnes et al. suggest, however, that tidal heating is too low to keep plate tectonics active on the planet, unless radiogenic heating is somewhat higher than expected.[35]

    Gliese 581d is probably too massive to be made only of rocky material. It may have originally formed on a more distant orbit as an icy planet that then migrated closer to its star.[20][36]

    Hello from Earth

    As part of the 2009 National Science Week celebrations in Australia, Cosmos magazine launched a website called "Hello from Earth" to collect messages for transmission to Gliese 581d. The maximum length of the messages was 160 characters, and they were restricted to the English language. In total, 25,880 messages were collected from 195 countries around the world. The messages were transmitted from the DSS-43 70 m radio telescope at the Canberra Deep Space Communication Complex at Tidbinbilla, Australia, on 28 August 2009.[37]

    Gliese 581d is the setting for the Doctor Who episode "Smile".[38] It is also shown in Into the Universe with Stephen Hawking's Episode 3: The Story of Everything, and in Episode 3 and 8 of Season 2 of How the Universe Works.

    See also

    References

    1. 1 2 Udry, 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 8 MEarth) 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. S2CID 119144195. Archived from the original (PDF) on October 8, 2010.
    2. 1 2 3 4 5 6 7 8 Vogt, S. S.; et al. (2010). "The Lick-Carnegie Exoplanet Survey: A 3.1 MEarth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581". The Astrophysical Journal. 723 (1): 954–965. arXiv:1009.5733. Bibcode:2010ApJ...723..954V. doi:10.1088/0004-637X/723/1/954. S2CID 3163906.
    3. ↑ "PHL's Exoplanets Catalog - Planetary Habitability Laboratory @ UPR Arecibo". Planetary Habitability Laboratory. Archived from the original on 2019-05-21. Retrieved 2014-01-09.
    4. 1 2 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 (6195): 440–444. arXiv:1407.1049. Bibcode:2014Sci...345..440R. CiteSeerX 10.1.1.767.2071. doi:10.1126/science.1253253. PMID 24993348. S2CID 206556796.
    5. ↑ Trifonov, T.; Kürster, M.; et al. (February 2018). "The CARMENES search for exoplanets around M dwarfs. First visual-channel radial-velocity measurements and orbital parameter updates of seven M-dwarf planetary systems". Astronomy & Astrophysics. 609: A117. arXiv:1710.01595. Bibcode:2018A&A...609A.117T. doi:10.1051/0004-6361/201731442.
    6. ↑ "The HARPS search for southern extra-solar planets" Archived 2010-10-08 at the Wayback Machine, S. Udry. Xavier Bonfils, Xavier Delfosse, Thierry Forveille, M. Mayor, C. Perrier, François Bouchy, Christophe Lovis, Francesco Pepe, D. Queloz, J.-L. Bertaux, The Extrasolar Planets Encyclopaedia, April 4, 2007, accessed June 10, 2011
    7. ↑ Roman Baluev (2012). "The impact of red noise in radial velocity planet searches: Only three planets orbiting GJ581?". Monthly Notices of the Royal Astronomical Society. 429 (3): 2052–2068. arXiv:1209.3154. Bibcode:2013MNRAS.429.2052B. doi:10.1093/mnras/sts476.
    8. ↑ Vogt, S. S.; Butler, R. P.; Haghighipour, N. (2012). "GJ 581 update: Additional evidence for a Super-Earth in the habitable zone". Astronomische Nachrichten. 333 (7): 561. arXiv:1207.4515. Bibcode:2012AN....333..561V. doi:10.1002/asna.201211707. S2CID 13901846.
    9. 1 2 Makarov, Valeri V.; et al. (2012). "Dynamical evolution and spin-orbit resonances of potentially habitable exoplanets. The case of GJ 581d". The Astrophysical Journal. 761 (2): 83. arXiv:1208.0814. Bibcode:2012ApJ...761...83M. doi:10.1088/0004-637X/761/2/83. S2CID 926755.
    10. ↑ Quenqua, Douglas (7 July 2014). "Earthlike Planets May Be Merely an Illusion". New York Times. Retrieved 8 July 2014.
    11. ↑ "New doubt cast on 2 of the most Earth-like planets ever found". CBC News. Retrieved January 7, 2017.
    12. ↑ Anglada-Escudé, Guillem; Tuomi, Mikko (6 March 2015). "Comment on "Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581"". Science. 347 (6226): 1080–b. arXiv:1503.01976. Bibcode:2015Sci...347.1080A. doi:10.1126/science.1260796. PMID 25745156. S2CID 5118513.
    13. ↑ "Reanalysis of data suggests 'habitable' planet GJ 581d really could exist". Astronomy Now. 9 March 2015. Retrieved 27 May 2015.
    14. ↑ "Gliese 581d: New Study Confirms Existence of One of Most 'Earth-Like' Extrasolar Planets Known". 8 March 2015.
    15. ↑ Robertson, Paul; Mahadevan, Suvrath; Endl, Michael; Roy, Arpita (2015). "Response to Comment on "Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581"". Science. 347 (6226): 1080. arXiv:1503.02565. Bibcode:2015Sci...347.1080R. doi:10.1126/science.1260974. PMID 25745157. S2CID 206562664.
    16. ↑ Suárez Mascareño, A.; et al. (September 2015), "Rotation periods of late-type dwarf stars from time series high-resolution spectroscopy of chromospheric indicators", Monthly Notices of the Royal Astronomical Society, 452 (3): 2745–2756, arXiv:1506.08039, Bibcode:2015MNRAS.452.2745S, doi:10.1093/mnras/stv1441, S2CID 119181646.
    17. ↑ Hatzes, A. P. (2016). "Periodic Hαvariations in GL 581: Further evidence for an activity origin to GL 581d". Astronomy & Astrophysics. 585: A144. arXiv:1512.00878. Bibcode:2016A&A...585A.144H. doi:10.1051/0004-6361/201527135. S2CID 55623630.
    18. ↑ Newton, Elisabeth R.; Irwin, Jonathan; Charbonneau, David; Berta-Thompson, Zachory K.; Dittmann, Jason A. (2016). "The Impact of Stellar Rotation on the Detectability of Habitable Planets around M Dwarfs". Astrophysical Journal Letters. 821 (1): L 19 (6 pp.). arXiv:1604.03135. Bibcode:2016ApJ...821L..19N. doi:10.3847/2041-8205/821/1/L19. S2CID 73538034.
    19. ↑ Dodson-Robinson, Sarah E.; Delgado, Victor Ramirez; Harrell, Justin; Haley, Charlotte L. (2022-04-01). "Magnitude-squared Coherence: A Powerful Tool for Disentangling Doppler Planet Discoveries from Stellar Activity". The Astronomical Journal. 163 (4): 169. arXiv:2201.13342. Bibcode:2022AJ....163..169D. doi:10.3847/1538-3881/ac52ed. ISSN 0004-6256. S2CID 246430514.
    20. 1 2 von Bloh, W.; Bounama, C.; Cuntz, M.; Franck, S. (2007). "The Habitability of Super-Earths in Gliese 581". Astronomy & Astrophysics. 476 (3): 1365–1371. arXiv:0705.3758. Bibcode:2007A&A...476.1365V. doi:10.1051/0004-6361:20077939. S2CID 14475537.
    21. ↑ 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. S2CID 11492499.
    22. ↑ Heller, René; Barnes, Rory; Leconte, Jérémy. (August 2011). "Habitability of extrasolar planets and tidal spin evolution". Origins of Life and Evolution of Biospheres. Onlinefirst (6): 539–543. arXiv:1108.4347. Bibcode:2011OLEB...41..539H. doi:10.1007/s11084-011-9252-3. PMID 22139513. S2CID 10154158.
    23. 1 2 Mayor, 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. S2CID 2983930. Archived from the original (PDF) on 21 May 2009.
    24. ↑ "Lightest exoplanet yet discovered". eso.org. 2009-04-21. Archived from the original on 2009-07-05. Retrieved 2009-04-21.
    25. ↑ R. Stewart, (2013). " First View of Gliese-581 d; A preliminary Surface Survey (Part-1). Publisher-(IJAAR), (Issue 2013). At-http://www.journals-of-science.com/5-feature-research-pap.html
    26. ↑ STEWART, R., CELEBONOVIC, V., AGUIAR, O.A.,CHATZITHEODORIDIS, E., ACKERMAN, J., ROMAN, M.,ARTUSO, G., TRENTADUE, W., SAMADI, A., ARISTIZABAL,C.I.O. and others. Gliese 581d; Views of Its atmospheric, topographical, geological, and oceanic conditions (Part 2). Journal of The International Society of Peer Reviewers. Available at: "8-Astronomy - Stewart Research". Archived from the original on 2013-08-30. Retrieved 2013-09-01. . Which in summary shows its atmosphere geology topography and oceanic conditions.
    27. ↑ AGUIAR, O.A., STEWART, R., ARTUSO, G., SAMADI, A., ARISTIZABAL, C.I.O., ACKERMAN, J.R., TRENTADUE, W. The (possible) confirmation of the First exo-oceans. Pluralidade 1: 82–95, 2013. At- http://www.pluralidade.info/Eindex.html Archived 2014-07-29 at the Wayback Machine .
    28. 1 2 Wordsworth, R.; et al. (2011). "Gliese 581d is the first discovered terrestrial-mass exoplanet in the habitable zone". The Astrophysical Journal. 733 (2): L48. arXiv:1105.1031. Bibcode:2011ApJ...733L..48W. doi:10.1088/2041-8205/733/2/L48. S2CID 16349424.
    29. ↑ "Lightest exoplanet yet discovered", European Southern Observatory. April 21, 2009. Accessed June 10, 2011
    30. ↑ "Global Warming Frequently Asked Questions". Lwf.ncdc.noaa.gov. 2008-05-08. Archived from the original on 2009-01-19. Retrieved 2009-01-18.
    31. ↑ von Bloh, W.; et al. (2008). "Habitability of Super-Earths: Gliese 581c and 581d". Proceedings of the International Astronomical Union. 3: 503–506. arXiv:0712.3219. Bibcode:2008IAUS..249..503V. doi:10.1017/S1743921308017031. S2CID 113406160.
    32. ↑ "Centauri Dreams » Blog Archive » Gliese 581d: A Habitable World After All?". Centauri-dreams.org. 2007-12-13. Retrieved 2009-01-18.
    33. ↑ "New 'Super Earth 2' Discovered in Constellation Libra". Dailygalaxy.com. 2007-06-15. Archived from the original on 2009-01-11. Retrieved 2009-01-18.
    34. ↑ Wordsworth, Robin; François Forget; Franck Selsis; Ehouarn Millour; Benjamin Charnay; Jean-Baptiste Madeleine (2011). "Gliese 581d is the first discovered terrestrial-mass exoplanet in the habitable zone". The Astrophysical Journal. 733 (2): L48. arXiv:1105.1031. Bibcode:2011ApJ...733L..48W. doi:10.1088/2041-8205/733/2/L48. S2CID 16349424.
    35. ↑ Barnes, Rory; Jackson, Brian; Greenberg, Richard; Raymond, Sean N. (2009-06-09). "Tidal Limits to Planetary Habitability". The Astrophysical Journal. 700 (1): L30–L33. arXiv:0906.1785. Bibcode:2009ApJ...700L..30B. doi:10.1088/0004-637X/700/1/L30. S2CID 16695095.
    36. ↑ "Hopes Dashed for Life on Distant Planet". Space.com. 18 June 2007.
    37. ↑ Jenkins, Simon (2009-08-28). "Earth sends 25,000 hellos to outer space". Brisbane Times. Retrieved 2009-09-16.
    38. ↑ "Visit Doctor Who's spectacular colony world... In Valencia".
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