Astronomers have made a groundbreaking discovery using NASA’s James Webb Space Telescope (JWST), identifying the clearest evidence yet of an atmosphere surrounding a rocky exoplanet. The planet, known as TOI-561 b, is an ancient, extremely hot super Earth with a surface likely covered by molten rock. This significant finding, led by a team from Carnegie Science, was published in The Astrophysical Journal Letters.
TOI-561 b, with a mass approximately twice that of Earth, presents a stark contrast in other aspects. It orbits its star at a mere one-fortieth the distance between Mercury and the Sun, completing a full orbit in just 10.56 hours. Despite its star being slightly smaller and cooler than our Sun, the planet’s proximity results in one side being permanently exposed to daylight.
“Based on what we know about other systems, astronomers would have predicted that a planet like this is too small and hot to retain its own atmosphere for long after formation,” explained Nicole Wallack, a Carnegie Science Postdoctoral Fellow and the study’s second author. “But our observations suggest it is surrounded by a relatively thick blanket of gas, upending conventional wisdom about ultra-short-period planets.”
Unraveling the Mysteries of TOI-561 b
In our Solar System, small and intensely heated planets typically lose their original gas envelopes early in their history. However, TOI-561 b orbits an older star than the Sun, and despite its harsh conditions, it appears to have retained its atmosphere.
Low Density and Unusual Composition
The potential presence of an atmosphere may also explain the planet’s unexpectedly low density. “It’s not what we call a super-puff — or ‘cotton candy’ planet — but it is less dense than you would expect if it had an Earth-like composition,” said Johanna Teske, the study’s lead author from Carnegie Science.
Before analyzing the new data, the team considered whether the planet’s structure alone could account for this. One hypothesis suggested that TOI-561 b might have a smaller iron core and a mantle made of lighter rock compared to Earth. Teske noted, “TOI-561 b 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. It must have formed in a very different chemical environment from the planets in our own Solar System.”
JWST Temperature Data and Atmospheric Evidence
The research team proposed that a thick atmosphere could make the planet appear larger and therefore less dense. Using JWST’s Near-Infrared Spectrograph (NIRSpec), they measured the temperature of the planet’s dayside by observing its brightness in near-infrared light. This method, also used to study planets in the TRAPPIST-1 system, tracks how the system’s light changes when the planet moves behind its star.
If TOI-561 b had no atmosphere, its dayside temperature should reach nearly 4,900 degrees Fahrenheit (2,700 degrees Celsius). Instead, the measurements showed a lower temperature of about 3,200 degrees Fahrenheit (1,800 degrees Celsius).
While still extremely hot, this difference strongly suggests that heat is being redistributed across the planet.
Understanding the Atmospheric Dynamics
Winds, Clouds, and Volatile-Rich Atmosphere
To account for the cooler temperature, scientists explored several possibilities. A molten surface ocean could move some heat, but without an atmosphere, the nightside would likely remain solid, limiting heat transfer. A thin layer of vaporized rock might also exist, though it would not provide enough cooling on its own.
“We really need a thick volatile-rich atmosphere to explain all the observations,” said co-author Anjali Piette from the University of Birmingham, UK. “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.”
Although the evidence points strongly to an atmosphere, it raises a major question. How can a planet exposed to such intense radiation hold onto gas at all? Some material is likely escaping into space, but perhaps not as quickly as expected.
A “Wet Lava Ball” with a Recycling Atmosphere
One explanation is a balance between the planet’s molten interior and its atmosphere. “We think there is an equilibrium between the magma ocean and the atmosphere. 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,” said co-author Tim Lichtenberg from the University of Groningen in the Netherlands. “This planet must be much, much more volatile-rich than Earth to explain the observations. It’s really like a wet lava ball.”
Johanna Teske emphasized that the discovery raises as many questions as it answers: “What’s really exciting is that this new data set is opening up even more questions than it’s answering.”
Future Research and Implications
These results come from JWST’s General Observers Program 3860, which monitored the system for more than 37 hours as the planet completed nearly four orbits. Researchers are now analyzing the full dataset to map temperature patterns across the entire planet and better understand its atmospheric composition.
The work continues a long legacy of Carnegie Science involvement with JWST, dating back to the telescope’s early development and extending through multiple observation cycles. Since JWST began scientific operations, Carnegie researchers have led numerous teams studying exoplanets, galaxies, and other cosmic phenomena.
“These JWST powered breakthroughs tap directly into our long-standing strength in understanding how exoplanet characteristics are shaped by planetary evolution and dynamics,” said Michael Walter, Director of the Earth and Planets Laboratory. “There are more exciting results on the horizon and we’re poised for a new wave of Carnegie-led JWST science in the year ahead.”
