Free Astronomy Magazine July-August 2025

7 JULY-AUGUST 2025 ASTRO PUBLISHING T he size of the Earth compared to that of K2-18 b. The image of the latter is a pure figment of the imagi- nation, since we cannot know its true appearance. A stro- physicist Nikku Mad- husudhan poses for a portrait at the Institute of Astron- omy, Univer- sity of Cambridge, England, April 2025. [Atlantic Stu- dios via AP] but not granted, that this planet is a hycean, the only peculiar charac- teristic that it would have in com- mon with Earth would be that of having liquid water on the surface. For everything else, it is much differ- ent: it has a mass between 7 and 10 times greater, a radius 2.6 times that of Earth, and a density that is less than half that of our planet. Even the star that hosts K2-18 b (124 light years away from us) is very different from the Sun; it is, in fact, a red dwarf (M2.5V), around which the planet revolves in just 33 days at an average distance of 23.8 million km, or 14.8 million miles. (There is also a second known planet in the system, K2-18 c, which is irrelevant here.) Since the star K2-18 has approxi- mately half the mass and radius of the Sun, the energy flux per unit of surface area that K2-18 b receives is slightly lower than what the Earth receives from the Sun; it follows that K2-18 b orbits within the K2-18 habitable zone. This peculiarity has prompted researchers to study it since its discovery, which occurred in 2015 during the extended mission of T he banner of NASA's K2 mission, which, using the Kepler telescope, has pro- vided precise photometric data of many target fields in the ecliptic, discovering over 300 exoplanets and many hundreds more exoplanet candidates. [NASA] NASA's Kepler space telescope (K2), in the 18 th extrasolar system discov- ered along the ecliptic plane. The 2023 observations Although the sensational announce- ment of the possible existence of life on K2-18 b did not go around the world until last April, the presence of DMS in the atmosphere of that planet had already been proposed in 2023, also on that occasion by Madhusudhan’s team. It all started even earlier, in 2019, when the Hub- ble Space Telescope had taken spec- tra of K2-18 b, revealing a thick atmosphere rich in hydrogen (H 2 ), clouds, and also the possible detec- tion of water vapor (H 2 O). The rea- son why water vapor was indicated only as a “possible detection” is that Hubble can only see a small part of