Fundamental nature of sub-Neptune exoplanets

Determine whether sub-Neptune exoplanets (temperate exoplanets with sizes between Earth and Neptune) are more similar to (i) terrestrial planets with thin atmospheres and potentially habitable surfaces, (ii) mini-Neptunes that are less massive analogs of Solar System ice giants with deep hydrogen–helium atmospheres lacking a habitable surface, or (iii) intermediate volatile-rich configurations such as water worlds with substantial H2O interiors.

Background

Sub-Neptune exoplanets constitute a large fraction of known exoplanets but exhibit strong mass–radius degeneracies, allowing multiple interior structures to fit current observations. This degeneracy leads to uncertainty about whether these worlds are rocky with thin atmospheres, Neptune-like with deep H/He envelopes, or volatile-rich water worlds.

K2-18 b, a canonical temperate sub-Neptune, has recent JWST detections of CH4 and CO but a nondetection of CO, intensifying interest in using atmospheric composition to constrain interior structure. However, despite these advances, the overarching classification of sub-Neptunes as terrestrial-like, mini-Neptune-like, or water-world-like remains unresolved.