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L 98-59 d: sulphur-rich magma-ocean world — a new planet class

2 min read

Summary

A University of Oxford-led team reports that l-98-59-d, a 1.6 R⊕ planet orbiting the M dwarf l-98-59 ~35 light-years away in Volans, does not fit the rocky / gaseous / water-rich categories and instead represents a new planetary class: a persistent, global sulphur-rich silicate magma-ocean world. Coupled atmosphere–interior modelling plus transit and spectroscopic data indicate a thick sulphur-bearing atmosphere drives a runaway greenhouse that keeps the mantle molten over billions of years. The planet is lower density than expected for a terrestrial composition and likely evolved from a sub-Neptune precursor that lost much of its primordial envelope. Lead author harrison-nicholls argues current small-planet taxonomy is too simple. Co-author raymond-pierrehumbert frames the work as a demonstration that interior history of distant worlds can be reconstructed from mass, radius, and atmospheric data alone. The team expects more such molten worlds to be found by next-generation telescopes (jwst and successors).

Key Claims

  • L 98-59 d is ~1.6 R⊕, lower density than terrestrial expectation, 35 ly distant in Volans, orbiting M dwarf L 98-59.
  • It represents a brand-new planet class — molten, sulphur-rich, global silicate magma ocean — distinct from rocky, gaseous, and water-rich categories.
  • A thick atmosphere of heavy sulphur-bearing molecules drives an intense greenhouse effect preventing crust formation.
  • The magma ocean persists for billions of years (unlike early Earth, which cooled to a solid crust).
  • Formation pathway: likely began as a sub-Neptune that shrank and lost much of its primordial atmosphere.
  • Methodology combines advanced atmosphere–interior modelling with observational data; interior reconstructed from mass, radius, and atmospheric composition.
  • Implication: current small-planet taxonomy is too coarse; sulphur magma worlds are likely a broader population.
  • Habitability: the surface is uninhabitable, but the result widens the diversity catalogue framing biosignatures / false-positive-biosignatures interpretation.

Notable Quotes

“This discovery suggests that the categories astronomers currently use to describe small planets may be too simple.” — harrison-nicholls

“What’s exciting is that we can use computer models to uncover the hidden interior of a planet we will never visit … it is possible to reconstruct the deep past of these alien worlds — and discover types of planets with no equivalent in our own Solar System.” — raymond-pierrehumbert

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