Scientists discover a new type of molten sulphur world

Observations from the James Webb Space Telescope (JWST) reveal that the exoplanet L 98-59 d has a remarkably low density and an atmosphere containing sulphur molecules. Simulations by scientists from the University of Groningen (UG) and other institutions now show that the planet, with approximately 1.6 times the mass and diameter of Earth, has a permanent magma ocean containing a large amount of sulphur. The exoplanet may be the first example of a new category of exoplanets and does not fit into existing theories about the formation and evolution of ‘super-Earths’. The research has been published in Nature Astronomy on 16 March.

FSE Science Newsroom | Text NOVA

To explain the planet’s properties, the scientists simulated the development of L 98-59 d from the moment of its formation, some five billion years ago. Their computer model – PROTEUS – calculated the interactions between the planet’s deep interior and its atmosphere over billions of years. The simulations took into account the intense ultraviolet radiation from the nearby star and the planet’s internal chemical and physical evolution. A combination of these processes shaped the planet as it is today.

The simulations, combined with observations, show that the surface of L 98-59 d is extremely hot and covered by an ocean of magma in which large quantities of sulphur are stored. The JWST has detected hydrogen sulphide and sulphur dioxide high in the planet’s atmosphere, the first time this has been observed on a planet of this size. This has recently been confirmed by telescopes on Earth. According to the scientists, the sulphur has escaped from the magma ocean and converted into sulphur dioxide in the atmosphere under the influence of ultraviolet radiation.

New category of exoplanets

Exoplanets with a diameter between 1.5 and 4 times that of Earth have, until now, fallen into roughly two categories. They are either rocky planets with a hydrogen-rich atmosphere (‘gas dwarfs’), or they possess a ‘steam atmosphere’ and a large amount of water in their interiors (‘water worlds’). Based on the composition and density of its atmosphere, L 98-59 d does not appear to fit into either category.

The scientists conclude that they have characterised the first planet of a new class of super-Earths. This must have formed and evolved in a fundamentally different way from the gas dwarfs and water worlds. 'The measurements and simulations of L 98-59 d show that the population of super-Earths – the most common type of exoplanet – is likely to be much more diverse than previously assumed,' says Tim Lichtenberg, assistant professor at the Kapteyn Institute of the University of Groningen and co-author of the scientific article.

Planet L 98-59 d was discovered in 2019 orbiting the dwarf star L 98-59, which is known to have five planets. The planetary system is located just under 35 light-years from the Solar System in the constellation Piscis Austrinus. Given the distance, it is impossible to visit these exoplanets. Astronomers must rely on observations – in this case, by the JWST and the Very Large Telescope in Chile.       

Molten sulphur world

'Observations of this kind provide information about global properties, such as the planet’s size and mass and the composition of its atmosphere,' says Lichtenberg. 'By developing realistic computer models, we can now gain insight into the interiors of exoplanets, their formation history and the evolution of their climate.'

The astronomers will now apply their simulations to observations of other exoplanets. 'With our simulation model PROTEUS, we can determine the formation history of many more planets with different compositions and climatic histories,' says Lichtenberg. 'This provides information about the surface of these worlds and whether they are theoretically suitable for life as we know it.' Due to its molten surface, the likelihood of extraterrestrial life on L 98-59 d appears to be low.

'Our research into this molten sulphur world shows that we are entering a new era in planetary science. We are discovering new families of planets that are turning our understanding of planetary evolution on its head,' concludes Harrison Nicholls, lead author of the study. He works as a postdoc at the University of Cambridge (UK).

The JWST is currently providing a wealth of new information about exoplanets, which will be further supplemented in the coming years by the Extremely Large Telescope on Earth and the Ariel and PLATO space missions. This could yield more new types of exoplanets, whose interiors and climates can be studied in this way.

Reference: Harrison Nicholls, Tim Lichtenberg, Richard D. Chatterjee, Claire Marie Guimond, Emma Postolec & Raymond T. Pierrehumbert: Volatile-rich evolution of molten super-Earth L 98-59 d. Nature Astronomy, 16 March 2026