Forty light-years away from Earth, in the faint light of the ultracool red dwarf TRAPPIST‑1, a rocky planet slightly larger than our own is providing its first cautious hints on whether it can retain an atmosphere. TRAPPIST‑1e, the fourth planet from its parent star and firmly in the so-called habitable zone, has been the focus of an intensive campaign with NASA’s James Webb Space Telescope, whose infrared vision is beginning to lift the shroud of ignorance over this potentially life-hospitable world.
In four separate observations between mid- and late-2023, Webb’s Near-Infrared Spectrograph (NIRSpec) picked up the weakest of signals as TRAPPIST‑1e passed in front of its star. This method, called transmission spectroscopy, detects the way starlight passes through the atmosphere of a planet, if it has one, leaving a characteristic signature of absorbed wavelengths. “Webb’s infrared instruments are giving us more detail than we’ve ever had access to before,” said Space Telescope Science Institute principal investigator Néstor Espinoza. Those details, while preliminary, already enable scientists to exclude some possibilities.
The evidence strongly suggests that TRAPPIST-1e has shed its native hydrogen-helium envelope, a fate likely for planets orbiting active M-dwarfs whose intense flares tend to drive light-weight gases away. Whether a heavier, secondary atmosphere has formed in their place is still an open question. 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, said MIT’s Kavli Institute’s Ana Glidden. However, the measurements do not preclude enough CO₂ to maintain a moderate greenhouse effect—potentially stabilizing surface liquid water.
The planet’s climate is affected by tidal locking, with one half of the planet in perpetual daylight and the other in constant night. The models inform us that water, if it exists, may form a global ocean or be confined to a sunlit region at “perpetual noon,” surrounded by ice. Three‑dimensional models of tidally locked planets demonstrate that greenhouse gases such as CO₂ and CH₄ are capable of transferring heat from daytime to nighttime but the balance is fragile: high levels of methane cause formation of haze, altering the radiation budget and cooling the surface. In modern climate modeling, a thin haze at a CH₄/CO₂ ratio of 0.1 warmed the dayside by nearly 5 K, but denser hazes did the opposite.
Chasing these planetary signatures out of Webb’s spectra is complicated by “stellar contamination.” TRAPPIST‑1’s surface is streaked with starspots—cooler, magnetically active regions—that can masquerade or destroy atmospheric signals. It took the DREAMS (Deep Reconnaissance of Exoplanet Atmospheres using Multi‑instrument Spectroscopy) team over a year to factor in the effects, thanks to an innovative observational approach: observing back-to-back transits of TRAPPIST‑1b and TRAPPIST‑1e. Because planet b is considered to be a rocky body without an atmosphere, its spectrum may serve as a template for the star’s emission, so any further absorption in the spectrum of planet e can be accounted for by its atmosphere.
The approach also solves the second technical issue: distinguishing between real atmospheric absorption and instrument systematics. Webb’s NIRSpec prism mode sees 0.6–5 μm in a single point, reducing the necessity to “stitch” data from two epochs—a procedure that can introduce mismatches if the star’s output is changing. For later campaigns, combining NIRSpec with the Mid-Infrared Instrument’s low-resolution spectrograph might extend coverage to 12 μm, sampling additional molecular bands and resolving degeneracies between temperature structure and gas abundances.
Geophysical modeling offers another aspect to the scenario of habitability. Predictions of tidal heating—energy generated as the planet’s interior deforms to the star’s gravitation—indicate that TRAPPIST‑1e avoids the runaway greenhouse limit which would boil surface water. Its total heat flux, from stellar irradiation and internal dissipation, is less than critical, and stable oceans are favored. In contrast, inner siblings TRAPPIST‑1b and c likely experience Io‑like volcanism, while outer planets f, g, and h may be frozen over with subsurface seas.
For now, the verdict on TRAPPIST‑1e’s atmosphere remains open. “We’re at a very early stage,” said Ryan MacDonald of the University of St Andrews. “The atmosphere model is an excellent fit to what we see, but our errors are large enough that we can’t rule out a bare rock.” With 15 more transits scheduled, the team is looking to tighten its constraints, potentially detecting specific molecules such as CO₂ or CH₄ and measuring surface temperature. As Glidden said, “We’re in a new era of exploration that’s really exciting to be part of.”
