James Webb Space Telescope detects thick atmosphere around broiling lava world 

Researchers have detected the strongest evidence yet for an atmosphere on a rocky planet outside our solar system using NASA’s James Webb Space Telescope. Observations of the ultra-hot super-Earth TOI-561 b suggest that the exoplanet is surrounded by a thick blanket of gases above a global magma ocean. The results help explain the planet’s unusually low density and challenge the prevailing wisdom that relatively small planets so close to their stars are not able to sustain atmospheres. University of Groningen astronomer Tim Lichtenberg and PhD student Emma Postolec are co-authors of the paper on this boiling lava world.

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With a radius roughly 1.4 times Earth’s, and an orbital period less than 11 hours, TOI-561 b falls into a rare class of objects known as ultra-short period exoplanets. Although its host star is only slightly smaller and cooler than the Sun, TOI-561 b orbits so close to the star — less than one million miles, which is one-fortieth the distance between Mercury and the Sun — that it must be tidally locked, with the temperature of its permanent dayside far exceeding the melting temperature of typical rock.

A very old star

‘What really sets this planet apart is its anomalously low density,’ says Johanna Teske, staff scientist at Carnegie Science Earth and Planets Laboratory and lead author on a paper published Thursday in The Astrophysical Journal Letters. ‘It’s not a super-puff, but it is less dense than you would expect if it had an Earth-like composition.’

One explanation the team considered for the planet’s low density was that it could have a relatively small iron core and a mantle made of rock that is not as dense as rock within Earth. Teske notes that this could make sense: ‘TOI-561 b is distinct among ultra-short period planets in that it orbits a very old star, twice as old as the Sun. I is an iron-poor star in a region of the Milky Way known as the thick disk and must have formed in a very different chemical environment from the planets in our own solar system.’ The planet's composition could be representative of planets that formed when the universe was relatively young. 

But an exotic composition can’t explain everything. The team also suspected that TOI-561 b might be surrounded by a thick atmosphere that makes it look larger than it actually is. Although small planets that have spent billions of years baking in blazing stellar radiation are not expected to have atmospheres, some show signs that they are not just bare rock or lava. 

Far cooler than expected

To test the hypothesis that TOI-561 b has an atmosphere, the team used Webb’s Near-Infrared Spectrograph (NIRSpec) to measure the planet’s dayside temperature based on its near-infrared brightness. The technique, which involves measuring the decrease in brightness of the star-planet system as the planet moves behind the star, is similar to that used to search for other rocky worlds. 

If TOI-561 b is a bare rock with no atmosphere to carry heat around to the nightside, its dayside temperature should be approaching 2,700 degrees Celsius. But the NIRSpec observations show that the planet’s dayside appears to be closer to 1,800 degrees Celsius — still extremely hot, but far cooler than expected.

To explain the results, the team considered a few different scenarios. The magma ocean could circulate some heat, but without an atmosphere, the nightside would probably be solid, which limits flow away from the dayside. A thin layer of rock vapor on the surface of the magma ocean is also possible, but would likely have a much smaller cooling effect on its own than observed. 

Gases escaping to space

‘We really need a thick volatile-rich atmosphere to explain all the observations,’ said Anjali Piette, coauthor from the University of Birmingham (UK). ‘Strong winds would cool the dayside by transporting heat over to the nightside. Gases like water vapor would absorb some wavelengths of near-infrared light emitted by the surface before they make it all the way up through the atmosphere. This means the planet would look colder because the telescope detects less light. It’s also possible that there are bright silicate clouds that cool the atmosphere by reflecting starlight.’

While the Webb observations provide compelling evidence for such an atmosphere, the question remains: How can a small planet exposed to such intense radiation retain any atmosphere at all, let alone one so substantial? Some gases must be escaping to space, but perhaps not as efficiently as expected. 

‘We think there is an equilibrium between the magma ocean and the atmosphere,’ says co-author Tim Lichtenberg from the University of Groningen. ‘At the same time that gases are coming out of the planet to feed the atmosphere, the magma ocean is sucking them back into the interior. This planet must be much more volatile-rich than Earth to explain the observations. It's really like a wet lava ball.’

For this study, the system was observed continuously for more than 37 hours while TOI-561 b completed nearly four full orbits of the star. The team is currently analyzing the full data set to map the temperature all the way around the planet and narrow down the composition of the atmosphere. Teske: ‘What’s really exciting is that this new data set is opening up even more questions than it’s answering.’ 

Reference: Johanna K. Teske et al: A Thick Volatile Atmosphere on the Ultrahot Super-Earth TOI-561 b. Astrophysical Journal Letters, 11 December 2025

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