A Planet of Fire and Gas: How Magma Oceans May Explain TOI-270 d’s Mysterious Atmosphere

The sub-Neptune TOI-270 d, located around a cool M-dwarf star, has puzzled astronomers with its unexpectedly rich atmosphere. Matthew C. Nixon et al. investigate whether interactions between a molten interior and the surrounding gas, called magma-atmosphere interactions, can explain the chemical signatures that the James Webb Space Telescope (JWST) has detected. These signatures include large amounts of water (H₂O), methane (CH₄), and carbon dioxide (CO₂).

Introduction: A New Kind of World

Planets between Earth and Neptune in size, known as sub-Neptunes, are common in the galaxy but have no direct counterparts in our Solar System. Observations suggest that some have thick hydrogen-helium envelopes, while others may contain water-rich layers. JWST has revealed that many of these worlds possess atmospheres far richer in heavy elements, hundreds of times more “metallic” than the Sun, than scientists expected. TOI-270 d, for instance, has a metallicity about 225 times solar. Traditionally, this was thought to result from accreting icy material when the planet formed beyond the solar system’s “snow line.” Nixon and colleagues, however, explore an alternative: could ongoing chemical exchange between a molten mantle and its atmosphere naturally create these same signatures?

Modeling Approach: Linking the Deep and the Shallow

To test this, the authors developed a multi-layered model that connects a planet’s deep magma ocean to its upper observable atmosphere. Starting with an equilibrium chemistry model developed by Schlichting & Young (2022), they simulate how the core, mantle, and atmosphere exchange elements like hydrogen, carbon, and oxygen under extreme pressures. This output then feeds into HELIOS, a radiative-convective model that calculates temperature profiles, and FastChem Cond, which predicts how gases condense or remain in the atmosphere. Finally, they incorporate Photochem, which accounts for sunlight-driven reactions and vertical mixing, before producing synthetic JWST transmission spectra with the Aura-3D tool.

The team varied parameters such as the temperature at the core-mantle boundary, the temperature where magma meets the atmosphere, and the proportion of iron in the planet’s interior. These variables determine how readily elements react and move between layers, ultimately shaping the atmosphere’s metallicity and carbon-to-oxygen ratio (C/O).

Results: Magma Interactions Reproduce JWST Observations

Across dozens of simulated cases, Nixon et al. found that magma-atmosphere interactions alone can produce enhanced metallicity and low C/O ratios consistent with TOI-270 d’s measured values. Models with hotter interiors and higher iron content yielded upper atmospheres rich in H₂O and CH₄, matching JWST’s findings without requiring any additional delivery of icy material.

The models also predicted that vertical mixing within the atmosphere keeps gases like CO₂ and CO abundant at high altitudes, even though they form deeper down. When compared directly to the observed JWST spectrum, the best-fit model, with boundary temperatures of 3000 K and 4000 K and an iron fraction of about 33%, matched the data remarkably well. The team even explored variations in nitrogen abundance, showing that its depletion could explain the non-detection of ammonia (NH₃) while leaving other molecules largely unaffected.

Discussion: A Planet Forged by Fire

These findings suggest that TOI-270 d’s atmosphere may not require icy origins or late bombardment by comets to explain its composition. Instead, ongoing reactions between a molten mantle and gaseous envelope could gradually enrich its air with carbon and oxygen compounds. The study also highlights that both magma-driven and ocean-driven scenarios might lead to similar observable spectra, meaning that distinguishing between a “planet of fire” and a “planet of water” will demand further modeling and observations.

Conclusions: The Broader Implications

By linking planetary interiors and observable atmospheres in a single self-consistent framework, Nixon et al. show that magma-ocean interactions can naturally produce the molecular features JWST detects on sub-Neptunes like TOI-270 d. This work expands the toolkit for interpreting exoplanet spectra and demonstrates that interior processes can shape what telescopes see. As JWST continues to survey more sub-Neptunes, these results underscore a growing realization: the chemistry of a planet’s atmosphere is often written deep within its molten heart.

Source: Nixon