JWST data leave open possibility of life-supporting atmosphere on nearby Earth-sized world

New observations with the James Webb Space Telescope (JWST) have produced the first strong constraints on the atmosphere of TRAPPIST-1e, an Earth-sized planet about 40 light-years away, but do not yet determine whether the world is habitable.

Astronomers using JWST’s Near-Infrared Spectrograph (NIRSpec) recorded four transits of TRAPPIST-1e, measuring changes in the star’s light as the planet passed in front of it. The transmission spectrum is inconsistent with a primordial hydrogen-helium envelope and with a thin, carbon-dioxide–dominated atmosphere like Mars or Venus, the research team reported, but it is consistent with two remaining possibilities: the planet either has no substantial atmosphere or it hosts a heavier, nitrogen-rich secondary atmosphere that could moderate surface temperatures and permit liquid water under the right conditions.

TRAPPIST-1e orbits an ultra-cool M dwarf star known as TRAPPIST-1 and completes a full orbit in about 6.1 Earth days. The planet has a mass roughly 0.692 times that of Earth and lies at roughly 3% of the Earth–Sun distance from its host star; because TRAPPIST-1 is much cooler than the Sun, that close orbit places the planet in the system’s habitable zone. The star is magnetically active, producing flares and starspots that complicate precise spectroscopic measurements.

"The amount of light that would shine through an atmosphere like our own is incredibly small," said Professor Hannah Wakeford of the University of Bristol, a co-author on the study. The team spent about a year collecting data and correcting for stellar activity to reach the sensitivity needed to search for changes at the parts-per-million level.

Dr. Ryan MacDonald of the University of St Andrews, another study co-author, summarized the remaining interpretations: "We are seeing two possible explanations. The most exciting possibility is that TRAPPIST-1e could have a so-called secondary atmosphere containing heavy gases like nitrogen." A nitrogen-rich atmosphere would produce greenhouse warming that could keep temperatures stable, particularly if the planet is tidally locked and concentrates liquid water on the star-facing hemisphere while ice persists on the night side.

Planetary scientists expect that small planets forming close to their stars first acquire a hydrogen-helium primordial atmosphere from the protoplanetary disk, but intense stellar activity can strip that light gas away early in a system’s history. If TRAPPIST-1e retains any atmosphere today, the researchers argue, it would likely be a secondary atmosphere produced by volcanic outgassing and other geological processes, analogous in broad terms to the processes that created Earth’s early atmosphere.

The JWST observations rule out a thick primordial envelope and a thin CO2-rich atmosphere but cannot yet discriminate between a heavier secondary atmosphere and the non-atmosphere case. The current result is based on four transits; the team plans to increase the dataset to 20 transits in coming years, which the researchers say should allow them to distinguish between the two scenarios and place tighter limits on the composition and pressure of any atmosphere.

The new constraints on TRAPPIST-1e follow a separate JWST transit study of TRAPPIST-1d, another planet in the same system, which found no evidence for an Earth-like atmosphere there. Together, those results illustrate the diversity of atmospheric outcomes in compact systems around ultra-cool dwarfs and demonstrate JWST’s capability to probe small, temperate exoplanets.

The authors caution that the present measurements do not address the presence of life or the planet’s suitability for human visitation. While a nitrogen-rich atmosphere could permit liquid water under certain configurations, detecting biosignatures or assessing long-term habitability will require further observations and complementary studies of the planet’s surface and interior state.

TRAPPIST-1 is a system of seven known planets orbiting an ultra-cool dwarf about 40 light-years away in the constellation Aquarius. The planets are tightly packed and, because of the star’s low luminosity, several lie in regions where temperatures could permit liquid water under favorable atmospheric and geological conditions. The new JWST results mark an early milestone in the detailed atmospheric study of such small, temperate worlds and set the stage for additional JWST time and future instruments to refine assessments of habitability across the system.