Washington, DC—In a groundbreaking discovery, a team of astronomers led by Carnegie Science has identified compelling evidence of an atmosphere around a rocky exoplanet, defying long-held scientific assumptions. Their findings, published in The Astrophysical Journal Letters, utilized NASA’s James Webb Space Telescope (JWST) to reveal an unexpected atmospheric presence on TOI-561 b, an ancient, ultra-hot super-Earth potentially hosting a magma ocean.
TOI-561 b, a rocky planet about twice the mass of Earth, orbits its host star at a distance only a fraction of that between Mercury and the Sun. Despite its proximity to a star slightly less massive and cooler than our Sun, the planet completes an orbit in just 10.56 hours, resulting in perpetual daylight on one side.
“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, Carnegie Science Postdoctoral Fellow and second author of the paper. “But our observations suggest it is surrounded by a relatively thick blanket of gas, upending conventional wisdom about ultra-short-period planets.”
Challenging Conventional Wisdom
In our Solar System, smaller and hotter planets have historically been unable to retain their primordial gaseous envelopes. However, TOI-561 b’s host star, much older than our Sun, still supports an intact atmosphere. This atmospheric presence could elucidate the planet’s surprisingly 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,” noted Johanna Teske, Carnegie Science astronomer and lead author of the study.
The research team hypothesized that TOI-561 b’s low density might be attributed to a small iron core and a mantle composed of less dense rock than Earth’s. Teske elaborated, “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.”
Testing the Atmospheric Hypothesis
To verify the existence of TOI-561 b’s atmosphere, the team employed JWST’s Near-Infrared Spectrograph (NIRSpec) to measure the planet’s dayside temperature. The technique, akin to methods used for the TRAPPIST-1 system, involves assessing the brightness decrease as the planet moves behind its star.
Without an atmosphere, TOI-561 b’s dayside temperature should approach 4,900 degrees Fahrenheit (2,700 degrees Celsius). Yet, NIRSpec observations indicate a significantly cooler temperature of approximately 3,200 degrees Fahrenheit (1,800 degrees Celsius).
“We really need a thick volatile-rich atmosphere to explain all the observations,” said Anjali Piette, co-author and 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.”
Theories and Implications
While JWST’s observations provide compelling evidence for an atmosphere, questions remain about how a small planet under intense radiation retains such a substantial atmosphere. Some gases are likely escaping, but not as efficiently as anticipated.
“We think there is an equilibrium between the magma ocean and the atmosphere,” explained Tim Lichtenberg, co-author and member of the Carnegie-led 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.”
These findings are part of JWST’s General Observers Program 3860, which involved over 37 hours of continuous observation as TOI-561 b completed nearly four full orbits. The team is now analyzing the complete data set to map the planet’s temperature and refine the atmospheric composition.
Looking Ahead
Johanna Teske concluded, “What’s really exciting is that this new data set is opening up even more questions than it’s answering.” The research underscores Carnegie Science’s long-standing excellence in exoplanetary studies, dating back to JWST’s conception three decades ago.
Since JWST began its mission, Carnegie scientists, including Teske and Wallack, have led numerous teams, yielding groundbreaking insights into exoplanet atmospheres and galaxy formation. “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, Earth and Planets Laboratory Director. “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.”
