Hydrogen Atmospheres on Scorching Planets: The Volcano Secret express
Many planets around other stars — exoplanets — have elongated orbits. With each close approach, the star’s gravity squeezes and stretches them, like kneading dough. Friction heats up the interior to the point where rock melts, creating underground oceans of magma.
In this scorching mix, water from minerals breaks apart and releases hydrogen. The lightweight gas tries to escape, like steam from a pot, but non-stop volcanic eruptions belch out fresh supplies. The planet resembles a boiling soup: evaporation is offset by adding broth. This sustains a thin hydrogen envelope that would otherwise be whisked away into space.
James Webb detects such worlds when they pass in front of the star’s disk. The transmission spectroscopy method reveals absorption in a hydrogen haze — a sign of relentless volcanism. If there’s no envelope, then the interior has little water or the magma has cooled. Most remarkably, from the thickness of this layer, astronomers can gauge the eruption strength on a planet they’ll never see through a telescope.
Original abstract
as written by the authors arXiv:2510.07378
H$$_2$$-dominated terrestrial exoplanets are highly accessible to atmospheric characterization via transmission spectroscopy, but such atmospheres are generally thought to be unstable to escape. Here, we propose that close-in, eccentric terrestrial exoplanets can sustain H$$_2$$-dominated atmospheres due to intense tidally-driven volcanic degassing. We develop an interior-atmosphere framework to assess whether volcanic outgassing can sustain \ch{H2}-dominated atmospheres over geologic timescales ($$\geq$$1 Gyr). We incorporate interior redox state, tidal heating, volatile inventory, and planetary parameters to compute outgassing fluxes and confront them with energy-limited hydrodynamic escape. We demonstrate that to sustain an H$$_2$$-dominated atmosphere, a terrestrial exoplanet must have a water-rich basal magma ocean and reduced melts, in addition to high eccentricity. We additionally demonstrate that detection of a specifically thin H$$_2$$-dominated atmosphere is a sign of current magmatic outgassing. We delineate an "outgassing zone" (OZ) most favorable to the existence of such planets, and identify the most observationally compelling targets. We propose combining precise mass-radius-eccentricity measurements with JWST constraints on atmospheric mean molecular mass $$\mu$$ to search for thin H$$_2$$-dominated atmospheres. Inversely, we argue that robust atmospheric non-detections on OZ exoplanets can constrain the planetary interior, including melt redox state, mantle melt fraction and volatile inventory, and tidal heat flux.
On Jupiter’s moon Io, tidal heating powers hundreds of active volcanoes — the most powerful eruptions in the Solar System.