L 98-59 d: new sulfur-rich molten exoplanet found — SkimNews
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- University of Oxford researchers, with the University of Groningen, University of Leeds and ETH Zurich, ran simulations reconstructing nearly five billion years of evolution for L 98-59 d, a planet about 1.6 times Earth's size orbiting a red dwarf 35 light-years away.
- L 98-59 d has a mantle of molten silicate forming a global magma ocean thousands of kilometers deep that stores enormous amounts of sulfur and helps the planet retain a hydrogen-rich atmosphere laced with hydrogen sulfide (H2S) that would otherwise be stripped away by stellar X-ray radiation.
- James Webb Space Telescope observations from 2024 detected sulfur dioxide and other sulfur gases high in L 98-59 d's upper atmosphere, which the team's models show are produced when ultraviolet light from the host star triggers chemistry buffered by the magma ocean below.
- L 98-59 d does not fit existing exoplanet categories of rocky gas-dwarf or water world, and may have originally resembled a larger sub-Neptune before shrinking as it cooled and shed atmosphere, according to the simulations.
- Lead author Dr. Harrison Nicholls said current categories for small planets 'may be too simple,' while co-author Prof. Raymond Pierrehumbert noted the work shows it is possible to reconstruct the deep interior of a planet 'we will never visit.'
- Co-author Dr. Richard Chatterjee flagged hydrogen sulfide—'responsible for the smell of rotten eggs'—as playing a starring role, adding that further investigation may show such 'rather pungent planets are surprisingly common.'
- Because magma oceans are the universal starting state of all rocky planets, the findings also inform models of Earth and Mars's primordial history; the team plans to apply the simulations to upcoming Ariel and PLATO mission data using machine learning.
Why it matters: By overturning the two-bucket classification of small exoplanets (rocky gas-dwarf vs. water world), the Oxford-led study expands the inventory of known planetary architectures and hands astronomers a new template for interpreting JWST spectra—while also providing a window into the molten early Earth and Mars, since all rocky planets begin life with magma oceans.
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