Lead
Using observations from NASA’s James Webb Space Telescope and ground-based facilities, researchers report a previously unrecognized class of world: a low-density, sulfur-rich exoplanet with a global magma ocean. The target, L 98-59 d, orbits a red dwarf about 35 light-years from Earth and is roughly 1.6 times Earth’s radius. Spectra reveal abundant sulfur-bearing gases—including hydrogen sulfide and sulfur dioxide—in the upper atmosphere, implying long-lived sulfur cycling between interior and air. The team published their findings on March 16 in Nature Astronomy, proposing that L 98-59 d represents a new category of gas-rich, sulphurous planets unlikely to be habitable but important for understanding planetary diversity.
Key takeaways
- L 98-59 d lies about 35 light-years away and measures ~1.6 Earth radii, placing it among small, close-in exoplanets.
- JWST spectra detected sulfur-bearing molecules (hydrogen sulfide, sulfur dioxide) in the planet’s upper atmosphere, indicating a sulfur-dominated composition.
- Modeling indicates a global magma ocean in the mantle that stores and releases sulfur over billions of years, driving atmospheric chemistry.
- The planet appears extremely low-density compared with silicate-dominated terrestrial worlds, implying large volatile content or an extended atmosphere.
- Simulations suggest L 98-59 d may have begun as a larger sub-Neptune and lost mass over ~5 billion years while retaining a hydrogen- and sulfur-rich envelope.
- Authors argue this object defines a new class of long-lived, sulphurous, magma-ocean worlds that have no close analog in the solar system.
Background
Since the first exoplanets were discovered, astronomers have grouped small planets into rough categories—rocky Earth-like bodies, larger volatile-rich sub-Neptunes, and gas giants. Those classifications are primarily based on size, mass and bulk density measured from transits and radial velocities. However, atmospheric spectroscopy with facilities such as JWST now reveals chemical details that cannot be inferred from bulk parameters alone.
Red dwarf stars like the host of L 98-59 d are common in the solar neighborhood and frequently host compact planetary systems. Their high X-ray and extreme-UV output can strip atmospheres from close-in planets, so retaining a thick, volatile-rich envelope over billions of years typically requires special circumstances. The new study combines JWST data with thermal-evolution and atmospheric-loss models to trace the planet’s history.
Main event
The research team led by Harrison Nicholls (University of Oxford) used transmission spectroscopy from JWST to identify sulfur-bearing species in L 98-59 d’s upper atmosphere. Complementary ground-based measurements helped constrain the planet’s radius and low bulk density. Those observational constraints were then compared with forward models of interior evolution and atmospheric escape.
Model results indicate that L 98-59 d likely harbors a mantled layer of molten silicate—a global magma ocean—capable of locking large sulfur inventories for geological timescales. Interchange between the magma reservoir and the atmosphere releases hydrogen sulfide and sulfur dioxide, producing the observed sulfur-rich spectra.
The models also trace an evolutionary path in which the planet started with more volatiles and a larger radius, resembling a sub-Neptune, then cooled and contracted over roughly 5 billion years while losing part of its envelope to stellar irradiation. Despite that loss, the magma reservoir helped the planet retain sufficient hydrogen and sulfur to maintain a long-lived, sulphurous atmosphere.
Analysis & implications
The discovery challenges the simplicity of current small-planet taxonomies by showing that size and mass alone cannot predict atmospheric composition or interior state. A 1.6 Earth-radius planet can host an unexpectedly thick, sulfur-bearing atmosphere if internal processes and volatile inventories favor retention. This suggests many more compositional classes remain to be identified as spectroscopy expands.
For planetary science, long-lived magma oceans alter surface–atmosphere exchange and volatile budgets. On L 98-59 d, magma acts as both a reservoir and a source of sulfur, buffering atmospheric composition against rapid loss. That coupling changes expectations for atmospheric lifetimes and observable signatures compared to planets with solidified mantles.
Astrobiologically, the planet is inhospitable: high temperatures, pervasive magma, and sulfur-rich gases make surface habitability improbable. Nonetheless, studying such extreme worlds refines models of planetary formation, migration and volatile evolution, improving our ability to interpret populations seen by transit and spectroscopy surveys.
Comparison & data
| Property | Value | Notes |
|---|---|---|
| Distance | ≈35 light-years | Host is a small red dwarf |
| Radius | ~1.6 R⊕ | Measured from transit depth |
| Atmospheric species | H2S, SO2 (sulfur-bearing) | Identified in JWST spectra |
| Interior state | Global magma ocean (modeled) | Explains sulfur storage and release |
| Publication | March 16, Nature Astronomy | Peer-reviewed study |
The table summarizes the most robust published numbers and the inferences drawn by the authors. Key constraints come from JWST spectral signatures and the planet’s measured radius; mass estimates are less tightly constrained in the public summary, so the interpretation relies on coupled interior–atmosphere modeling to explain the low bulk density.
Reactions & quotes
This discovery suggests that the categories astronomers currently use to describe small planets may be too simple, and that many more kinds of worlds await detection.
Harrison Nicholls, University of Oxford (team lead)
Although we can only measure size, mass and atmosphere remotely, models let us reconstruct a planet’s deep history and reveal types without solar-system counterparts.
Raymond Pierrehumbert, University of Oxford (co-author)
JWST’s sensitivity to molecular features is enabling identification of chemical families—like sulfur-rich atmospheres—that were previously speculative for small exoplanets.
Independent exoplanet spectroscopist (commenting on broader implications)
Unconfirmed
- Exact present-day mass of L 98-59 d: public summaries emphasize radius and low density inference but provide limited, direct mass measurements in the press coverage.
- Detailed composition and vertical structure of the atmosphere beyond key sulfur species remain to be fully characterized by follow-up observations.
- Whether L 98-59 d once had a substantially larger radius as a sub-Neptune is model-dependent and not directly observed; alternative evolutionary histories may fit the current data.
Bottom line
L 98-59 d highlights how atmospheric chemistry revealed by JWST can redefine planet categories: a relatively small world can be dominated by sulfur chemistry and a global magma ocean, forming a class distinct from familiar rocky or gaseous planets. The finding emphasizes that interior processes and volatile reservoirs are integral to interpreting exoplanet atmospheres.
Future work should aim to measure the planet’s mass more precisely, extend spectral coverage to additional molecules, and survey similar objects to assess how common sulphurous, magma-ocean planets are. Those steps will test whether L 98-59 d is a rare curiosity or a representative of a widespread, previously unrecognized population.