WASHINGTON, DC—A groundbreaking discovery by a Carnegie-led team of astronomers has unveiled the presence of a thick atmosphere around a rocky exoplanet, challenging long-standing assumptions about such celestial bodies. The findings, published in The Astrophysical Journal Letters, were made possible through observations by NASA’s James Webb Space Telescope (JWST), revealing an alien atmosphere on TOI-561 b, an ancient and ultra-hot super-Earth.
TOI-561 b, a rocky planet with twice the mass of Earth, orbits its host star at a distance forty times closer than Mercury is to the Sun. This proximity results in a year lasting just 10.56 hours, with one side of the planet perpetually bathed in daylight. Despite these harsh conditions, the planet retains a significant atmosphere, defying conventional wisdom.
Challenging Conventional Wisdom
Carnegie Science Postdoctoral Fellow Nicole Wallack, the paper’s second author, explained the significance of the discovery. “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,” she stated. “But our observations suggest it is surrounded by a relatively thick blanket of gas, upending conventional wisdom about ultra-short-period planets.”
In our own Solar System, smaller and hotter planets typically lose their primordial gas envelopes shortly after formation. However, TOI-561 b’s host star, much older than our Sun, has allowed its atmosphere to remain intact. This atmosphere may also account for the planet’s unusually low density.
Decoding the Planet’s Composition
Carnegie Science astronomer Johanna Teske, the paper’s lead author, noted, “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.” The team’s observations suggest that TOI-561 b might have a smaller iron core and a mantle composed of less dense rock than Earth’s interior.
“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,” Teske added. “It must have formed in a very different chemical environment from the planets in our own Solar System.”
Testing Atmospheric Theories
The research team used JWST’s Near-Infrared Spectrograph (NIRSpec) to measure the planet’s dayside temperature, which revealed surprising results. If TOI-561 b were a bare rock, its dayside temperature should approach 4,900 degrees Fahrenheit (2,700 degrees Celsius). However, observations indicated a temperature closer to 3,200 degrees Fahrenheit (1,800 degrees Celsius), suggesting the presence of a substantial atmosphere.
Anjali Piette, a co-author from the University of Birmingham, explained, “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. 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.”
Implications for Exoplanet Studies
The discovery raises questions about how such a small planet can maintain a thick atmosphere despite intense radiation. Some gases may be escaping to space, but not as efficiently as expected. Tim Lichtenberg from the University of Groningen, a member of the Carnegie-led AEThER project, suggested, “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.”
These findings are part of the first results from JWST’s General Observers Program 3860, which involved continuous observation of the system for over 37 hours. The team is analyzing the full data set to further understand the planet’s atmospheric composition and temperature distribution.
Carnegie Science’s leadership in this research continues a tradition of excellence dating back to JWST’s earliest conception. The telescope’s capabilities have enabled groundbreaking discoveries in exoplanet atmospheres, galaxy formation, and more.
Michael Walter, Director of the Earth and Planets Laboratory, remarked, “These JWST-powered breakthroughs tap directly into our long-standing strength in understanding how exoplanet characteristics are shaped by planetary evolution and dynamics. 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.”
