An article (file in PDF format) accepted for publication in the journal “The Astrophysical Journal Letters” reports the results of a study of the exoplanet K2-18 b, including the discovery of carbon compounds such as methane in its atmosphere. A team of researchers led by Nikku Madhusudhan of the British University of Cambridge used observations conducted with the James Webb Space Telescope’s NIRISS and NIRSpec instruments to probe the atmosphere of K2-18 b. It’s an exoplanet already considered interesting due to its characteristics and its position in its planetary system’s habitable zone. Spectral analyzes also suggest the possible presence of dimethyl sulfide (DMS), a compound considered a biomarker. These are discoveries that add to the previous ones regarding an exoplanet that led to the proposal of the Hycean class.
About 120 light-years away from Earth, the exoplanet K2-18 b has been considered interesting since its discovery because it’s in its planetary system’s habitable zone. However, its mass is about 8.6 times the Earth’s, and this means that it’s a sub-Neptunian with a hydrogen-rich atmosphere, therefore conditions very different from those on Earth. Despite this, interest continued and indeed was taken as an example for a new possible class of potentially habitable planets.
Observations of the exoplanet K2-18 b conducted with the James Webb Space Telescope were awaited and they finally arrived, offering new information that’s very interesting. In particular, the NIRISS (Near-Infrared Imager and Slitless Spectrograph) and NIRSpec (Near-Infrared Spectrograph) instruments were used to find the traces that the various compounds present in the atmosphere of K2-18 b leave in the electromagnetic emissions that arrive on Earth from it.
The detection of methane and carbon dioxide along with the scarcity of ammonia support the hypothesis that there may be an ocean beneath the hydrogen-rich atmosphere of the exoplanet K2-18 b. These are the two characteristics that led to the proposal of the Hycean class of planets. The authors of this study include the ones who published the study containing that proposal but other scientists are also becoming interested in it and the habitability potential of exoplanets of this type.
The spectrum shown in the bottom image (NASA, CSA, ESA, J. Olmstead (STScI), N. Madhusudhan (Cambridge University)) also shows possible traces of dimethyl sulfide, a compound that on Earth is produced only by living organisms. For this reason, it was indicated as a possible marker of the presence of life on an exoplanet, be it a super-Earth or a Hycean. Its detection is currently uncertain and more spectrometric observations will be necessary to try to verify its presence.
Nikku Madhusudhan explained that his team’s ultimate goal is to identify life on a habitable exoplanet, an achievement that would transform our understanding of our place in the universe. It’s a very ambitious goal, and it’s really interesting considering that it’s focused on a type of planet with characteristics very different from those of Earth.
Savvas Constantinou, another of the authors of this study and the proponents of the Hycean class of planets, pointed out that these results were obtained with only two observations of the exoplanet K2-18 b using the James Webb Space Telescope and that this means that this is just an early demonstration of what Webb can observe in habitable-zone exoplanets.
For this reason, the researchers intend to conduct follow-up observations of K2-18 b using another Webb instrument, MIRI (Mid-Infrared Instrument). This will allow to probe this exoplanet in other infrared frequencies and obtain new information on its habitability potential. All the information collected will also be useful for the search for other mini-Neptunes that may belong to the Hycean class they can study.