Webb Telescope Detects Thick Atmosphere Around Broiling Lava World

Observations of the ultra-hot super-Earth exoplanet TOI-561 b show the strongest evidence yet for an atmosphere on a rocky planet outside our Solar System.

Researchers using NASA’s James Webb Space Telescope have detected the strongest evidence yet for an atmosphere on a rocky planet outside our Solar System, as NASA leads the world in exploring the universe from the Moon to Mars and beyond. Observations of the ultra-hot super-Earth TOI-561 b suggest that the exoplanet is surrounded by a thick blanket of gases above a global magma ocean. The results help explain the planet’s unusually low density and challenge the prevailing wisdom that relatively small planets so close to their stars are not able to sustain atmospheres.

With a radius roughly 1.4 times Earth’s, and an orbital period less than 11 hours, TOI-561 b falls into a rare class of objects known as ultra-short period exoplanets. Although its host star is only slightly smaller and cooler than the Sun, TOI-561 b orbits so close to the star — less than one 1.6 million kilometers (one-fortieth the distance between Mercury and the Sun) — that it must be tidally locked, with the temperature of its permanent dayside far exceeding the melting temperature of typical rock.

“What really sets this planet apart is its anomalously low density,” said  Johanna Teske, staff scientist at Carnegie Science Earth and Planets Laboratory and lead author on a paper published Thursday in The Astrophysical Journal Letters. “It’s not a super-puff, but it is less dense than you would expect if it had an Earth-like composition.”

An artistic representation of lava world TOI-561 b and its host star. (Credit: NASA, ESA, CSA, Ralf Crawford, STScI)

One explanation the team considered for the planet’s low density was that it could have a relatively small iron core and a mantle made of rock that is not as dense as rock within Earth. Teske notes that this could make sense: “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.” The planet’s composition could be representative of planets that formed when the universe was relatively young.

But an exotic composition can’t explain everything. The team also suspected that TOI-561 b might be surrounded by a thick atmosphere that makes it look larger than it actually is. Although small planets that have spent billions of years baking in blazing stellar radiation are not expected to have atmospheres, some show signs that they are not just bare rock or lava.

To test the hypothesis that TOI-561 b has an atmosphere, the team used Webb’s NIRSpec (Near-Infrared Spectrograph) to measure the planet’s dayside temperature based on its near-infrared brightness. The technique, which involves measuring the decrease in brightness of the star-planet system as the planet moves behind the star, is similar to that used to search for atmospheres in the TRAPPIST-1 system and on other rocky worlds.

If TOI-561 b is a bare rock with no atmosphere to carry heat around to the nightside, its dayside temperature should be approaching 2,700 degrees Celsius. But the observations from the NIRSpec instrument show that the planet’s dayside appears to be closer to 1,800 degrees Celsius — still extremely hot, but far cooler than expected.

To explain the results, the team considered a few different scenarios. The magma ocean could circulate some heat, but without an atmosphere, the nightside would probably be solid, limiting flow away from the dayside. A thin layer of rock vapor on the surface of the magma ocean is also possible, but on its own would likely have a much smaller cooling effect than observed.

“We really need a thick volatile-rich atmosphere to explain all the observations,” said Anjali Piette, coauthor from the University of Birmingham, United Kingdom. “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.”

While the Webb observations provide compelling evidence for such an atmosphere, the question remains: How can a small planet exposed to such intense radiation hold on to any atmosphere at all, let alone one so substantial? Some gases must be escaping to space, but perhaps not as efficiently as expected.

“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.”

These are the first results from Webb’s General Observers Program 3860, which involved observing the system continuously for more than 37 hours while TOI-561 b completed nearly four full orbits of the star. The team is currently analyzing the full data set to map the temperature all the way around the planet and narrow down the composition of its atmosphere.

Lisa Dang, Professor at the University of Waterloo and former IREx postdoctoral researcher, is a co-author of the study.

Samuel Boucher, UdeM undergraduate student and Prof. Dang’s summer intern, is also a co-author of the study.

Canadian researchers played a key role in interpreting Webb’s surprising view of TOI-561 b. Prof. Lisa Dang (University of Waterloo), formerly a postdoctoral researcher at UdeM and IREx, helped design the project, carried out an independent data analysis, and focused on placing TOI-561 b in the broader context of how rocky planets gain, lose, and recycle their atmospheres.

“In the past year, there’s been a strong focus on what drives atmospheric escape,” said Dang. “What’s exciting here is that lava worlds remind us that the planet’s interior matters just as much. For TOI-561 b, it’s fascinating to see hints that a rocky planet could sequester so many volatiles in its mantle, and that Webb can now reveal these deep geologic processes.”

Her work was complemented by Samuel Boucher, an undergraduate student at UdeM and her IREx summer intern, who performed an independent analysis of the full-orbit phase curve. His contribution helped address key peer-review questions and strengthened confidence in the paper’s eclipse-based results. Boucher and Dang are now leading a follow-up study using the complete phase-curve dataset to probe the planet’s day–night temperature contrast and further constrain its atmosphere.

Dang also noted the striking contrast between these findings and the recent study on the ultra-hot Jupiter WASP-121 b she co-authored, which is losing its atmosphere at an extreme rate.“I am really amazed that with Webb, we are finding evidence of both extreme atmospheric retention and extreme atmospheric loss,” exclaims Dang. “Together, these discoveries are reshaping our understanding of what promotes escape or preservation across exoplanets.”

The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our Solar System, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our Universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the CSA (Canadian Space Agency).

Press release written by Margaret W. Carruthers (STScI) and Hannah Braun (STScI), adapted by Nathalie Ouellette (IREx).

For more information

• Scientific paper on The Astrophysical Journal Letters
• Scientific paper on arXiv
• NASA Press Release
• Press Release on WASP-121 b

Scientific Contact

Lisa Dang
Professor
University of Waterloo
lisa.dang@uwaterloo.ca

Media Contact

Nathalie Ouellette
JWST Outreach Scientist
Université de Montréal
nathalie@astro.umontreal.ca