Scientists have identified a previously unknown type of planet beyond our solar system – one capable of storing vast quantities of sulphur deep beneath a permanent ocean of molten rock.
The study, led by scientists at University of Oxford and published in Nature Astronomy, focuses on the distant world L 98-59 d.
The planet orbits a small red dwarf star, L 98-59, around 35 light years from Earth.
Observations using the James Webb Space Telescope (JWST) and ground-based observatories suggested something unusual; despite being about 1.6 times the size of Earth, the planet has an unexpectedly low density and an atmosphere containing significant amounts of hydrogen sulphide.
Until now astronomers would typically place planets of this type into one of two categories – rocky ‘gas dwarfs’ with hydrogen atmospheres, or water-rich worlds covered by deep oceans and ice.
However, the new research indicates that L 98-59 d fits neither description. Instead, it appears to represent an entirely different class of planet dominated by heavy sulphur compounds.
Using advanced computer simulations, researchers from University of Oxford, University of Groningen, University of Leeds and ETH Zurich reconstructed the planet’s history from shortly after its formation nearly five billion years ago.
By linking telescope observations to detailed models of planetary interiors and atmospheres, the team attempted to determine what might be happening deep within the world.
Their results suggest the mantle of L 98-59 d is likely made of molten silicate – similar to lava on Earth – forming a global magma ocean stretching thousands of kilometres below the surface.
This enormous reservoir of molten rock could allow the planet to store huge amounts of sulphur within its interior over geological timescales.
It may also help the planet retain a thick hydrogen-rich atmosphere containing sulphur-bearing gases such as hydrogen sulphide (H₂S), which would otherwise gradually escape into space under the influence of X-ray radiation from its host star.
Over billions of years, chemical exchanges between the molten interior and the atmosphere appear to have shaped the world observed by astronomers today.
The researchers believe L 98-59 d could be the first recognised example of a broader population of gas-rich, sulphur-bearing planets sustained by long-lived magma oceans. If confirmed, it would suggest the variety of planets in our galaxy is even greater than previously thought.
Lead author Dr Harrison Nicholls, from the Department of Physics at the 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?”
Observations by the James Webb Space Telescope in 2024 indicated the presence of sulphur dioxide and other sulphur gases high in the planet’s upper atmosphere.
According to the team’s models, these gases may form when ultraviolet light from the red dwarf star L 98-59 triggers chemical reactions. At the same time, the magma ocean beneath the surface acts as a vast reservoir, storing and gradually releasing volatile gases over billions of years.
Together, these processes could explain the unusual characteristics detected by astronomers.
The simulations also suggest that L 98-59 d may initially have formed with far greater amounts of volatile material and might once have resembled a larger “sub-Neptune” planet. Over time, as it cooled and lost part of its atmosphere, it gradually shrank to its present size.
Co-author Professor Raymond Pierrehumbert, also from the 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.”
Scientists say a growing stream of data from the James Webb Space Telescope will soon be joined by observations from upcoming missions including Ariel exoplanet mission and PLATO mission.
The team plans to apply their simulations to these future observations using machine-learning techniques, in an effort to map the diversity of planets beyond the Solar System and better understand how they form and evolve.
Dr Richard Chatterjee of the University of Leeds and the University of Oxford concluded: “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 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.
“Further investigation may yet show that rather pungent planets are surprisingly common.”
MORE: England’s gonorrhoea hotspots revealed — with one London borough crowned ‘clap capital’
MORE: Chick-fil-A is opening in London — and the full menu has been revealed
MORE: Most true-to-life ear has been 3D printed and ‘feels just like the real thing’
