A strong case just emerged for an atmosphere around a molten, rocky exoplanet. But the headline isn’t the end of the story—it reshapes how we think about tiny worlds so close to their stars.
Since the James Webb Space Telescope began science operations in 2022, it has steadily advanced our ability to detect atmospheres around exoplanets. Webb has already revealed carbon dioxide in WASP-39b, water vapor in WASP-96 b, and even heavier elements like oxygen and carbon in HD 149026b. The latest findings push the boundary further: they present the strongest evidence yet for an atmosphere around a rocky planet.
The planet in question is TOI-561 b, an ultra-hot, rocky world with a radius about 1.4 times that of Earth. It orbits a Sun-like star roughly 275 light-years away and completes an orbit in under 11 hours. This puts it in the rare class of ultra-short period exoplanets. Webb’s Near-Infrared Spectrograph (NIRSpec) data suggest TOI-561 b is cloaked by a global magma ocean beneath a thick atmospheric blanket. The team behind the study argues that these observations challenge the prevailing idea that small, close-in planets cannot sustain atmospheres.
Led by Johanna Teske and researchers from the Carnegie Institution for Science’s Earth and Planets Laboratory, the project included collaborators from the Waterloo Centre for Astrophysics, the Trottier Institute of Exoplanet Science, the Kapteyn Astronomical Institute, the Oxford Atmospheric, Oceanic, and Planetary Physics group, and several universities. The team’s results were published on December 11 in The Astrophysical Journal Letters.
An artist’s concept illustrates what a dense atmosphere above a vast magma ocean on TOI-561 b might look like. Credit: NASA, ESA, CSA, Ralf Crawford (STScI)
Because TOI-561 b orbits so close to its star—less than one‑fortieth the Earth–Sun distance—the researchers conclude the planet is tidally locked, with one dayside permanently facing the star. That configuration yields dayside temperatures above rock’s melting point, creating a persistent magma surface. Measurements of the planet’s size and mass reveal a surprisingly low density, which could indicate a relatively small iron core and a mantle composed of rocks denser or lighter than Earth’s, offering a glimpse into a potentially distinct internal structure.
Teske explained in a NASA press release that TOI-561 b is unusual among ultra-short period planets because it orbits an ancient, iron-poor star in the Milky Way’s thick disk. This implies the planet formed in a chemical environment very different from our solar system, potentially representing worlds that formed much earlier in the universe’s history. With an estimated stellar age around 10.5 billion years, TOI-561 b could exemplify planets that formed when the universe was younger. Alternatively, it’s possible the planet’s apparent size is inflated by a thick atmosphere, akin to some “super-puff” gas giants that orbit very close to their stars.
To test this atmospheric hypothesis, the team used Webb’s NIRSpec to gauge the planet’s dayside temperature. They observed the system for more than 37 hours as TOI-561 b completed nearly four orbits. The analysis included watching the planet pass behind its star—a secondary eclipse—to measure the emitted light directly. This approach is the mirror image of the transit technique that detects planets by dips in starlight and is somewhat similar to methods used to search for atmospheres on rocky planets around red dwarfs (like TRAPPIST-1).
If the planet lacked an atmosphere, heat would be unevenly distributed between dayside and nightside, and the dayside temperature would be around 2,700 °C (4,900 °F). Instead, the observed dayside temperature is closer to 1,800 °C (3,200 °F). Co-author Dr. Anjali Piette notes that a thick, volatile-rich atmosphere is necessary to explain the data: strong winds could redistribute heat, water vapor would absorb specific infrared wavelengths, and the atmosphere could even appear cooler if bright silicate clouds reflect starlight. In short, a substantial atmosphere helps reconcile the temperature measurements with the planet’s expected physics.
Another puzzle is how a small, tidally locked world can retain a dense atmosphere under intense radiation. Tim Lichtenberg of the University of Groningen suggests a dynamic balance: a magma ocean releases gases that feed the atmosphere, while the molten surface simultaneously absorbs some of those gases back into the interior. The team proposes that TOI-561 b is far more volatile-rich than Earth—essentially a “wet lava ball.”
These results originate from Webb’s General Observers Program 3860, part of Cycle 2. The researchers are still analyzing the full data set to map temperatures on both sides of the planet and to better characterize the atmosphere’s composition.
For further details, see NASA’s summary of Webb’s detection of a thick atmosphere around this broiling lava world.
Follow-up question: Do you think discoveries like this should push us to rethink how common atmospheres are on rocky planets that live so close to their stars? Share your perspective in the comments.
