The new observations with the NASA/ESA/CSA James Webb Space Telescope suggest that TOI-561b is surrounded by a thick blanket of gases above a global magma ocean.
This artist’s concept shows what TOI-561b and its star could look like. Image credit: NASA / ESA / CSA / Ralf Crawford, STScI.
TOI-561 is a bright star located 280.5 light-years away in the constellation of Sextans.
The star is approximately 10 billion years old, and has a mass and size about 80% that of the Sun.
Otherwise known as TYC 243-1528-1, it belongs to a rare population of stars called the Galactic thick disk stars.
TOI-561 hosts at least three exoplanets — 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 has a mass and a radius of 3.2 and 1.45 times that of the Earth, and a density of 5.5 g/cm3, consistent with a rocky composition.
“What really sets this planet apart is its anomalously low density,” said Dr. Johanna Teske, an astronomer at the Carnegie Institution for Science.
“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 astronomers 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.
“TOI-561b is distinct among ultra-short period planets in that it orbits a very old (twice as old as the Sun), iron-poor star in a region of the Milky Way known as the thick disk,” Dr. Teske said.
“It must have formed in a very different chemical environment from the planets in our own Solar System.”
The researchers also suspected that TOI-561b 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.
To test the hypothesis that TOI-561b has an atmosphere, they used Webb’s NIRSpec (Near-Infrared Spectrograph) instrument 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 atmospheres in the TRAPPIST-1 system and on other rocky worlds.
If TOI-561b is a bare rock with no atmosphere to carry heat around to the nightside, its dayside temperature should be approaching 2,700 degrees Celsius (4,900 degrees Fahrenheit).
But the NIRSpec observations show that the planet’s dayside appears to be closer to 1,800 degrees Celsius (3,200 degrees Fahrenheit) — still extremely hot, but far cooler than expected.
An emission spectrum captured by Webb in May 2024 shows the brightness of different wavelengths of near-infrared light emitted by exoplanet TOI-561b. Image credit: NASA / ESA / CSA / Ralf Crawford, STScI / Johanna Teske, Carnegie Science Earth and Planets Laboratory / Anjali Piette, University of Birmingham / Tim Lichtenberg, Groningen / Nicole Wallack, Carnegie Science Earth and Planets Laboratory.
To explain the results, the authors considered a few different scenarios.
The magma ocean could circulate some heat, but without an atmosphere, the nightside would probably be solid, limiting flow away from the dayside.
A thin layer of rock vapor on the surface of the magma ocean is also possible, but on its own would likely have a much smaller cooling effect than observed.
“We really need a thick volatile-rich atmosphere to explain all the observations,” said Dr. Anjali Piette, an astronomer at the University of Birmingham.
“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.”
“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 can hold on to 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,” said Dr. Tim Lichtenberg, an astronomer at 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, much more volatile-rich than Earth to explain the observations. It’s really like a wet lava ball.”
The study appears today in the Astrophysical Journal Letters.
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Johanna K. Teske et al. 2025. A Thick Volatile Atmosphere on the Ultrahot Super-Earth TOI-561b. ApJL 995, L39; doi: 10.3847/2041-8213/ae0a4c