JWST Finds Atmospheres Where None Should Exist, Rewriting the Rules for Rocky Exoplanets
Two March discoveries reveal a 10-billion-year-old lava world with a thick atmosphere and a sulfur-dominated planet unlike anything in existing classification systems.
Overview
In the span of a single week, the James Webb Space Telescope has delivered two discoveries that challenge fundamental assumptions about which rocky planets can hold onto atmospheres. On March 16, a team led by the University of Oxford published findings in Nature Astronomy revealing that exoplanet L 98-59 d harbors a sulfur-rich atmosphere above a global magma ocean, representing what researchers call a previously unknown class of planet. Six days later, on March 22, a Carnegie Science-led team announced in The Astrophysical Journal Letters that the ancient super-Earth TOI-561 b possesses a thick atmosphere despite orbiting so close to its star that a year lasts barely 10 hours.
Taken together, the findings suggest that the categories astronomers currently use to describe small rocky worlds may be far too simple.
A Lava World That Should Be Bare
TOI-561 b is, by most measures, a planet that should have been stripped of any atmospheric envelope long ago. With roughly twice Earth’s mass and an orbital period of just 10.56 hours, it circles its star at less than one million miles — a fraction of Mercury’s distance from the Sun. One hemisphere is locked in permanent daylight at rock-melting temperatures. The star itself is twice as old as the Sun, meaning the planet has endured billions of additional years of intense stellar radiation.
Yet JWST’s Near-Infrared Spectrograph (NIRSpec) measured a dayside temperature of approximately 3,200 degrees Fahrenheit (1,800 degrees Celsius) — dramatically cooler than the 4,900 degrees Fahrenheit (2,700 degrees Celsius) expected for a bare rock. The 1,700-degree discrepancy points to an atmosphere redistributing heat from the dayside to the nightside through strong winds.
“It’s super old and ultrahot. This planet should not have an atmosphere. And it has one,” said Tim Lichtenberg, a planetary scientist at the University of Groningen who contributed to the research. Lead author Johanna Teske of the Carnegie Science Earth and Planets Laboratory noted that the planet’s density is lower than expected for an Earth-like composition, suggesting a volatile-rich interior. The team describes TOI-561 b as a “wet lava ball” in which a churning magma ocean continuously feeds and recycles atmospheric gases.
A Sulfur World That Defies Classification
The discovery of L 98-59 d, located just 35 light-years from Earth, presents a different but equally confounding puzzle. At roughly 1.6 times Earth’s size, the planet orbits a red dwarf star and has a surprisingly low density for its mass. JWST observations detected hydrogen sulfide and sulfur dioxide high in the planet’s upper atmosphere — compounds that, combined with the planet’s other properties, do not match any known planetary category.
According to findings published in Nature Astronomy, L 98-59 d hosts a mantle of molten silicate similar to lava on Earth, with a magma ocean extending thousands of kilometers deep. This enormous reservoir allows the planet to trap large quantities of sulfur within its interior over geological timescales, gradually releasing sulfur-bearing gases into the atmosphere. The result is what lead author Harrison Nicholls of the University of Oxford and colleagues describe as a “sulfur-dominated” world — a type of planet that has never been observed before.
The research team, which included collaborators from the University of Groningen, University of Leeds, and ETH Zurich, argues that L 98-59 d may be the first confirmed example of a broader population of gas-rich, sulfur-dominated planets that sustain long-lived magma oceans, according to a report from Phys.org.
What We Don’t Know
Both discoveries raise more questions than they answer. For TOI-561 b, the mechanism by which such an ancient, irradiated planet retains a thick atmosphere remains unclear. The magma ocean recycling hypothesis — in which volatiles are continuously outgassed from the interior to replenish what stellar wind strips away — is the leading explanation, but has not been directly confirmed. Alternative explanations, such as unusual mineral compositions that lower the planet’s thermal emission, have not been fully ruled out.
For L 98-59 d, the origin of such abundant sulfur is an open question. Whether the sulfur was present from the planet’s formation or was concentrated through later geological processes remains to be determined. The broader population of sulfur-dominated worlds that the team predicts has yet to be identified.
Neither planet is considered habitable in any conventional sense. But the discoveries redefine the boundary conditions for atmospheric retention on rocky worlds, suggesting that planets previously dismissed as too hot, too small, or too old to hold atmospheres may harbor complex atmospheric systems sustained by interior geological processes that Earth-centric models did not anticipate.