James Webb Telescope Finds Many Sub-Neptunes May Be Rocky Within

Sub-Neptunes—the most common exoplanets found by Kepler—were long assumed to host enduring magma oceans beneath hydrogen-rich atmospheres. New modelling now overturns that blanket assumption, finding that many such worlds likely possess solid silicate surfaces. The result reshapes how astronomers interpret James Webb Space Telescope (JWST) spectra of these planets.

What the new study shows

Researchers Bodie Breza, Matthew C. Nixon and Eliza M.-R. Kempton modelled hundreds of thousands of interior–atmosphere combinations for sub-Neptunes. They report that a significant fraction should lack present-day magma oceans; pressures at the envelope–mantle boundary can force silicates into solid post-perovskite phases, yielding “rock floors” instead of lava seas.

Levers that tip magma to solid

Three controls dominate: a large envelope (mantle) mass fraction combined with high atmospheric mean molecular weight, cooler photospheric temperatures or thinner atmospheres, and higher pressures at the radiative–convective boundary that rise as planets age. Together these can stabilise a solid surface where a magma ocean was expected.

Evidence from benchmark worlds

GJ 1214 b, the archetypal mini-Neptune, shows a hazy, heavy (high-MMW) atmosphere in recent JWST analyses, conditions that could compress and solidify underlying silicates. Similar high-MMW inferences for TOI-270 d and other sub-Neptunes strengthen the case that molten surfaces are not universal.

Exam Oriented Facts

• Sub-Neptunes sit between Earth and Neptune in size and are the galaxy’s most common planets.
• Mean molecular weight (MMW) is the average mass of atmospheric molecules; higher MMW raises surface pressure.
• JWST’s transmission spectra probe upper atmospheres, constraining composition and aerosol layers.
• The radiative–convective boundary marks where energy transport switches from radiation to convection, influencing interior pressure.

Implications for JWST interpretations

If many sub-Neptunes have solid, not molten, surfaces, retrieved atmospheric metallicities and molecules may reflect limited magma–atmosphere exchange. New frameworks that couple interiors to observables—tested on TOI-270 d—aim to distinguish lava-ocean worlds from solid-surface planets, refining target lists and climate models for future JWST cycles.