NASA's James Webb Space Telescope has detected signs of a thick atmosphere on the ultra-hot exoplanet TOI-561 b, challenging assumptions about such worlds. This rocky planet, orbiting its star in under 11 hours, shows lower temperatures and density than expected, suggesting a layer of gases above a magma ocean. The findings, published on December 11, highlight how intense radiation might not strip away all atmospheres from small, close-in planets.
Astronomers using the James Webb Space Telescope have identified the strongest evidence yet for an atmosphere enveloping TOI-561 b, a super-Earth located far beyond our solar system. This planet, with a radius 1.4 times that of Earth, races around its host star—a star slightly smaller and cooler than the Sun—in less than 11 hours. At a distance of under one million miles, or about one-fortieth the Earth-Sun separation, TOI-561 b is likely tidally locked, with one side perpetually facing the star and experiencing scorching heat that exceeds the melting point of rock.
The observations reveal that the planet's dayside temperature reaches around 3,200 degrees Fahrenheit (1,800 degrees Celsius), cooler than the anticipated 4,900 degrees Fahrenheit (2,700 degrees Celsius) for a bare rocky surface. This discrepancy, measured via Webb's NIRSpec instrument during a 37-hour monitoring period under General Observers Program 3860, points to heat redistribution, possibly by strong winds in a volatile-rich atmosphere.
Lead author Johanna Teske, a staff scientist at Carnegie Science Earth and Planets Laboratory, noted the planet's unusual traits: "What really sets this planet apart is its anomalously low density. It is less dense than you would expect if it had an Earth-like composition." TOI-561 b orbits an ancient, iron-poor star in the Milky Way's thick disk, twice the age of our Sun, suggesting it formed in a distinct chemical environment from solar system planets.
Co-author Dr. Anjali Piette from the University of Birmingham explained the atmospheric role: "We really need a thick volatile-rich atmosphere to explain all the observations. Strong winds would cool the dayside by transporting heat over to the nightside." The team proposes a substantial gas layer atop a global magma ocean, where gases cycle between the atmosphere and interior, maintaining equilibrium despite intense stellar radiation.
Co-author Tim Lichtenberg from the University of Groningen added: "We think there is an equilibrium between the magma ocean and the atmosphere. While 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."
These results, detailed in The Astrophysical Journal Letters on December 11, reshape understandings of ultra-short period exoplanets and their potential to retain atmospheres over billions of years. Further analysis of the full dataset aims to map temperature variations and atmospheric composition more precisely.