Free Astronomy Magazine July-August 2025

6 JULY-AUGUST 2025 ASTRO PUBLISHING P lastic representation of the mole- cules of dimethyl sulfide (DMS) on the left and dimethyl disulfide (DMDS) on the right. [ResearchGate] T his artist's illustration shows what the red dwarf star system K2-18 might look like based on scientific data. K2-18 b is the planet in the foreground on the right, while the star is in the background at lower left. Between the two is the small inner planet K2-18 c. [NASA, CSA, ESA, J. Olmstead (STScI), N. Mad- husudhan (University of Cambridge)] This scenario has convinced scientists to include DMS among the most in- teresting biosignature molecules (about twenty in total) that we can expect to recognize in the atmos- pheres of exoplanets that host life, making it an important molecule to look for during transmission spec- troscopy experiments. This technique, applicable to most exoplanets with atmospheres that transit in front of their star along our line of sight, es- sentially consists in subtracting the spectrum of the star when the exo- planet is not transiting from the one taken during a transit. The difference provides information on the nature of the gases present in the exoatmos- phere. In practice, a tiny fraction of the starlight passes through the at- mosphere of the exoplanet during transits before reaching the Earth. This passage leaves faint traces in the stellar spectrum, which astronomers can piece together to determine the gases that make up the exoplanet’s atmosphere. What exactly is K2-18 b? Exactly, no one knows. The various teams of researchers who have stud- ied the planet so far have come to conflicting conclusions. Depending on the comparison models, K2-18 b has turned out to be a super-Earth rather than a mini-Neptune or sub- Neptune. There is currently a con- troversial tendency to classify it as a “hycean” planet. This term was coined about four years ago by the same team of researchers, led by Professor Nikku Madhusudhan, who announced the discovery of DMS in the K2-18 b atmosphere. Hycean is a syncretism of hydrogen and ocean , and indicates a planet en- tirely and solely covered by a very deep ocean of water, overlaid by a thick atmosphere composed essen- tially of hydrogen. Astronomers don’t know for sure whether these worlds actually exist, but the models that describe them fit at least in part to the case of K2-18 b. Granted,