Free Astronomy Magazine November-December 2023

NOVEMBER-DECEMBER 2023 are unlike anything in our solar sys- tem. This lack of equivalent nearby planets means that these ‘sub-Nep- tunes’ are poorly understood, and the nature of their atmospheres is a matter of active debate among as- tronomers. The suggestion that the sub-Nep- tune K2-18 b could be a Hycean ex- oplanet is intriguing, as some as- tronomers believe that these worlds are promising environments to search for evidence for life on exo- planets. “Our findings underscore the im- portance of considering diverse habitable environments in the search for life elsewhere,” ex- plained Nikku Madhusudhan, an as- tronomer at the University of Cambridge and lead author of the paper announcing these results. “Traditionally, the search for life on exoplanets has focused primarily on smaller rocky planets, but the larger Hycean worlds are significantly more conducive to atmospheric ob- servations.” The abundance of methane and car- bon dioxide, and shortage of am- monia, support the hypothesis that there may be a water ocean under- neath a hydrogen-rich atmosphere in K2-18 b. These initial Webb ob- servations also provided a possible detection of a molecule called di- methyl sulfide (DMS). On Earth, this is only produced by life. The bulk of the DMS in Earth’s atmosphere is emitted from phytoplankton in ma- rine environments. The inference of DMS is less robust and requires further validation. “Upcoming Webb observations should be able to confirm if DMS is indeed present in the atmosphere of K2-18 b at significant levels,” ex- plained Madhusudhan. While K2-18 b lies in the habitable zone, and is now known to harbor carbon-bearing molecules, this does not necessarily mean that the 29 ASTRO PUBLISHING