Planets orbiting stars beyond the solar system come in all shapes and sizes but mostly fit into a few categories: large and hot gas giants, super-Earths that are slightly larger than our planet, Neptunian planets with gassy atmospheres and rocky cores, and small terrestrial planets made up of silicate or water. New observations of a nearby exoplanet, however, suggest that it cannot be mixed in with the regular types and instead belongs to a previously unknown class of planets.
Using the Webb space telescope, astronomers believe they have identified a new type of molten exoplanet with volatile gases in its atmosphere and a permanent magma ocean. The exoplanet, known as L 98-59 d, was previously classified as a super-Earth or a sub-Neptune, but this would appear to be wrong.
“This discovery suggests that the categories astronomers currently use to describe small planets may be too simple,” Harrison Nicholls, a researcher at the University of Oxford’s physics department and lead author of a new study published in Nature Astronomy, said in a 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?”
A whole new world
L 98-59 d orbits a small, cool star along with four other planets. It’s around twice the mass of Earth and is located around 35 light-years away from our planet.
The planet’s discovery was first announced in 2019, but follow-up observations by Webb in 2024 revealed an abundance of sulfur dioxide and other sulfur gases in its atmosphere. The team of researchers behind the study sought to reconstruct the planet’s history, tracing its evolution from shortly after its birth until today.
Using computer simulations to recreate what’s likely been happening inside the planet over the past five billion years, the researchers discovered that the mantle of L 98-59 d is likely molten silicate (similar to the lava found on Earth) with a global magma ocean that extends thousands of miles below.
The simulations revealed that ultraviolet light from the host star triggers chemical exchange reactions between the planet’s molten interior and atmosphere, while the magma ocean acts as a massive reservoir for storing and releasing sulfur dioxide into the upper atmosphere. Sulfur is famously stinky—think of rotten eggs and farts.
The combination of volatile gases stored within its interior and ultraviolet-driven atmospheric chemistry explains the planet’s odd properties, which differentiate it from other types of planets.
“What’s exciting is that we can use computer models to uncover the hidden interior of a planet we will never visit,” Raymond Pierrehumbert, physics professor at the University of Oxford and co-author of the study, said in a statement. “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.”
New planet category just dropped
Based on the computer simulations, L 98-59 d may have started off as a larger sub-Neptune planet. It later evolved over billions of years, gradually shrinking and cooling as it lost some of its atmosphere.
Normally, its entire atmosphere would have been lost over time due to radiation from its host star. The planet’s magma ocean, however, has helped it retain its thick hydrogen-rich atmosphere with sulfur-bearing gases, producing the oddball planet we see today.
The researchers behind the study suggest L 98-59 d may be the first recognized member of a broader population of gas-rich sulfurous planets, and that there may be more planets like it out there in the cosmos. “More observations are needed to understand this planet and others like it,” Richard Chatterjee, a researcher at the University of Oxford and co-author of the study, said in a statement. “Further investigation may yet show that rather pungent planets are surprisingly common.”
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