Of the seven Earth-sized planets circling the red dwarf star TRAPPIST-1, one world has become a special focus for astronomers. This planet, TRAPPIST-1e, orbits within the star's "Goldilocks zone" -- a region where temperatures could allow liquid water to exist on the surface -- but only if the planet has an atmosphere to help regulate those conditions. Where liquid water can persist, the possibility of life follows naturally.
Two recent scientific papers report the first detailed observations of the TRAPPIST-1 system made with NASA's James Webb Space Telescope. These studies, published in the Astrophysical Journal Letters, come from a research team that includes Sukrit Ranjan of the University of Arizona's Lunar and Planetary Laboratory. The authors carefully examine the data collected so far and describe several plausible possibilities for what TRAPPIST-1e's atmosphere and surface might be like.
A third paper, however, urges restraint. While the early findings are encouraging and represent a major step toward understanding one of the closest potentially Earth-like exoplanets, Ranjan argues that stronger evidence is needed. In particular, he calls for more rigorous studies to test whether TRAPPIST-1e actually possesses an atmosphere and whether the tentative hints of methane seen by James Webb really come from the planet, rather than from its host star.
A Compact Planetary System Close to Home
The TRAPPIST system gets its name from the survey that first identified it -- the "Transiting Planets and Planetesimals Small Telescope project." This planetary family lies roughly 39 light-years from Earth. It can be thought of as a scaled-down version of our own solar system, since the star and all seven planets would fit comfortably inside the orbit of Mercury. Years pass very quickly there: each TRAPPIST planet completes an orbit around the star in just a few Earth days.
"The basic thesis for TRAPPIST-1e is this: If it has an atmosphere, it's habitable," said Ranjan, who is an assistant professor at LPL. "But right now, the first-order question must be, 'Does an atmosphere even exist?'"
How Webb Searches for an Atmosphere
To investigate that question, the team used the James Webb Space Telescope's powerful Near-Infrared Spectrograph (NIRSpec). They pointed the instrument at the TRAPPIST system while TRAPPIST-1e transited -- i.e. passed in front of -- its host star. During a transit, some of the starlight passes through any atmosphere surrounding the planet and certain wavelengths are absorbed. By measuring this filtered starlight, astronomers can infer which gases are present. Repeating this process over multiple transits gradually sharpens the picture of the planet's atmospheric chemistry.
Across four such transits of TRAPPIST-1e, the team saw faint indications of methane. However, TRAPPIST-1 is a so-called M dwarf star, only about one tenth the size of the sun and just slightly larger than Jupiter. Because stars of this type have different physical properties from our sun, Ranjan notes that scientists must be especially cautious when interpreting any potential planetary signal.
"While the sun is a bright, yellow dwarf star, TRAPPIST-1 is an ultracool red dwarf, meaning it is significantly smaller, cooler and dimmer than our sun," he explained. "Cool enough, in fact, to allow for gas molecules in its atmosphere. We reported hints of methane, but the question is, 'is the methane attributable to molecules in the atmosphere of the planet or in the host star?'"
Probing the Methane Mystery
To explore that question, Ranjan and his colleagues modeled a range of possible atmospheres for TRAPPIST-1e, focusing on scenarios rich in methane. They then calculated how likely each case would be, given the Webb data. In the most plausible of the tested situations, TRAPPIST-1e ended up looking broadly similar to Titan, Saturn's methane-rich moon. Even so, the analysis showed that this scenario remained very unlikely.
"Based on our most recent work, we suggest that the previously reported tentative hint of an atmosphere is more likely to be 'noise' from the host star," Ranjan said. "However, this does not mean that TRAPPIST-1e does not have an atmosphere -- we just need more data."
Ranjan also emphasizes that, despite its remarkable capabilities, the James Webb Space Telescope was not built with small, Earth-sized exoplanets as its primary targets.
"It was designed long before we knew such worlds existed, and we are fortunate that it can study them at all," he said. "There's only a handful of Earth-sized planets in existence for which it could potentially ever measure any kind of detailed atmosphere composition."
New Missions and Techniques on the Horizon
Future observations may help resolve the uncertainties. One promising effort is NASA's Pandora mission, a small satellite now being developed and scheduled to launch in early 2026. Led by Daniel Apai, a professor of astronomy and planetary sciences at the U of A's Steward Observatory, Pandora is specifically intended to study exoplanet atmospheres and their host stars. The spacecraft will track stars that host potentially habitable planets before, during and after planetary transits, improving scientists' ability to separate stellar effects from true atmospheric signals.
In parallel, the TRAPPIST-1e research team is working on a larger program of observations and applying new analysis methods that might finally clarify whether the planet has an atmosphere. A key approach is known as dual transit. In this method, astronomers observe the star at times when both TRAPPIST-1e and TRAPPIST-1b, the system's innermost and airless planet, cross in front of the star at the same time.
"These observations will allow us to separate what the star is doing from what is going on in the planet's atmosphere -- should it have one," Ranjan said.
