By Will Dunham
WASHINGTON, March 16 (Reuters) - Astronomers have spotted a planet orbiting a star in our neighborhood of the Milky Way galaxy that presents a unique hellscape - covered with a perpetual ocean of magma and enveloped by a noxious and fiercely hot sulfur-rich atmosphere.
The molten planet's diameter is more than 60% greater than Earth, though its density is only about 40% that of our planet. It orbits a star smaller and dimmer than the sun located about 34 light-years from Earth in the constellation
Volans. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).
"The planet lacks distinct structure within its magma ocean, so there is no crust, upper mantle and lower mantle. The magma ocean is a single deep, mushy layer," said Harrison Nicholls, a postdoctoral researcher at the University of Cambridge Institute of Astronomy and lead author of the research published on Monday in the journal Nature Astronomy.
Small crystals of solid rock may be trapped within the turbulent fluid magma comprising the mantle, Nicholls said.
The planet's metallic core appears to be relatively small, with the magma ocean comprising 70-90% of the planetary interior radius - reaching a depth between 2,775 and 3,565 miles (4,465-5,740 km).
Its thick atmosphere is composed primarily of hydrogen, but has a very high sulfur content. About 10% of the atmosphere is the toxic gas hydrogen sulfide, which gives off the stench of rotten eggs. The atmosphere has caused a runaway greenhouse effect, trapping heat from the star, that keeps the planet's surface so hot that it remains molten.
"Your nose can smell hydrogen sulfide at concentrations of something like one part per billion, so this would be overwhelmingly stinky. But you wouldn't survive long enough in this hot atmosphere to notice," said planetary scientist and study co-author Raymond Pierrehumbert of the University of Oxford and Massachusetts Institute of Technology.
The atmosphere's composition suggests a high sulfur content in the planetary interior, the researchers said.
"We do not know the exact composition of the molten material but our interpretation of the observations suggests that the high sulfur content would likely mean a mineralogical composition different to our solar system's planets," Nicholls said.
The planet, named L 98-59 d, was discovered in 2019, then was observed by the James Webb Space Telescope in 2024 and by ground-based telescopes in 2025. The researchers used advanced computer simulations to reconstruct its history spanning nearly five billion years, making it somewhat older than Earth.
It orbits a common type of star called a red dwarf. The star's mass is just under 30% that of the sun and its luminosity about 1% that of the sun. In terms of orbital distance, L 98-59 d is the third of five planets known to orbit this star.
More than 6,100 planets beyond our solar system, called exoplanets, have been discovered since the 1990s, though none quite like this one. Its unique combination of a magma ocean and a sulfur-laden atmosphere puts it in a class by itself, for now.
Other molten planets are known, some orbiting so close to their host stars that their surfaces are broiled. That is not the dynamic with L 98-59 d.
"A key point to recognize is that all planets - including Earth - start out molten, but this one has remained so because of a combination of factors," Nicholls said.
Its hot atmosphere plays the leading role in preventing the magma from hardening into rock, and its magma ocean helps maintain the harsh atmosphere. Two secondary factors in heating up L 98-59 d are the star's radiation and gravitational interactions with the system's other planets.
As scientists seek evidence of possible life on exoplanets, this one is not a promising candidate.
"This planet's surface is in excess of 1,500 degrees Celsius (2,732 degrees Fahrenheit), so it would not harbor life as we know it," Nicholls said.
"The era of exoplanet discovery keeps showing us new kinds of worlds, indeed 'strange new worlds,' generally stranger than anything in 'Star Trek,'" Pierrehumbert said, using a phrase from the "Star Trek" prologue. "This offers all sorts of exciting opportunities to put together fundamental physics in very novel ways."
(Reporting by Will Dunham, Editing by Rosalba O'Brien)
