A research team led by the University of Oxford has uncovered evidence for a previously unknown kind of planet beyond our Solar System -- one that locks away large quantities of sulfur deep inside a long lasting ocean of molten rock. The results were published on March 16 in Nature Astronomy.
The world, called L 98-59 d (an exoplanet, meaning it orbits a star outside our Solar System), circles a small red star about 35 light-years from Earth. Data from the James Webb Space Telescope (JWST) and ground based observatories revealed something unusual. For a planet about 1.6 times the size of Earth, it has a surprisingly low density and an atmosphere rich in hydrogen sulfide.
Until now, scientists would have grouped a planet like L 98-59 d into one of two categories. It could be a rocky "gas-dwarf" with a hydrogen dominated atmosphere, or a water rich world covered by deep oceans and ice.
New evidence shows it fits neither category. Instead, L 98-59 d appears to belong to a completely different class of planet dominated by heavy sulfur compounds.
To understand this unusual world, researchers from the University of Oxford, the University of Groningen, the University of Leeds and ETH Zurich used advanced computer simulations to trace its evolution from shortly after formation to today, spanning nearly five billion years. By combining telescope observations with detailed models of planetary interiors and atmospheres, they were able to infer what is happening deep inside the planet.
Their findings suggest that L 98-59 d has a mantle made of molten silicate, similar to lava on Earth. Beneath its surface lies a vast magma ocean extending thousands of kilometers deep. This enormous reservoir allows the planet to trap large amounts of sulfur within its interior over long periods of time.
The magma ocean also helps maintain a thick hydrogen rich atmosphere that contains sulfur bearing gases such as hydrogen sulfide (H2S). Normally, radiation from the host star would gradually strip these gases away into space through X-ray driven processes.
Over billions of years, ongoing chemical exchanges between the molten interior and the atmosphere have shaped the planet's current appearance. These interactions explain the unusual signals detected by telescopes.
Researchers suggest that L 98-59 d may be the first identified example of a broader population of gas rich sulfur dominated planets that sustain long lived magma oceans. If that is the case, it points to a much wider variety of planetary types across the galaxy than previously recognized.
Lead author Dr. Harrison Nicholls (Department of Physics, University of Oxford) said: "This discovery suggests that the categories astronomers currently use to describe small planets may be too simple. 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 planet are waiting to be uncovered?"
JWST observations from 2024 detected sulfur dioxide along with other sulfur gases high in the upper atmosphere of L 98-59 d. According to the team's models, these gases form when ultraviolet radiation from the host star, the red dwarf L 98-59, drives chemical reactions.
At the same time, the magma ocean below acts as a massive storage system for volatile materials, absorbing and releasing gases over billions of years after the planet formed. This combination of deep interior storage and ultraviolet driven chemistry explains the planet's distinctive properties.
Simulations indicate that L 98-59 d likely formed with a large supply of volatile material and may once have resembled a larger sub-Neptune type planet. Over time, it cooled, lost part of its atmosphere, and became smaller.
Scientists note that magma oceans are thought to be the initial state of all rocky planets (including the Earth and Mars). Studying these environments on distant worlds can provide insight into the earliest stages of our own planet's history.
Co-author Professor Raymond Pierrehumbert (Department of Physics, University of Oxford) said: "What's exciting is that we can use computer models to uncover the hidden interior of a planet we will never visit. 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."
JWST is already delivering a growing stream of data, and future missions such as Ariel and PLATO are expected to expand that dataset even further. The research team plans to apply their models to these observations using machine learning to map the diversity of planets beyond our Solar System and link them to their early development.
By doing so, scientists hope to better understand how planets form and evolve, and to identify which types of worlds might be capable of supporting life.
Dr. Richard Chatterjee (University of Leeds/ University of Oxford) said: "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. Hydrogen sulfide 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. Further investigation may yet show that rather pungent planets are surprisingly common."
