A molten lava world encased in a thick envelope of steamy rock may be the strongest evidence yet of a rocky exoplanet with an atmosphere outside our solar system.
The planet TOI-561 b is an ultra-hot super-Earth with what appears to be a global magma ocean beneath a thick atmosphere of volatile chemicals, according to a new study led by Carnegie Science researchers.
TOI-561 b is also an ancient astrophysical enigma that challenges what we know about hot exoplanets caught in a dizzyingly fast dance around their stars.
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The exoplanet orbits its star at a distance of less than 1.6 million kilometers (0.99 million miles), or just one-fortieth the distance between the Sun and Mercury, making it a tidal hellscape with one side bathed in eternal light and the other plunged in eternal darkness.
Interestingly, it has somehow held on to its atmosphere for billions of years, despite intense stellar radiation that strips similar planets of their gaseous cloaks and leaves them as bare, smoky rocks, if not molten balls of lava.
TOI-561 b, top, should not be atmospheric according to conventional measures of its radiation. (Teske et al., ApJL2025)
“Based on what we know about other systems, astronomers predicted that such a planet would be too small and hot to maintain its own atmosphere long after it formed,” says Carnegie Science astronomer Nicole Walleck.
TOI-561 b is known as an ultra-short period (USP) planet due to its tight orbit, which takes less than 11 hours to complete. In terms of size, it is twice the mass of Earth and 1.4 times the radius of Earth.
It orbits a very old star that is slightly less massive and cooler than the Sun. This star is low in iron and rich in alpha elements like oxygen, magnesium, silicon, which were fused by massive stars in the early universe.
It also lies in the dense disk of the Milky Way, a galactic region akin to a stellar retirement community. These factors indicate that the star is about 10 billion years old, twice the age of the Sun.
The researchers also noted that TOI-561 b has an unusually low density, only four times the density of water. That may be because TOI-561 b has a relatively small iron core and may be made of less dense rocks than Earth’s crust, a composition that suggests TOI-561 b formed in the early universe, when there was less iron.
On the other hand, it could also be because TOI-561 b has an atmosphere that makes it appear larger than it actually is.
To find out if TOI-561 b’s lower-than-expected density is due to its atmosphere, the researchers used data from JWST, which observed the planetary system for 37 hours and about four complete orbits around its star.
By measuring the dayside brightness of TOI-561 b with Webb’s NIRSpec (near-infrared spectrograph), the researchers could calculate its temperature, and therefore whether it was likely to have an atmosphere.
Without an atmosphere, TOI-561 b should be about 2,700 °C (4,900 °F), but measurements have shown it to be closer to 1,800 °C.
The researchers speculate that the atmosphere may be ‘cooling’ the star-side of TOI-561 b in some way: winds in the atmosphere may transport some of the daytime heat to the nightside, while water vapor may absorb near-infrared light from the planet’s surface, making it appear cooler.
But how did TOI-561 b manage to maintain this dense atmosphere for billions of years while flying so close to its host star?
The team thinks the exoplanet may have maintained an equilibrium between its atmosphere and the magma ocean covering the surface, which would have solidified on the night rim without the atmosphere.
Instead, researchers think that gases may have been released from the exoplanet’s crust to feed the atmosphere, some inevitably escaping into space. At the same time, the vast magma ocean may have been acting as a sink, drawing gases into the planet’s interior.
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The exoplanet’s iron content may play a role here: the same element that binds oxygen to our red blood cells may help TOI-561 maintain its atmosphere by trapping volatile chemicals in its magma ocean or core.
“From a sample of rocky planets with dayside bright temperature constraints, it appears that planets with radiative temperatures greater than ∼2000 K are able to replenish volatile envelopes faster than they are lost,” the researchers write in their paper.
However, “more theoretical and observational research is needed to pinpoint exactly why TOI-561 b has a dense atmosphere.”
This research is published in Astrophysical Journal Letters.
