A magma ocean world is a small-planet class proposed by harrison-nicholls, raymond-pierrehumbert et al. (university-of-oxford) in 2026 (src-l-98-59-d-sulphur-magma-world-2026-05). Unlike early Earth — which underwent a transient magma-ocean phase before its crust cooled — magma ocean worlds maintain a persistent, global silicate magma ocean over billions of years, sustained by a thick heavy-molecule atmosphere driving a runaway greenhouse effect that prevents crust formation.
Prototype
l-98-59-d — 1.6 R⊕, lower density than expected for terrestrial composition; orbits the M dwarf l-98-59 ~35 ly away in Volans. Atmosphere dominated by heavy sulphur-bearing molecules (sulphur-atmosphere). Likely formed via atmospheric loss from a sub-Neptune precursor.
Significance
- Adds a fourth category to the rocky / gaseous / water-rich planetary taxonomy.
- Surface is uninhabitable, but the class widens the planetary diversity catalogue used to interpret biosignatures and false-positive-biosignatures.
- Demonstrates that coupled atmosphere–interior modelling can reconstruct deep planetary history from mass, radius, and atmospheric composition alone.
- Sulphur-magma worlds may be a broad population once more next-generation telescope characterization is available (jwst and successors).