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University of Bristol astrophysicists are helping shed new light on an Earth-sized exoplanet 40 light years away where liquid water in the form of a global ocean or icy expanse might exist on its surface. That would only be possible if an atmosphere is present—a big mystery that the scientists are attempting to unravel and now even closer to solving using the largest telescope in space.

Deploying NASA's JWST, the researchers have reached these discoveries as part of a major international project which is probing the atmosphere and surface of TRAPPIST-1e, also more simply known as planet e in the system, orbiting within the habitable zone of red dwarf star TRAPPIST-1.

Exoplanets are highly varied planets which orbit stars outside the solar system. Planet e is of particular interest because the presence of liquid water—not too hot or cold—is theoretically viable, but only if the planet has an atmosphere.

Researchers aimed JWST's powerful NIRSpec (Near-Infrared Spectrograph) instrument at the system as planet e passed in front of its star. Starlight passing through the planet's atmosphere, if there is one, will be partially absorbed and the corresponding dips in the light spectrum that reaches JWST tell astronomers what chemicals are found there. With each additional transit, the atmospheric contents become clearer.

Initial results, published today in two scientific papers in The Astrophysical Journal Letters, indicate several potential scenarios, including the possibility of an atmosphere.

Dr. Hannah Wakeford, Associate Professor in Astrophysics at the University of Bristol, is a leading member of the JWST Transiting Exoplanet team who helped design the observational set-up for the telescope to ensure scientists obtain vital data.

Dr. Wakeford said, "What we have found with JWST in these first four observations helps refine the earlier Hubble measurements and reveals there might now be hints of an atmosphere, but we cannot yet rule out the possibility there is nothing to detect."

"JWST's infrared instruments are providing unprecedented detail, helping us understand much more about what determines a planet's atmosphere and surface environment, and what they're composed of. It's incredibly exciting to be peeling back the curtain on these fascinating other worlds, measuring the details of starlight around Earth-sized planets to ascertain what it might be like, and if life could be possible. Through a careful process of elimination and comparison, we're uncovering great new insights."

Although various possibilities remain open for planet e, the researchers are confident the planet does not have its original atmosphere.

Co-author of both studies, Dr. David Grant, a former Senior Research Associate at the University of Bristol, explained, "The findings also further rule out the presence of a primordial hydrogen-based atmosphere. This is the gaseous envelope, mainly comprising hydrogen, that surrounded a planet in its early stages of formation. Such atmospheres are believed to be common for both giant planets and terrestrial planets in the early solar system."

Dr. Wakeford added, "Since TRAPPIST-1 is a very active star, with frequent flares, it's not surprising that any hydrogen-helium atmosphere the planet may have formed would be stripped off by stellar radiation. Many planets, including Earth, build up a heavier secondary atmosphere after losing their primary atmosphere. It is possible planet e was never able to do this and doesn't have a secondary atmosphere, but there's an equal chance one does exist."

The presence of a secondary atmosphere means liquid water could also exist on the surface and if that's the case, researchers understand it would be accompanied by a greenhouse effect, akin to that of Earth, in which various gases, especially carbon dioxide, keep the atmosphere stable and the planet warm.

The second paper details work on the theoretical interpretation and lead author Dr. Ana Glidden, a post-doctoral researcher at Massachusetts Institute of Technology, explained: "It is unlikely the atmosphere of planet e is dominated by carbon dioxide, like the thick atmosphere of Venus and the thin atmosphere of Mars. But it's also important to note there are no direct parallels with our solar system. TRAPPIST-1 is a very different star from our sun, and the planetary system around it is also distinct."

Dr. Wakeford added, "A little greenhouse effect can go a long way and the new measurements do not rule out sufficient carbon dioxide to sustain some liquid water on the surface. The liquid water could take the form of a global ocean, or cover a smaller area of the planet where the star is at perpetual noon, surrounded by ice. This would be possible because, owing to TRAPPIST-1's planets' sizes and close orbits to their star, they are all tidally locked, with one side always facing the star and the other side in perpetual darkness."

Next steps in the research will involve further detailed observations, comparing data from another exoplanet—planet b—orbiting closest to TRAPPIST-1 in order to make more revelations.

One of the principal investigators of the research team focused on TRAPPIST-1e Dr. Néstor Espinoza, an Associate Astronomer and Mission Scientist for Exoplanet Science at the Space Telescope Science Institute (STScI) in Baltimore, Maryland, said, "Webb's infrared instruments are giving us more detail than we've ever had access to before, and the initial four observations we've been able to make of planet e are showing us what we will have to work with when the rest of the information comes in."

The JWST is the world's premier space science observatory, capable of observing distant worlds and stars, and probing the mysterious structures of our universe. It is an international program led by NASA, the European Space Agency, and the Canadian Space Agency.

The project is part of the JWST-TST DREAMS program, led by Dr. Nikole Lewis, Associate Professor of Astronomy at Cornell University in Ithaca, New York. This international project involves more than 30 scientists from the U.K., U.S., and India, five of whom are members or former members of Dr. Wakeford's team. It includes the breakthrough detection of Quartz clouds in the atmosphere of a hot exoplanet, as shown in a 2023 study, led by Dr. Grant and co-authored by Dr. Wakeford.