Astronomers might be close to confirming the presence of an Earth-like atmosphere on an exoplanet for the first time, if more detailed analyses verify preliminary observations from the James Webb Space Telescope.
The planet is part of a planetary system about 40 light-years away from Earth called TRAPPIST-1, which five Belgian astronomers discovered in 2016 and named after their favorite beer. The system has been heavily studied ever since.
“As a planetary system, it’s as alien as it gets,” said Néstor Espinoza, an astronomer at the Space Telescope Science Institute in Baltimore. “The star is very, very small — the size of Jupiter — and it has at least seven rocky planets orbiting around it. Three of those are in what we call the habitable zone, which means they are close enough to the star that if they had an atmosphere, they could sustain liquid water.”
Espinoza and his colleagues focused on TRAPPIST-1 e, the fourth planet from the star in the system, in a study published last week in The Astrophysical Journal Letters. Four observations performed in 2023 with the Webb telescope were unable to rule out the presence of an atmosphere, prompting optimism.
“Based on the first four observations, we cannot tell that (this planet) doesn’t have an atmosphere, so the dream is still alive — it could still have an atmosphere,” Espinoza said, “and that’s super exciting, because we do have a follow-up program of 15 more observations.”
The Webb telescope has only been able to rule out any atmosphere on TRAPPIST-1 b, the innermost planet, but the jury is still out on the other six, Espinoza said, with TRAPPIST-1 e seen as one of the best candidates for liquid surface water.
“Three years ago, before the launch of James Webb, these kinds of studies were science fiction,” Espinoza said about the possibility of detecting atmospheres on faraway planets. “Now I’m pretty confident we’ll be able to see what kind of atmosphere TRAPPIST-1 e could have — and if it has an Earth-like atmosphere, we’ll be able to tell.”
TRAPPIST-1 e is similar to Earth in size and orbits its star every six days — a much faster rate than Earth orbits the sun. That’s because the star is much smaller than our sun, and all the planets are close to it. “If you could magically bring the TRAPPIST-1 star to our solar system,” Espinoza said, “all the planets and their orbits would fit within the orbit of Mercury.”
When looking for an atmosphere, astronomers wait for a planet to transit in front of its star and notice any minor changes in the starlight that filters through. They search for telltale signs of an atmosphere while learning about its chemical composition.
With the four transits observed by the Webb telescope in 2023, astronomers were able to rule out the presence of a primary, hydrogen-based atmosphere on TRAPPIST-1 e, which was likely blown away by significant amounts of radiation emitted by its star. (Earth similarly lost its primordial, primary atmosphere early on but then built a secondary one; astronomers hope that TRAPPIST-1 e could have done the same.)
This scenario is detailed in a second study, also published last week in The Astrophysical Journal Letters. It notes that the planet is unlikely to have an atmosphere rich in carbon dioxide like those of Venus and Mars, and points toward a nitrogen-heavy one, more similar to Earth and Saturn’s icy moon Titan.
“TRAPPIST-1e remains one of our most compelling habitable-zone planets, and these new results take us a step closer to knowing what kind of world it is,” Sara Seager, a professor of planetary science at the Massachusetts Institute of Technology and coauthor of both studies, said in a statement. “The evidence pointing away from Venus- and Mars-like atmospheres sharpens our focus on the scenarios still in play.”
Espinoza said that his team plans to finish the new round of 15 observations by the end of the year, with two-thirds already completed. If the team finds irrefutable signs of an atmosphere, even more Webb telescope observations could be warranted in the hopes of detecting specific chemical signatures of gases such as methane, which is associated with life on Earth.
Confirmation of an atmosphere would be groundbreaking, Espinoza said. “It would settle a huge debate going on right now on whether these red dwarf systems can sustain an atmosphere or not,” he said. “Red dwarfs are actually the majority of the stars in the universe. So, if it can happen there, it can happen anywhere. The possibilities for life just kind of multiply.”
However, even if an atmosphere was ruled out, the result would still be exciting, he said, because it would make life on Earth even more special. It would also set the stage for future research to look at exoplanets orbiting around yellow dwarf stars such as the sun, using telescopes even more powerful than Webb that are still in the planning stages.
These first James Webb results for TRAPPIST-1e are immensely exciting, said Michaël Gillon, research director of the Astrobiology Research Unit at Belgium’s University of Liège. Gillon, who led the original discovery of the TRAPPIST-1 system, was not part of the recent study.
“The data are not yet conclusive,” he added in an email, “but they prove that JWST has the power to detect an Earth-like atmosphere if it exists. For the first time in history, we are truly within reach of discovering an atmosphere around a potentially habitable rocky exoplanet.”
The planets in TRAPPIST-1 have a range of conditions similar to the terrestrial planets in our solar system. These conditions make them potential hosts for surface liquid water, but they require an atmosphere to prevent the water from freezing on the surface or sublimating to space, said Eric Agol, a professor in the department of astronomy at the University of Washington. Agol was not involved with the study but is working on the new set of observations with Espinoza and his colleagues.
TRAPPIST-1 e is the most promising of the seven planets for hosting an Earth-like atmosphere, according to Agol.
“The results are interesting, but also incomplete,” he added in an email. “I am agnostic about whether TRAPPIST-1 e may have an Earth-like atmosphere or not, and the current study hasn’t affected my opinion.”
According to Howard Chen, an assistant professor of space sciences at the Florida Institute of Technology, TRAPPIST-1e sits right at a “tipping point” between opposite states, based on computer simulations that he detailed in a new study that published Wednesday in The Astrophysical Journal Letters. Based on its history, the planet can emerge as either extremely dry or water-rich: “That means both ‘bare rock’ and ‘atmosphere-bearing’ scenarios remain physically plausible,” he added in an email. Chen was not involved with the new research.
TRAPPIST-1e could have formed as a desert world, like its inner neighbors, or as a wet, ocean-covered planet, he concluded. “JWST’s current data can’t distinguish between those scenarios yet, but that ambiguity is exactly what our simulations predict,” Chen said. “The fact that TRAPPIST-1e can plausibly be either very dry or very wet means the next few JWST (observations) could deliver a dramatic reveal.”
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