Are sub-Neptunes rocky cores with hydrogen or water worlds?
In plain words
A planet two or three times Earth's size can be a rocky ball with a thin hydrogen layer or a world with a large fraction of water, and mass and radius alone cannot tell which. Their atmospheres carry clues.
Precise statement
For planets with $R \sim 2-3.5\,R_{\mathrm{earth}}$ and $M \sim 5-15\,M_{\mathrm{earth}}$, determine the bulk composition: rocky cores with H/He envelopes of $\sim 1-3\text{ percent}$ by mass, water mass fractions of $\sim 10-50\text{ percent}$ in supercritical layers, or magma oceans that dissolve and mix with hydrogen. The answer is a classification of individual planets and the population fractions, using JWST transmission spectra (mean molecular weight, H2O, CH4, CO2, NH3 abundances) together with interior models.
What would settle it
High signal-to-noise transmission and emission spectra of a sample of temperate and warm sub-Neptunes fitted with coupled interior-atmosphere models.
Related problems
- More general than How do magma oceans change sub-Neptune atmospheres?
- More general than Do sub-Neptunes with more than 10 percent water exist?