2025MNRAS.543..789R

A recent observation of the exoplanet K2-18b sparked interest among scientists - large amounts of carbon dioxide and methane were detected in an H$_2$-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 N$_2$ such as HCN, NH$_3$, and HC$_3$N. 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 N$_2$ present from 10 ppm to 10 per cent. If the bulk nitrogen-containing gas in the atmosphere is N$_2$, photochemistry produces only trace amounts of the aforementioned species. However, if ammonia is the main source of nitrogen, then the quantities of NH$_3$, CH$_5$N, and HCN approach detectable range. HC$_3$N 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 CO$_2$ abundance. This result combined with the non-detection of ammonia by James Webb Space Telescope (JWST) suggests that the planet is not a typical sub-Neptune, but could be indeed a hycean world or magma ocean planet. We also found that C$_2$H$_6$ is produced in significant amounts - if it is detected in the future, it could serve as a proxy for DMS presence.