New observations from the NASA/ESA/CSA James Webb Space Telescope suggest that TOI-561b is surrounded by a thick layer of gases above a global magma ocean.
This artist's concept shows what TOI-561b and its star might look like. Image credit: NASA/ESA/CSA/Ralph Crawford, STScI.
TOI-561 is a bright star located 280.5 light years away in the constellation Sextanov.
The star is about 10 billion years old, and its mass and size are about 80% of the mass of the Sun.
Also known as TYC 243-1528-1, it belongs to a rare population of stars called Galactic thick disk stars.
TOI-561 owners has at least three exoplanets – TOI-561b, c and d – and is one of the oldest and metal-poor planetary systems yet discovered in the Milky Way.
The inner planet of TOI-561b is a so-called super-Earth with an orbital period of just 0.44 days.
Its mass and radius are 3.2 and 1.45 times greater than those of Earth, and its density of 5.5 g/cm3 corresponds to a rocky composition.
“What really sets this planet apart is its abnormally low density,” said Dr. Johanna Teske, an astronomer at the Carnegie Institution for Science.
“It's not super fluff, but it's less dense than you'd expect if it had a composition similar to Earth's.”
One explanation that astronomers considered for the planet's low density was that it could have a relatively small iron core and a mantle of rock, which is not as dense as rock on Earth.
“TOI-561b differs from ultra-short-period planets in that it orbits a very old (twice the Sun's age) iron-poor star in a region of the Milky Way known as the thick disk,” Dr Teske said.
“It must have formed in a completely different chemical environment than the planets in our solar system.”
The researchers also suspected that TOI-561b may be surrounded by a thick atmosphere, making it appear larger than it actually is.
Although small planets baked for billions of years by scorching stellar radiation are expected to have no atmosphere, some are showing signs that they are more than just bare rock or lava.
To test the hypothesis that TOI-561b has an atmosphere, they used Webb's NIRSpec instrument (near-infrared spectrograph) to measure a planet's daytime temperature based on its near-infrared brightness.
The technique, which involves measuring the decrease in brightness of a star-planet system as a 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 bare rock with no atmosphere carrying heat to the night side, its daytime temperature should approach 2,700 degrees Celsius (4,900 degrees Fahrenheit).
But NIRSpec observations show the planet's dayside appears to be closer to 1,800 degrees Celsius (3,200 degrees Fahrenheit)—still very hot, but much cooler than expected.
An emission spectrum taken 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/Ralph Crawford, STScI/Joanna Teske, Carnegie Laboratory for Earth and Planetary Sciences/Anjali Piette, University of Birmingham/Tim Lichtenberg, Groningen/Nicole Wallach, Carnegie Laboratory for Earth and Planetary Sciences.
To explain the results, the authors looked at several different scenarios.
An ocean of magma would circulate some heat, but without an atmosphere the night side would likely be solid, limiting the flow from the day side.
A thin layer of rock vapor on the surface of a magma ocean is also possible, but itself would likely have a much smaller cooling effect than observed.
“We really need a dense, volatile-rich atmosphere to explain all the observations,” said Dr Anjali Piett, an astronomer at the University of Birmingham.
“Strong winds will cool the day side, transferring heat to the night side.”
“Gases such as water vapor will absorb some wavelengths of near-infrared light emitted by the surface before they pass through the atmosphere.”
“It's also possible that there are bright silicate clouds that cool the atmosphere by reflecting starlight.”
While Webb's observations provide compelling evidence for the existence of such an atmosphere, the question remains: How can a small planet exposed to such intense radiation possibly retain any atmosphere at all, let alone such a substantial one? Some gases must escape into 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.
“While gases escape from the planet to feed the atmosphere, an ocean of magma sucks them back in.”
“To explain the observations, this planet must be much richer in volatiles than Earth. It really does look like a wet lava ball.”
study appears today in Letters in an astrophysical journal.
_____
Johanna K. Teske etc.. 2025. A dense, volatile atmosphere on the super-hot super-Earth TOI-561b. ApJL 995, L39; two: 10.3847/2041-8213/ae0a4c
