Astronomers have discovered a strange new world just 35 light-years from Earth – one permanently covered in a vast ocean of molten rock.
The exoplanet, known as L 98-59 d, defies existing models of planetary formation and may represent an entirely new class of planet, according to a study published in Nature Astronomy.
The findings come following observations by the James Webb Space Telescope (JWST) and other ground-based observatories, which had flagged some unusual characteristics.
Most notably, the planet, which is 1.6 times the size of Earth, had a very low density and contained a large amount of hydrogen sulfide in its atmosphere.
This placed L 98-59 d outside of normal categorisation for a planet of its size. Typically, such planets are either designated as rocky 'gas-dwarfs' with a hydrogen atmosphere or 'water worlds' made of oceans and ice. To astronomers, it was clear L 98-59 d was neither.
To work out what it actually was, a team led by the University of Oxford used computer simulations to reconstruct the planet's entire history, winding the clock back nearly five billion years.
Their models revealed that L 98-59 d likely has a mantle of molten silicate rock, with a global magma ocean extending thousands of kilometres into its interior. This vast reservoir allows the planet to store enormous quantities of sulphur, explaining the unusual atmospheric chemistry spotted by JWST.
"This discovery suggests that the categories astronomers currently use to describe small planets may be too simple," said lead author Dr Harrison Nicholls. "What other types of planet are waiting to be uncovered?"
The findings also have implications closer to home. “All planets start in a molten state because of how they form. Some cool down, like Earth, and some stay molten, like L 98-59 d,” Nicholls told BBC Science Focus.
“We can view these observations as an opportunity to learn about the early history of our own planet (and the origin of life on it) by studying the common physics which also shapes these 'alien' worlds.”
Looking ahead, Nicholls believes L 98-59 d may be just the beginning. “This planet might indeed be the first representative member of a wider population of magma ocean worlds… It might turn out that 'magma oceans' are a common occurrence.”
Further data from upcoming space missions, including the European Space Agency’s Ariel and PLATO missions, should reveal whether L 98-59 d is a cosmic oddity or the first known member of a much larger family of worlds.
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