Webb Reveals a Sulfur-Shrouded Super-Earth With a Permanent Magma Ocean, Defining a New Class of Exoplanet
JWST observations of super-Earth L 98-59 d reveal a sulfur-dominated atmosphere sustained by a perpetual magma ocean, a combination that fits no existing planetary category and may define a new class of exoplanet.
Overview
A rocky planet slightly larger than Earth, orbiting a red dwarf star just 35 light-years away, has upended the way astronomers classify small worlds. Observations from the James Webb Space Telescope show that L 98-59 d possesses a thick atmosphere dominated by hydrogen sulfide and sulfur dioxide, sustained by a global ocean of molten rock that has persisted for billions of years. The findings, published on March 16 in Nature Astronomy by a team led by Harrison Nicholls at the University of Oxford, describe a planet that belongs to neither of the two categories scientists have traditionally used for sub-Neptune worlds and may instead be the founding member of an entirely new class.
What We Know
L 98-59 d was first identified in 2019 as part of a multi-planet system orbiting the M-dwarf star L 98-59. At roughly 1.6 times Earth’s radius and 1.64 times its mass, it qualifies as a super-Earth, but its density is unexpectedly low for a world of that size. The JWST data reveal why: rather than being a dense ball of rock and metal, the planet harbors a deep, permanent magma ocean beneath a volatile-rich atmosphere.
The atmospheric signature is unlike anything previously observed on a small exoplanet. JWST detected abundant hydrogen sulfide and sulfur dioxide, the latter produced when ultraviolet radiation from the host star strikes sulfur-bearing gases in the upper atmosphere. The result is a world that, in the words of the research team, would smell overwhelmingly of rotten eggs.
The magma ocean plays a critical role in sustaining this atmosphere. Molten silicate rock thousands of kilometers deep acts as a long-term reservoir, trapping and continuously releasing sulfur compounds into the atmosphere over geological timescales. This mechanism explains how a relatively small planet can maintain a thick, gas-rich envelope despite the intense stellar radiation that would otherwise strip it away.
Until now, astronomers classified small planets with low densities and detectable atmospheres into two bins: gas dwarfs, which retain a primordial hydrogen-helium envelope accreted during formation, and water worlds, covered by deep oceans and ice. L 98-59 d fits neither category. Its atmosphere is dominated by heavy sulfur compounds rather than hydrogen, and there is no evidence of the water-rich interior expected of an ocean world.
“This discovery suggests that the categories astronomers currently use to describe small planets may be too simple,” said Nicholls.
The research involved collaborators from the University of Groningen, the University of Leeds, and ETH Zurich. Their planet-evolution models show that as L 98-59 d cooled after formation and atmospheric escape shrank its gaseous envelope, the magma ocean stepped in as a steady sulfur source, with photochemistry converting hydrogen sulfide into sulfur dioxide in the upper atmosphere.
What We Don’t Know
Several fundamental questions remain open. The precise depth and temperature profile of the magma ocean have not been directly measured; the team inferred its existence from the planet’s low bulk density and the atmospheric chemistry that a magma reservoir best explains. Whether the sulfur cycling mechanism is stable on timescales of tens of billions of years, or whether it will eventually exhaust the interior’s volatile budget, is not yet clear.
Perhaps the most consequential unknown is how common these sulfur-dominated, magma-ocean worlds are. The researchers suggest L 98-59 d may be the first identified example of a broader population, but confirming that hypothesis will require JWST observations of other low-density super-Earths to search for similar atmospheric fingerprints. If such worlds prove abundant, the two-category framework that has guided exoplanet science for over a decade will need significant revision.
The L 98-59 system, with at least four known planets orbiting a nearby M-dwarf, remains a prime target for continued characterization. Future observations may clarify whether the other planets in the system share any of L 98-59 d’s unusual properties, or whether its sulfurous atmosphere is a product of conditions unique to its orbital position.