K2-18 b is so far the smallest habitable-zone world with a confirmed water vapour detection (Tsiaras et al. 2019).
This detection has sparked intense debate regarding the nature of this world: water world or sub-Neptune? Seemingly sitting in the sub-Neptune region of the radius valley, the internal structure of K2-18b remains unknown. Large uncertainties on the radius of the star have caused the planet radius to be poorly defined, also affecting the calculated density. Hence, while the atmosphere of K2-18 b could contain a large amount of hydrogen and/or helium, it could also be a water world.
While these worlds are at the limits of our capabilities with current instrumentation in transmission spectroscopy, they will be within reach of future facilities such as JWST, ELTs and Ariel (Changeat et al. 2021).
UCL has produced a press release
An artist's impression of the planet K2-18b. Credit ESA/Hubble