Discovered in 2017, the TRAPPIST-1 system, located 40 light-years away, consists of at least seven rocky planets comparable in size to Earth orbiting a small red dwarf star. Of these seven planets, four receive enough energy to theoretically support liquid water on their surfaces, a key criterion in the search for life elsewhere in the Universe!

Since its launch, the JWST has been observing the planets in this system to detect possible atmospheres. The first observations focused on the planets closest to the star, TRAPPIST-1b and c. They made it possible to measure their temperatures and rule out the presence of certain dense atmospheres similar to those of Venus and Earth.

In this new study, scientists used the JWST’s NIRSpec instrument for the first time to observe the transit spectra of TRAPPIST-1d. This is the third planet from the star and receives the most Earth-like insolation. The aim was to detect the spectral signatures of a possible atmosphere.

Artistic rendition of TRAPPIST-1 d passing in front of its turbulent star, with other members of the closely packed system shown in the background. © NASA, ESA, CSA, Joseph Olmsted

This work was made particularly difficult by the activity of the star TRAPPIST-1. Like many red dwarfs, the star is subject to variations in brightness (due in particular to the presence of spots) which can ‘contaminate’ the observed signal, thus complicating the identification of the planet’s true contribution.

The observations did not reveal any clear signatures of molecules characteristic of a dense atmosphere (such as Earth’s atmosphere), such as water vapour, carbon dioxide or methane. This absence of signals suggests that TRAPPIST-1d either has no atmosphere, is enveloped in an extremely thin atmosphere (like Mars), or is covered by thick clouds at high altitude, like Venus.

This result marks a major milestone: it is the first time that the JWST has been able to examine a temperate, rocky planet outside the Solar System in such detail. New observations are currently being analysed for the planets further away from TRAPPIST-1. Due to their distance from the system and their higher gravity, they may have better preserved their atmospheres.

This research is part of the effort to better understand the conditions necessary for the emergence of habitable worlds and refines our view of the diversity of rocky exoplanets in the Universe.

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Reference
Caroline Piaulet-Ghorayeb et al., Strict Limits on Potential Secondary Atmospheres on the Temperate Rocky Exo-Earth TRAPPIST-1 d, 2025 ApJ 989 181

Contact
Martin Turbet, Laboratoire de météorologie dynamique (LMD-IPSL) •

Source: CNRS Terre & Univers.