COCONUTS-2b

COCONUTS-2b

COCONUTS-2b with unWISE. The planet in the center of the image stands out due to its red color. The host star is not pictured here.
Discovery
Discovered by	Zhoujian Zhang Michael Liu Zach Claytor William Best Trent Dupuy Robert Siverd [1]
Discovery date	August 2011 (as a free-floating brown dwarf) [2] July 2021 (as a planet) [3]
Detection method	Direct imaging
Designations
Alternative names	WISEPA J075108.79-763449.6 [2]
Orbital characteristics
Semi-major axis	7,506+5,205 −2,060  AU [4] (1.123×1012km)
Orbital period (sidereal)	1.1+1.3 −0.4×106 [4] years
Star	L 34-26
Physical characteristics
Mean radius	1.11+0.03 −0.04  RJ [5]
Mass	8±2  MJ [5]
Surface gravity	log(g) = 4.19+0.18 −0.13  cgs [5]
Temperature	483+44 −53  K [5]
Spectral type	T9.5±0.5 [5]

COCONUTS-2 b is a gas giant exoplanet that orbits the M-type star L 34-26. With a mass of 8 Jupiters, ^[5] it takes over one million years to complete one orbit around the star orbiting 7,506 AU away from it. ^[1]

The planet was discovered in 2011 and was initially identified as a T9 free-floating brown dwarf WISEPA J075108.79−763449.6. ^[2] During the COol Companions ON Ultrawide orbiTS (COCONUTS) survey, its association with L 34-26 was announced in 2021. ^[6] At a distance of 35.5 light-years (10.9 parsecs ), COCONUTS-2b was the closest directly imaged exoplanet to Earth ^[7] 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 may instead have formed separately. ^{[4] [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. ^[5]

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. ^{[2] [4]}

Due to its large orbital separation, COCONUTS-2b is a great laboratory to study the atmosphere and composition of young gas-giant exoplanets. ^[3] Astronomers estimate the planet's temperature to be around 434 K (161 °C; 322 °F). ^[4]

Observations with Gemini/Flamingos-2 showed a spectral type of T9.5±0.5, near the T/Y transition. The spectrum is also more consistent with disequilibrium chemistry 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. ^[5]

Host star

L 34-26

L 34-26 (COCONUTS-2A) and its planetary companion in unWISE
Observation data Epoch J2000.0       Equinox J2000.0
Constellation	Chamaeleon
Right ascension	07h 49m 12.678s [10]
Declination	−76° 42′ 06.72″ [10]
Apparent magnitude  (V)	11.3 [11]
Characteristics
Evolutionary stage	Red dwarf
Spectral type	M3V [11]
Astrometry
Radial velocity (Rv)	1.19±0.61 [12]  km/s
Proper motion (μ)	RA: -102.154  mas/yr [10] Dec.: -192.918  mas/yr [10]
Parallax (π)	91.8263 ± 0.0185  mas [10]
Distance	35.519 ± 0.007  ly (10.890 ± 0.002  pc)
Details
Mass	0.37±0.02 [4]   M☉
Radius	0.388±0.011 [4]   R☉
Temperature	3,406±69 [4]   K
Age	150–800 [4]   Myr
Other designations
L 34-26, COCONUTS-2A, WISEPA J075108.79-763449.6, 1RXS J074912.9-764202, 2MASS J07491271-7642065, NLTT 18592, TIC 272232401, TYC 9381-1809-1, UCAC4 067-006518
Database references
SIMBAD	data

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. ^[13]

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. ^[14] 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. ^[9]

Gallery

• 
  The COCONUTS-2 system with unWISE
• 
  The planet COCONUTS-2b with Gemini-South
• 
  Allwise image by the discoverers, showing planet COCONUTS-2b

References

1. "Massive COCONUTS exoplanet discovery led by UH grad student | University of Hawaiʻi System News". University of Hawaiʻi News. University of Hawaiʻi at Mānoa. Archived from the original on 15 December 2021. Retrieved 21 December 2021.
2. 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. arXiv: 1108.4677 . Bibcode:2011ApJS..197...19K. doi:10.1088/0067-0049/197/2/19. ISSN   0067-0049. S2CID   16850733.
3. Kooser, Amanda. "Massive exoplanet 'Coconuts-2b' could help reveal the secrets of young gas giants". CNET . Archived from the original on 21 December 2021. Retrieved 21 December 2021.
4. 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". The Astrophysical Journal. 916 (2): L11. arXiv: 2107.02805 . Bibcode:2021ApJ...916L..11Z. doi: 10.3847/2041-8213/ac1123 . ISSN   0004-637X.
5. Zhang, Zhoujian; et al. (2025). "Disequilibrium Chemistry, Diabatic Thermal Structure, and Clouds in the Atmosphere of COCONUTS-2b". The Astronomical Journal. 169 (1) 9. arXiv: 2410.10939 . Bibcode: 2025AJ....169....9Z . doi: 10.3847/1538-3881/ad8b2d .
6. "Exoplanet-catalog". Exoplanet Exploration: Planets Beyond our Solar System. Archived from the original on 2021-11-22. Retrieved 2021-11-22.
7. Siegel, Ethan. "Astronomers Go Nuts For Closest Exoplanet Directly Imaged Ever: COCONUTS-2b". Forbes. Archived from the original on 2021-11-22. Retrieved 2021-11-22.
8. Marley, Mark S.; Fortney, Jonathan J.; Hubickyj, Olenka; Bodenheimer, Peter; Lissauer, Jack J. (2007-01-01). "On the Luminosity of Young Jupiters". The Astrophysical Journal. 655 (1): 541–549. arXiv: astro-ph/0609739 . Bibcode:2007ApJ...655..541M. doi:10.1086/509759. ISSN   0004-637X. S2CID   7793365.
9. Marocco, Federico; Kirkpatrick, J. Davy; Schneider, Adam C.; et al. (2024-04-22). "Thirteen New M Dwarf + T Dwarf Pairs Identified with WISE/NEOWISE". The Astrophysical Journal. 967 (2): 147. arXiv: 2404.14324 . Bibcode:2024ApJ...967..147M. doi: 10.3847/1538-4357/ad3f1d .
10. Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv: 2208.00211 . Bibcode:2023A&A...674A...1G. doi: 10.1051/0004-6361/202243940 . S2CID   244398875. Gaia DR3 record for this source at VizieR.
11. Martin, Pierre-Yves (2021). "Planet COCONUTS-2 b". Extrasolar Planets Encyclopaedia . Retrieved 2023-11-28.
12. "Simbad - Object view". simbad.cds.unistra.fr. Retrieved 2023-11-28.
13. "Giant Exoplanet Orbits Its Host Star Once Every 1.1 Million Years | Astronomy | Sci-News.com". Breaking Science News | Sci-News.com. Archived from the original on 2021-11-22. Retrieved 2021-11-22.
14. Stelzer, B.; Bogner, M.; Magaudda, E.; Raetz, St. (2022). "Flares and rotation of M dwarfs with habitable zones accessible to TESS planet detections". Astronomy and Astrophysics. 665: A30. arXiv: 2207.03794 . Bibcode:2022A&A...665A..30S. doi:10.1051/0004-6361/202142088. S2CID   249662585.