Scientists May Have Discovered a New Type of Planet Beyond the Solar System. The Catch? It Smells Like Rotten Eggs
L98-59d seems to be a molten planet with an atmosphere full of hydrogen sulfide
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Exoplanet L98-59d, located far beyond our solar system, might not sound like someone’s dream destination. It likely has an underground ocean of magma, a surface temperature that reaches around 3,500 degrees Fahrenheit and a constant stench of rotten eggs. But the smelly world might offer something exciting to scientists: a new class of planet.
In a study published March 16 in the journal Nature Astronomy, researchers detail the evolutionary history of the exoplanet and suggest it belongs to a previously unknown type of planet, marked by a molten rock interior and lots of atmospheric hydrogen sulfide—the chemical that produces the odor of rotten eggs.
“This discovery suggests that the categories astronomers currently use to describe small planets may be too simple,” says study co-author Harrison Nicholls, an astrophysicist at the University of Oxford in England, in statement. “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?”
The planet is about 1.6 times the size of Earth but has a low density. Most celestial bodies with those features fall into one of two categories: “gas dwarfs,” which are rocky planets with hydrogen atmospheres, or “water worlds,” which are full of oceans and ice. But recent James Webb Space Telescope observations of L98-59d, which was first reported in 2019, hinted that its characteristics didn’t fit either of those two classes.
Did you know? Not the first stinky exoplanet
In 2024, researchers reported another far-off world that smells like rotten eggs. HD 189733b, some 64 light-years from Earth, seems to be a Jupiter-sized gas giant with trace amounts of hydrogen sulfide in its atmosphere.
So, Nicholls and his colleagues used computer simulations to piece together the oddball’s roughly five-billion-year history, based on telescope data and prior knowledge about the interiors and atmospheres of planets. The analyses suggested that a deep global magma ocean maintained for billions of years could store the chemical ingredients that make L98-59d’s atmosphere stinky. What’s more, ultraviolet light from the planet’s host star—a red dwarf—can trigger chemical reactions to make sulfur gases in the atmosphere.
The new study provides a plausible explanation for this strange planet, Jo Barstow, a planetary scientist at the Open University in England, who was not involved in the study, tells the Guardian’s Hannah Devlin.
“We talked about it possibly being an exoplanet that resembles Jupiter’s moon Io, with lots of volcanoes caused by tidal heating,” she says. “This work suggests it could be even more extreme.”
Rocky planets tend to start out in a mushy, molten state, Nicholls tells Tom Howarth at BBC Science Focus. While Earth cooled down, some, like L98-59d, stay hot. Therefore, “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,” he says.
Unfortunately for us—and fortunately for our noses—humans and other earthly beings probably couldn’t survive on L98-59d.
“If there are aliens out there that could live in lava, that would be amazing, but I don’t think it’s likely that it’s habitable,” Nicholls tells the Guardian. “It’s nice to revel in the alienness of the planet itself.”