NASA’s James Webb Space Telescope has revealed a new type of magma planet that is rich in sulphur, following a recent analysis by University of Oxford researchers.
Some 35 light-years from our planet, the magma-covered L 98-59 d orbits a small red star, according to observations made by Webb and supplemental data. Now, according to a recent paper published in Nature Astronomy, the planet has been revealed to possess a shockingly low density and a large amount of hydrogen sulfide in its atmosphere.
James Webb Spots an Unusual Planet
The magma planet, based on its radius and mass, falls between the sizes of Earth and Neptune, which are a mysterious class of exoplanets, according to Dr. Harrison Nicholls, the lead author of a new study detailing the recent findings.
“It is unclear what sub-Neptune and super-Earth exoplanets are made of, or how they are structured, because we don’t have any examples in the Solar System,” Nicholls told The Debrief in an email. “Scientists have generally considered two distinct categories to explain them: a) as ‘gas dwarfs’, which have iron cores, rocky mantles, and hydrogen-rich atmospheres, and b) as ‘water worlds’ which have rocky interiors, various layered ‘shells’ of high-pressure water-ice, and then water vapour (steam) atmospheres on top.”
But the planet L 98-59 d, as seen by the James Webb Space Telescope, contains heavy sulphur molecules, indicating a separate type of planet formation from the other two, placing it in a new and distinct category.
Simulating a Magma World
A team with members at the University of Oxford, the University of Groningen, the University of Leeds, and ETH Zurich combined their efforts to leverage computer simulations to reconstruct the planet’s history. Combining these five billion years of planetary history with direct observations from the James Webb Space Telescope and elsewhere, the team was able to extrapolate the processes occurring inside the planet.
The planet’s mantle is made up of molten silicate similar to lava on Earth, with magma oceans extending for thousands of kilometers beneath it. All of this lava allows the planet to retain a vast amount of sulphur, either directly in the magma, or as atmospheric hydrogen sulfide, which its host star would typically cause to be lost to space over time.
The present planet was shaped by billions of years of chemical exchanges between the atmosphere and molten interior. The team believes that this is likely only the first recognized in what is likely an entire class of sulphur-rich magma gas planets.
“This discovery suggests that the categories astronomers currently use to describe small planets may be too simple,” Dr. Nicholls said. “While this molten planet is unlikely to support life, it reflects the wide diversity of the worlds which exist beyond the Solar System. We may then ask: what other types of planets are waiting to be uncovered?”
James Webb Space Telescope Observations
The James Webb Space Telescope first detected signs of sulphur dioxide on the planet in 2024. According to the team’s models, ultraviolet light from the host star triggers chemical reactions that generate the gas. On large time scales, magma oceans store and release gases to produce the planet’s unique properties.
The team’s simulations suggest that L 98-59 d was once a larger sub-Neptune planet that shrank and lost its atmosphere over billions of years of cooling. Since magma oceans are the initial state of all rocky planets like Earth and Mars, how these planets formed into their present state offers unique insight into Earth’s development as well.
“What’s exciting is that we can use computer models to uncover the hidden interior of a planet we will never visit,” said co-author Professor Raymond Pierrehumbert of the University of Oxford. “Although astronomers can only measure a planet’s size, mass, and atmospheric composition from afar, this research shows that 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.”
Beyond James Webb and Beyond the Solar System
The team looks forward to new data from the upcoming Ariel and PLATO missions, which will supplement the existing James Webb Space Telescope data. This data, combined with their simulations, is expected to reveal much about the exoplanets beyond our solar system, including how they form and evolve.
“Our computer models simulate various planetary processes, effectively enabling us to turn back the clock and understand how this unusual rocky exoplanet, L 98-59 d, evolved, said Dr Richard Chatterjee of the University of Leeds and the University of Oxford. “Hydrogen sulphide gas, responsible for the smell of rotten eggs, appears to play a starring role there.”
“But, as always, more observations are needed to understand this planet and others like it,” Chatterjee added. “Further investigation may yet show that rather pungent planets are surprisingly common.”
The paper, “Volatile-rich Evolution of Molten Super-Earth L 98-59 d,” appeared in Nature Astronomy on March 16, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.