Revision 1

Superseded · first revision · record

Title: Do sub-Neptunes with more than 10 percent water exist?
Plain: 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: 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.
Settled by: 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.
Kind: well-posed
Answer type: yes-no
References: (empty)
Literature status: open
Status 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.
Posed since: (empty)