Exoplanet TRAPPIST-1e may hold Earth-like atmosphere, JWST finds

Astronomers have taken a huge leap towards discovering habitable worlds, and the world now eagerly anticipates a rocky planet orbiting a faint star 40 light-years from home. Scientists are homing in on TRAPPIST-1e using the James Webb Space Telescope (JWST), a world that will afford the first real chance to detect an atmosphere around an Earth-sized planet.

The findings are hardly conclusive but already mark a milestone on the way toward establishing whether there are Earth-like worlds elsewhere in the universe.

The TRAPPIST-1 system has fascinated scientists since they first discovered it in 2016 using telescopes in Belgium, where, in a pinging nod to their brewing heritage, they gave it their preferred tipple as a nickname. The system's star is small relative to our Sun, about the diameter of Jupiter, but it contains seven terrestrial planets.

At least three of them are within the "habitable zone," the Goldilocks zone in which temperatures could support liquid water. TRAPPIST-1e is smack in the middle of that zone and is the most promising for being habitable.

Néstor Espinoza, an astronomer with Space Telescope Science Institute in Baltimore, captured the system's allure. "As a planetary system, it's as alien as you can get," he said. "The star is tiny, tiny — Jupiter-sized — and it's got at least seven planets that are rocks around it. Three of them are in what we call the habitable zone."

Astronomers search for atmospheres by the technique of transmission spectroscopy. As a planet crosses in front of a star, starlight travels through the thin outer atmosphere of the planet. Atmospheric molecules absorb some wavelengths of light, producing extremely feeble "fingerprints" that are measurable by telescopes.

By employing JWST's extremely powerful NIRSpec instrument, scientists collected data over four individual transits of TRAPPIST-1e in 2023. The telescope took a photograph of light at a wide range of wavelengths, from 0.6 to 5 micrometers, and provided scientists with their best opportunity yet to detect gases like carbon dioxide or methane.

But it is not an easy task. TRAPPIST-1 and other red dwarf stars are notoriously finicky. They're blanketed in seething and freezing zones, flare quite frequently, and can make it all but impossible to separate stellar signals from planetary signals. The turbulent surface of the star left fingerprints on the data, creating noise that easily drowned out or even mimicked the planet's atmospheric signatures.

The four transits gave four varying stories. Two harmonized with the cooler regions of the star, and two with hotter, brighter ones. One even cut through a stellar flare while in transit, further complicating the signal.

Traditional explanations of stellar activity were not easily able to explain this contamination. When scientists tried to extrapolate signals from wavelength to wavelength, the predictions fell apart. Something new clearly was needed.

Instead of being bound to fixed models, the group attempted a machine-learning technique known as Gaussian processes. By representing the star's activity as a statistical noise rather than a rigid input, they were then able to subtract it more dynamically. That allowed them to "marginalize" over the unwanted noise and focus on the planet.

The plan paid off. The evidence found that TRAPPIST-1e does not, most likely, possess a dense hydrogen-rich atmosphere like Neptune or Jupiter. The envelope would have left a solid mark in the evidence, and the researchers knew it eliminated with a certainty.

What is within our grasp is even more thrilling: a second, thinner atmosphere made of heavier molecules such as nitrogen or carbon dioxide. These atmospheres, formed through comet impacts or volcanism, are responsible for the rocky planets in our own solar system being habitable — or at least water-holding liquid.

The results stoked cautious optimism in astronomers. "Based on the first four observations, we cannot rule out that this planet has an atmosphere, so there is hope," Espinoza said. His group already had 15 more Webb observations scheduled, two-thirds completed.

Sara Seager, the MIT planetary scientist who was a co-author on both the papers, called the research a breakthrough. "TRAPPIST-1e is one of our most compelling habitable-zone worlds, and these new results bring us one step closer to understanding what kind of world it is," she wrote. "The evidence against Venus- and Mars-like atmospheres directs our focus onto the remaining possibilities that are still contenders."

Not everyone is willing to call it either way. Eric Agol, an astronomer at the University of Washington, described the findings as exciting but still incomplete. "I remain agnostic about whether TRAPPIST-1e could have an Earth-like atmosphere or not, and today's study hasn't changed my mind," he said.

This planet is a rocky one with its star to orbit within six days. If you placed the entire TRAPPIST-1 system within our system, all seven planets and their orbits would fit within the orbit of Mercury. That is why transits are frequent and less difficult to detect, and it is one of the reasons astronomers like red dwarf systems despite stellar tantrums.

The new data indicate TRAPPIST-1e is different from its closest siblings, possibly not having atmospheres. Being located in the habitable zone gives it a greater chance of having one, but the star itself has a tendency to radiate away gas in the long term.

Florida Tech astrophysicist Howard Chen referred to the planet as a "tipping point." Computer simulations suggest it could be an ocean planet or a dry rock, depending on what happened in its history. "That the possibility that TRAPPIST-1e can reasonably be very wet or very dry implies that future JWST observations over the next few may deliver a sensational revelation," Chen said.

The first four visits already demonstrate Webb is accurate enough to study atmospheres on Earth-sized worlds. Sensitivity had fallen as low as around 50 parts per million — an amazing achievement on a rocky world so far away. But even that level of precision isn't yet good enough to tell if TRAPPIST-1e's has a thin atmosphere, or none at all.

Additional data are on the way. Toward the end of 2025, astronomers anticipate that 15 additional transits will have been made. Some will look directly at TRAPPIST-1e and contrast it with airless brother TRAPPIST-1b in a bid to better account for stellar activity. If the signals remain uniform, any deviation may alert astronomers that gases encircle TRAPPIST-1e.

Michaël Gillon, one of the scientists on the discovery team of TRAPPIST-1, said that the new results further validate that Webb can deliver a solution to one of astronomy's largest mysteries. "For the first time in human history, we are literally within reach of finding an atmosphere on a potentially habitable rocky exoplanet," he explained.

Verification of an atmosphere on TRAPPIST-1e would be an astronomical breakthrough. It would prove that rocky planets around red dwarfs — the galaxy's most common type of star — are able to hold onto gas. That would significantly boost the number of regions where life is possible.

Even if TRAPPIST-1e were to be found to be uninhabited, the outcome would still impact future searches. Ruling out atmospheres in such planets would highlight the uniqueness of Earth and compel astronomers to center their searches on more Sun-like stars. Either outcome would impact the development of telescopes and inform the search for life for decades to come.

Research findings are available online in the journal The Astrophysical Journal Letters.

Like these kind of feel good stories? Get The Brighter Side of News' newsletter.