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COCONUTS-2b
Gas giant exoplanet orbiting L 34-26 From Wikipedia, the free encyclopedia
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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.
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Proposed formation scenarios
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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 in 2024 found that the metallicity of the planet is lower than that of the host star, which is inconsistent with it forming on its current location, like a binary system. 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]
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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]
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Host star
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L 34–26, also known as COCONUTS-2A and TYC 9381–1809–1, is a M3-type dwarf star located 35 light-years from Earth, 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
External links
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