A team of astronomers led by Carnegie has uncovered the clearest evidence yet that a rocky planet outside our Solar System has an atmosphere. Using NASA's James Webb Space Telescope (JWST), the researchers identified signs of gas surrounding an unusual target: an ancient, extremely hot super Earth that likely has a surface covered by molten rock. The findings were published in The Astrophysical Journal Letters.
The planet, known as TOI-561 b, has about twice the mass of Earth but is dramatically different in almost every other way. It orbits extremely close to its star, at a distance just one fortieth that of Mercury from the Sun. Even though its star is slightly smaller and cooler than our Sun, the planet's tight orbit means it completes a full year in only 10.56 hours. One side constantly faces the star, leaving it locked in permanent 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 Carnegie Science Postdoctoral Fellow Nicole Wallack, the paper'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."
In our Solar System, planets that are both small and intensely heated tend to lose their original gas envelopes early in their history. However, TOI-561 b orbits a much older star than the Sun, and despite its harsh conditions, it appears to have held onto its atmosphere.
Low Density Clues Point to an Unusual Composition
The possible presence of an atmosphere may help explain another puzzle: the planet's lower than expected 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 Carnegie Science astronomer Johanna Teske, the study's lead author.
Before analyzing the new data, the team considered whether the planet's structure alone could account for this. One idea was that TOI-561 b might have a smaller iron core and a mantle made of lighter rock compared to Earth.
Teske added that this idea fits with the planet's origins: "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."
This suggests the planet could resemble worlds that formed when the universe was much younger. Still, composition alone does not fully explain the observations.
JWST Temperature Data Reveals Hidden Atmosphere
The research team also proposed that a thick atmosphere could make the planet appear larger and therefore less dense. To investigate this, they used JWST's Near-Infrared Spectrograph (NIRSpec) to measure the temperature of the planet's dayside by observing its brightness in near infrared light. This method tracks how the system's light changes when the planet moves behind its star, a technique also used to study planets in the TRAPPIST-1 system.
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.
Winds, Clouds, and a 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, of University of Birmingham, United Kingdom -- a former Carnegie Science Postdoctoral Fellow. "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. (The planet would look colder because the telescope detects less light.) 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, who is also part of the Carnegie-led Atmospheric Empirical Theoretical and Experimental Research (AEThER) project team. "This planet must be much, much more volatile-rich than Earth to explain the observations. It's really like a wet lava ball."
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."
JWST Observations Open New Questions About Exoplanets
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 Earth and Planets Laboratory Director Michael Walter. "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."
