ASTRO In the literature: open

Do sub-Neptunes with more than 10 percent water exist?

In plain words

Planets that formed beyond the line where water freezes and then moved inward could be up to half water by mass. Whether any such water worlds exist among close-in sub-Neptunes has not been shown.

Precise statement

Determine whether any planet with $R \sim 1.5-3.5\ R_{\mathrm{earth}}$ and $P < 100\ \mathrm{d}$ has a water mass fraction above 0.1, demonstrated by a combination of bulk density and an atmospheric mean molecular weight and H2O abundance that a rocky core with an H/He envelope or a magma-ocean model cannot reproduce. The answer is yes or no, with the planets and water fractions if yes.

What would settle it

JWST spectra giving high mean molecular weight and H2O-dominated composition for a planet whose mass and radius are measured to better than 10 and 3 percent, fitted with interior models that include supercritical water and hydrogen-water mixing.

Status in the literature

Unverified note

A 2025 JWST analysis interpreted K2-18 b as water-rich (arXiv:2507.12622), but gas-rich mini-Neptune and magma-ocean models also fit its spectrum.