L 98-59 d orbits a red dwarf 35 light-years away. It's 1.6 times Earth's diameter but has an unusually low density for its size. JWST observations published in Nature Astronomy (Oxford, March 2026) detected hydrogen sulfide in its atmosphere — a gas that should be stripped away by the host star's X-ray radiation. Something is replenishing it.
The answer is a global magma ocean. The planet's mantle is molten silicate extending thousands of kilometers deep, and this reservoir dissolves and stores enormous quantities of sulfur. The magma slowly outgasses hydrogen sulfide into the atmosphere, replacing what radiation destroys. The atmosphere persists not because it's stable, but because the interior continuously rebuilds it.
The researchers propose L 98-59 d as the first recognized member of a new planetary class: gas-rich, sulfurous worlds sustained by permanent magma oceans. These planets look wrong by existing models — too much atmosphere for their size and stellar environment — because the models assume a solid surface. A molten interior changes the volatile budget entirely. The magma isn't just hot rock. It's a chemical reservoir that buffers the atmosphere against loss.
The structural insight is that planetary atmospheres can be driven from below rather than retained from above. Earth's atmosphere is primarily retained by gravity against solar wind erosion — a balance of holding and losing. L 98-59 d's atmosphere is primarily maintained by volcanic outgassing against radiation stripping — a balance of producing and losing. Same observable (atmospheric gases), different mechanism (retention versus generation). The distinction matters because generation-maintained atmospheres can exist around stars whose radiation would strip any purely retained atmosphere. The habitable zone calculation changes when the planet makes its own air.