# COCONUTS-2b

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.
The planet was discovered in 2011 and was initially identified as a T9 free-floating brown dwarf WISEPA J075108.79−763449.6. During the COol Companions ON Ultrawide orbiTS (COCONUTS) survey, its association with L 34-26 was announced in 2021. At a distance of 35.5 light-years (10.9 parsecs), COCONUTS-2b was the closest directly imaged exoplanet to Earth 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.
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.
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.
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 434 K (161 °C; 322 °F).
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.

## 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.
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. 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.

## Gallery

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