Dominant composition of K2-18 b’s volatile-rich interior layers

Determine whether the volatile-rich interior layers of the temperate sub-Neptune K2-18 b are predominantly composed of water (H2O) or hydrogen/helium (H2/He), using the planet’s measured mass (8.6 M⊕), radius (2.6 R⊕), and atmospheric constraints from JWST transmission spectroscopy to distinguish between water-rich and H2/He-rich interior scenarios.

Background

K2-18 b, a temperate sub-Neptune orbiting an M2.5V star, has a mass and radius that indicate substantial volatile-rich layers. However, interior structure models allow both water-rich and H2/He-rich possibilities, and transmission spectra alone do not directly reveal the bulk interior composition.

Resolving whether these layers are predominantly water or H2/He is central to assessing the planet’s capacity to host a liquid-water ocean and to understanding its formation history and volatile inventory.