COCONUTS-2b explained

COCONUTS-2 b is a gas giant exoplanet that orbits the M-type star L 34-26. With a mass of 8 Jupiters, it takes over one million years to complete one orbit around the star orbiting 7,506 AU away from it.^[4]

The planet was discovered in 2011 and was initially identified as a T9 free-floating brown dwarf WISEPA J075108.79−763449.6.^[1] During the COol Companions ON Ultrawide orbiTS (COCONUTS) survey, its association with L 34-26 was announced in 2021.^[5] At a distance of 35.5lk=onNaNlk=on, COCONUTS-2b was the closest directly imaged exoplanet to Earth^[6] until Epsilon Indi Ab was imaged in 2024.

Proposed formation scenarios

The researchers found that it is unlikely that COCONUTS-2b was formed inside the protoplanetary disk of the host star and it is more likely that the planet formed on its own via high entropy formation (aka hot-start process).^{[7] [8]}

The peculiar properties of COCONUTS-2b could be explained with different scenarios as proposed by Marocco et al. in 2024. The properties could be explained by a non-solar carbon-to-oxygen ratio, meaning that it formed inside a disk around L 34-26. In this scenario the most likely way COCONUTS-2b got in a higher orbit is by a stellar fly-by of two binaries or two planetary systems. In the second scenario L 34-26 is not actually young, but mimics youth due to tidal and/or magnetic interactions with an unseen companion. In this scenario COCONUTS-2b would be an old brown dwarf. In a third scenario COCONUTS-2b could be a captured old brown dwarf. This is however seen as unlikely due to the stellar fly-by requiring a low velocity.^[9]

Another study found that their preferred model showed a metallicity that is lower than the host star, which is inconsistent with in-situ binary-like formation. Only their third-preferred model is consistent with a binary-like formation, because in this model the metallicity of host star and planet agreed.

Atmosphere

The planet's spectral type suggests high amounts of methane, water vapor and low amounts of carbon monoxide in the atmosphere of COCONUTS-2b. It might also have both clouds and a non-equilibrium process in its atmosphere.^[1]

Due to its large orbital separation, COCONUTS-2b is a great laboratory to study the atmosphere and composition of young gas-giant exoplanets. Astronomers estimate the planet’s temperature to be around .

Observations with Gemini/Flamingos-2 showed a spectral type of T, near the T/Y transition. The spectrum is also more consistent with disequilibrium chemestry and the presence of clouds. Additionally the atmosphere shows a diabatic thermal structure, meaning the pressure-temperature profile is non-adiabatic. Adiabatic means here an increase of the temperature with pressure. The observation also indicate a sub- or near-solar metallicity.

Host star

L 34-26, also known as COCONUTS-2A and TYC 9381-1809-1, is a M3-type dwarf star located 35 light-years away, in the constellation of Chamaeleon. The star is about one-third the mass of the Sun, with an age between 150 and 800 million years old.^[10]

Researchers using TESS found that L 34-26 showed stellar flares about every 0.48 days. It was the most active planet hosting star in their sample. The team studying the host star also found that L 34-26 is fast rotating with a rotation period of 2.83 days. The planet should not be influenced by the flares, because of the large orbital separation.^[11] The star is seen almost equator-on with i = 81.8±5.8 deg and might belong to the proposed Ursa Major corona, which is 400 million years old.

Notes and References

1. Kirkpatrick . J. Davy . Cushing . Michael C. . Gelino . Christopher R. . Griffith . Roger L. . Skrutskie . Michael F. . Marsh . Kenneth A. . Wright . Edward L. . Mainzer . A. . Eisenhardt . Peter R. . McLean . Ian S. . Thompson . Maggie A. . Bauer . James M. . Benford . Dominic J. . Bridge . Carrie R. . Lake . Sean E. . 2011-12-01 . The First Hundred Brown Dwarfs Discovered by the Wide-field Infrared Survey Explorer (WISE) . The Astrophysical Journal Supplement Series . 197 . 2 . 19 . 10.1088/0067-0049/197/2/19 . 1108.4677 . 2011ApJS..197...19K . 16850733 . 0067-0049.
2. News: Kooser . Amanda . Massive exoplanet 'Coconuts-2b' could help reveal the secrets of young gas giants . 21 December 2021 . . en . 21 December 2021 . https://web.archive.org/web/20211221120020/https://www.cnet.com/news/meet-coconuts-2b-an-exoplanet-with-six-times-the-mass-of-jupiter/ . live .
3. Web site: COCONUTS-2 b . Exoplanet Exploration: Planets Beyond our Solar System . . 21 December 2021 . 22 November 2021 . https://web.archive.org/web/20211122022627/https://exoplanets.nasa.gov/exoplanet-catalog/7945/coconuts-2-b/ . live .
4. News: Massive COCONUTS exoplanet discovery led by UH grad student University of Hawaiʻi System News . 21 December 2021 . University of Hawaiʻi News . . 15 December 2021 . https://web.archive.org/web/20211215173853/https://www.hawaii.edu/news/2021/07/27/massive-coconuts-exoplanet-discovery-uh-grad-student/ . live .
5. Web site: Exoplanet-catalog. 2021-11-22. Exoplanet Exploration: Planets Beyond our Solar System. 2021-11-22. https://web.archive.org/web/20211122022623/https://exoplanets.nasa.gov/exoplanet-catalog/7945/coconuts-2-b/. live.
6. Web site: Siegel. Ethan. Astronomers Go Nuts For Closest Exoplanet Directly Imaged Ever: COCONUTS-2b. 2021-11-22. Forbes. en. 2021-11-22. https://web.archive.org/web/20211122022619/https://www.forbes.com/sites/startswithabang/2021/08/02/astronomers-go-nuts-for-closest-exoplanet-directly-imaged-ever-coconuts-2b/. live.
7. Zhang . Zhoujian . Liu . Michael C. . Claytor . Zachary R. . Best . William M. J. . Dupuy . Trent J. . Siverd . Robert J. . 2021-08-01 . The Second Discovery from the COCONUTS Program: A Cold Wide-orbit Exoplanet around a Young Field M Dwarf at 10.9 pc . 2021ApJ...916L..11Z . The Astrophysical Journal . 916 . 2 . L11 . 10.3847/2041-8213/ac1123 . 2107.02805 . 0004-637X . free .
8. Marley . Mark S. . Fortney . Jonathan J. . Hubickyj . Olenka . Bodenheimer . Peter . Lissauer . Jack J. . 2007-01-01 . On the Luminosity of Young Jupiters . 2007ApJ...655..541M . The Astrophysical Journal . 655 . 1 . 541–549 . 10.1086/509759 . astro-ph/0609739 . 7793365 . 0004-637X.
9. Marocco . Federico . Kirkpatrick . J. Davy . Schneider . Adam C. . Meisner . Aaron M. . Popinchalk . Mark . Gelino . Christopher R. . Faherty . Jacqueline K. . Burgasser . Adam J. . Caselden . Dan . Gagné . Jonathan . Aganze . Christian . Bardalez-Gagliuffi . Daniella C. . Casewell . Sarah L. . Hsu . Chih-Chun . Kiman . Rocio . 2024-04-22 . Thirteen New M Dwarf + T Dwarf Pairs Identified with WISE/NEOWISE . The Astrophysical Journal. 2404.14324 . Peter R. M. . Eisenhardt . Marc J. . Kuchner . Daniel . Stern . Léopold . Gramaize . Arttu . Sainio . Thomas P. . Bickle . Austin . Rothermich . William . Pendrill . Melina . Thévenot . Martin . Kabatnik . Giovanni . Colombo . Hiro . Higashimura . Frank . Kiwy . Elijah J. . Marchese . Nikolaj Stevnbak . Andersen . Christopher . Tanner . Jim . Walla . Zbigniew . Wedracki . The Backyard Worlds Collaboration. 967 . 2 . 147 . 10.3847/1538-4357/ad3f1d . free . 2024ApJ...967..147M . 3.
10. Web site: Giant Exoplanet Orbits Its Host Star Once Every 1.1 Million Years Astronomy Sci-News.com. 2021-11-22. Breaking Science News Sci-News.com. en-US. 2021-11-22. https://web.archive.org/web/20211122022624/http://www.sci-news.com/astronomy/coconuts-2b-exoplanet-09922.html. live.
11. 2022A&A...665A..30S . Flares and rotation of M dwarfs with habitable zones accessible to TESS planet detections . Stelzer . B. . Bogner . M. . Magaudda . E. . Raetz . St. . Astronomy and Astrophysics . 2022 . 665 . A30 . 10.1051/0004-6361/202142088 . 2207.03794 . 249662585 .