There might be magma beneath the clouds of TOI-270d

Sub-Neptunes are among the most common planets in our galaxy, yet their true nature remains mysterious. Observations of TOI-270 d with the James Webb Space Telescope have revealed a surprising twist — its atmosphere may be shaped not by icy origins but by chemical reactions between molten rock and gas. In today’s Astrobito, we explore how magma–atmosphere interactions could explain TOI-270 d’s water and carbon-rich skies, and what this means for our understanding of planet formation and the diversity of worlds beyond our Solar System!

Paper details:

Title: Magma ocean interactions can explain JWST observations of the sub-Neptune TOI-270 d

Authors: Matthew C. Nixon, R. Sander Somers, Arjun, B. Savel, et. al.

First-author institution: School of Earth and Space Exploration, Arizona State University, Tempe, USA

Status: Open access arXiv.

TOI-270d belongs to one of the most common yet least understood types of exoplanets in our galaxy: a sub-Neptune, a world larger than Earth but smaller than Neptune. These planets, abundant across the Milky Way but absent in our own Solar System, have puzzled scientists for years. What are they made of? Are they rocky like Earth, icy like Neptune, or something in between?

A new study looking into the atmosphere of TOI-270d might shed some light into understanding the chemical formation of sub-Neptune exoplanets. Simulations suggest intense magma–atmosphere exchanges that can generate the water, carbon dioxide, and methane detected by JWST on this planet, without the need for the planet to have formed from icy material further from its star.

TOI-270d

TOI-270d orbits a small red dwarf star about 73 light-years away, in a compact system with two smaller sibling planets. Roughly 2.1 times the radius of Earth and about 5 Earth masses, it sits in a category between super-Earths and mini-Neptunes, which are planets with thick atmospheres but possibly rocky or partially molten surfaces below.

Two research teams have used JWST to probe its atmosphere. One group reported signatures of water vapour (H2O), methane (CH4), and carbon dioxide (CO2), which are molecules often linked to a high-metallicity atmosphere, one rich in heavy elements compared to hydrogen and helium. While another team suggested the atmosphere might even sit atop a liquid water ocean. That claim sparked debate: could TOI-270d really host a layer of liquid water, or could the observed gases be produced by a more exotic, high-temperature process?

The research paper addressed in this article offers a compelling third option: a magma-ocean world, where molten rock and gas are locked in a constant exchange, shaping what telescopes see hundreds of light-years away.

The case for a molten world

On young, close-in planets like TOI-270d, temperatures deep below the atmosphere can exceed 3000–4000 K, hot enough to melt silicate rocks and create a global ocean of magma. These molten layers can react chemically with the atmosphere above, exchanging oxygen, carbon, and hydrogen.

In this research’s models, an initially hydrogen-dominated atmosphere reacts with this molten surface, producing secondary gases such as water and carbon dioxide while consuming much of the free hydrogen. Over time, these reactions raise the planet’s metallicity (the proportion of heavier elements) and alter its carbon-to-oxygen ratio (C/O).

JWST observations of TOI-270d (yellow data points) align closely with simulations suggesting an intense magma-atmosphere chemical exchange (coloured lines).

This is significant. Astronomers have long assumed that metal-rich exoplanet atmospheres must come from formation beyond the “snow line”, where icy material dominates and volatile-rich planets can form. But TOI-270d challenges that idea. Its atmospheric composition could instead be the natural consequence of magma–atmosphere interactions, meaning the planet may have formed much closer to its star and still ended up with a water- and carbon-rich sky.

Lessons from other sub-Neptunes

TOI-270d is not the only sub-Neptune to show this pattern. Another well-studied planet, K2-18b, has also revealed a mix of CO2 and CH4 in its atmosphere, prompting speculation about whether such worlds could host habitable oceans or, alternatively, magma-driven chemistry beneath dense clouds.

What’s striking is that both the liquid-ocean and magma-interaction scenarios can reproduce the same broad spectral features. This study emphasises that until scientists can identify unique chemical markers, distinguishing between “warm ocean” and “molten mantle” planets will remain challenging.

Still, the implications are profound. If magma–atmosphere chemistry proves common, it could mean that many sub-Neptunes are not icy mini-Neptunes at all, but rocky, volatile-processed worlds.

Broader implications

Magma–atmosphere interaction models may also explain a broader puzzle in exoplanet science: why so many sub-Neptunes show high metallicities despite orbiting close to their stars, where icy material is scarce.

If molten interiors can enrich their own atmospheres through chemical exchange, it could mean that metal-rich skies don’t require ice-rich beginnings. This would simplify models of planetary migration and help explain why sub-Neptunes come in such a diverse range of densities and compositions.

It may also hint at why our Solar System lacks sub-Neptunes altogether. The specific balance of mass, temperature, and formation history required for long-lived magma oceans may simply not have occurred around our Sun, making systems like TOI-270 unique laboratories for studying what might have been.

What comes next

The authors are clear that more data are needed. Future JWST observations, especially at longer infrared wavelengths, could reveal new molecular signatures or refine existing abundance estimates.

Beyond TOI-270 d, several other sub-Neptunes are now scheduled for JWST observation. If their spectra show similar chemical fingerprints (high water and CO2 with depleted hydrogen) it could signal that magma oceans are a common stage in the evolution of small planets.

Page updated

Google Sites

Report abuse