A molten world wrapped in a thick crust of vaporized rock may be the strongest evidence we have yet that a rocky exoplanet has an atmosphere outside our solar system.
The planet TOI-561 b is an ultra-hot super-Earth that appears to have 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 astrophysics mystery that challenges our understanding of scorching exoplanets trapped in a dizzyingly fast dance around their star.
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The exoplanet orbits its star at a distance of less than 1.6 million kilometers (990,000 miles), or just one-forty the distance between the Sun and Mercury, making it a tidally locked hellscape with one side bathed in eternal light and the other plunged into eternal darkness.
Strangely, it has somehow maintained its atmosphere for billions of years, despite intense stellar radiation thought to have stripped the planets of their gaseous cloaks, leaving them nothing more than bare, smoldering rocks if not lava balls.
According to conventional radiation measurement methods, TOI-561 b (top) should not have an atmosphere. (Teske et al., Apply JL2025)
“Based on what we know about other systems, astronomers would predict that a planet like this would be too small and hot to retain its own atmosphere long after formation,” said Carnegie Science astronomer Nicole Wolak.
TOI-561 b is known as an ultrashort period (USP) planet due to its tight orbit, which lasts less than 11 hours. Judging from the size, its mass is about twice the mass of the Earth and 1.4 times the radius of the Earth.
It orbits an extremely old star that is slightly less massive and slightly cooler than the Sun. The star is low in iron but rich in alpha elements such as oxygen, magnesium, and silicon, which were formed by the fusion of massive stars in the early universe.
It’s also located in the Milky Way’s thick disk, a region of the Milky Way that resembles a stellar retirement community. These factors suggest that the star is about 10 billion years old, more than twice the age of the Sun.
The researchers also noted that TOI-561 b has an unusually low density, only about four times denser than water. This may be because TOI-561 b has a relatively small iron core and may be composed of rocks that are less dense than those in the Earth’s crust, a composition that would make sense if TOI-561 b formed in the early universe when there was less iron.
On the other hand, it could also be that TOI-561 b’s atmosphere makes it appear larger than it actually is.
To determine whether TOI-561 b’s lower-than-expected density was due to its atmosphere, the researchers used data from JWST, which observed the planet’s system for 37 hours and orbited its star nearly four times.
By measuring TOI-561 b’s solar brightness using Webb’s NIRSpec (near-infrared spectrometer), researchers can calculate its temperature and therefore whether it might have an atmosphere.
Without an atmosphere, TOI-561 b’s temperature should be about 2,700 degrees Celsius (4,900 degrees Fahrenheit), but measurements show it is closer to a lower 1,800 degrees Celsius.
The researchers speculate that the atmosphere may be “cooling” the star side of TOI-561 b in several ways: Winds in the atmosphere may transport some of the heat from day to night, and water vapor may absorb near-infrared light from the planet’s surface, making it appear cooler.
But how does TOI-561 b maintain such a thick atmosphere over billions of years when it flies so close to its host star?
The research team believes that the exoplanet’s atmosphere and the magma ocean covering the surface may have reached equilibrium. Without the atmosphere, the magma ocean would freeze solid at night.
Instead, researchers believe gas may have seeped out of the exoplanet’s crust and into its atmosphere, with some of it inevitably escaping into space. At the same time, the giant magma ocean may act like a sink, sucking gas back into the Earth’s interior.
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The exoplanet’s iron content may play a role here: The same element that binds to the oxygen in our red blood cells may help TOI-561 retain its atmosphere by trapping volatile chemicals in its magma ocean or core.
“From a sample of rocky planets with daytime brightness temperature limits, planets with irradiation temperatures above 2000 K replenish their volatile envelope faster than it is lost,” the researchers wrote in the paper.
However, “further theoretical and observational studies are needed to pinpoint exactly why TOI-561 b has a thick atmosphere.”
This research was published in Astrophysical Journal Letters.
