TOI-561b, a rocky exoplanet so close to its parent star that its year lasts just over 10 hours, may be wrapped in a thick, reflective atmosphere, according to a new analysis of data gathered by the NASA/ESA/CSA James Webb Space Telescope.
An artist’s rendition of the TOI-561 planetary system. Image credit: Adam Makarenko / W.M. Keck Observatory.
TOI-561 is a bright star located 280.5 light-years away in the constellation of Sextans.
Also known as TYC 243-1528-1, the star is approximately 10 billion years old, and has a mass and size about 80% that of the Sun.
It hosts at least three transiting planets — named TOI-561b, c, and d — and is one of the oldest, most metal-poor planetary systems discovered yet in the Milky Way.
The inner planet, TOI-561b, is a so-called super-Earth with an orbital period of only 0.44 days.
It is about 1.4 times Earth’s radius and 2.2 times its mass, which gives it a surprisingly low density of about 4.3 grams/cm3.
Theory says worlds this heavily irradiated should have lost any substantial atmosphere long ago.
“Our discovery challenges current theories,” said Dr. Anjali Piette, an astronomer at the University of Birmingham.
“We would expect a heavily irradiated rocky planet like TOI-561b to have lost its atmosphere billions of years ago.”
“Instead, the evidence suggests it still has a significant atmosphere today.”
“This raises major questions such as whether the planet started with an unusually large supply of gases or if the ocean of molten rock covering its surface, acts as a planet-wide volcano that is constantly feeding its atmosphere.”
Using Webb’s NIRSpec instrument, Dr. Piette and colleagues tracked the planet’s glow through four secondary eclipses and three transits, building a ‘phase curve’ of how its brightness changes as it orbits.
They also modeled the star’s own flickering, caused by granulation, the churning of convective cells on its surface.
Fitting that stellar signal and the planet’s signal together gave a dayside emission spectrum.
The planet’s dayside has a brightness temperature of roughly 2,050 K, well below the roughly 3,200 K expected for a dark, bare rock with no heat transport.
To explain this brightness, the astronomers compared the observations with a computer simulation of the planet’s climate.
These suggest that clouds made of silicate minerals, including silicon dioxide and magnesium silicate, could exist on the planet’s dayside and act like sunscreen.
“We measured how the planet’s brightness and heat changed throughout its orbit,” said Professor Lisa Dang, an astronomer at the University of Waterloo, Ontario.
“This technique, known as a phase curve, revealed that the planet reflects a significant amount of light from its star, and that heat is being redistributed around the planet instead of only on the sunlit side. Together, these point to the presence of an atmosphere.”
“The next big question we will seek to answer is whether TOI-561b’s atmosphere resembles Venus or an extreme version of Io.”
“Observing the planet at longer wavelengths with Webb could give us insights and help explain how a planet this hot has held on to an atmosphere for billions of years.”
The team’s findings appear in the Astronomical Journal.
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Samuel Boucher et al. 2026. A Highly Reflective Atmosphere on the Lava World TOI-561b Revealed by JWST/NIRSpec Phase Curves. AJ 172, 239; doi: 10.3847/1538-3881/ae9763
