Nitrogen Chemistry Of Hycean Worlds On The Example Of K2-18b

A recent observation of the exoplanet K2-18b sparked interest among scientists – large amounts of carbon dioxide and methane were detected in an H₂-rich background atmosphere.

If the planet is a hycean world (liquid water ocean + hydrogen-dominated atmosphere), it could be habitable under certain conditions. The presence of carbon, hydrogen and oxygen was already confirmed, however, there was no detection of nitrogen or its compounds. Molecular nitrogen is difficult to detect directly.

This study concentrates on possible photochemical products of N₂ such as HCN, NH₃ and HC₃N. We set approximate limits on the amount of nitrogen bearing species by varying atmospheric parameters, such as the Eddy Diffusion coefficient and the amount of N₂ present from 10 ppm to 10%.

If the bulk nitrogen-containing gas in the atmosphere is N₂, photochemistry produces only trace amounts of the aforementioned species. However, if ammonia is the main source of nitrogen, then the quantities of NH₃, CH₅N and HCN approach detectable range. HC₃N and NO are bad tracers of the nitrogen source in the atmosphere, because they are produced in similar amounts in all tested scenarios. Assuming equilibrium chemistry at the surface of K2-18b results in underprediction of CO₂ abundance.

This result combined with the non-detection of ammonia by JWST suggests the planet is not a typical sub-Neptune, but could be indeed a hycean world or magma ocean planet. We also found that C₂H₆ is produced in significant amounts – if it is detected in the future, it could serve as a proxy for DMS presence.

Maja W. Radecka, Paul B. Rimmer

Comments: 21 pages, 23 figures, accepted for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2509.03455 [astro-ph.EP] (or arXiv:2509.03455v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2509.03455
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Submission history
From: Paul B. Rimmer
[v1] Wed, 3 Sep 2025 16:24:52 UTC (399 KB)
https://arxiv.org/abs/2509.03455
Astrobiology, Exoplanet,