TRAPPIST-1 d: Searching for Signs of Air on a Nearby Earth-Sized World

Caroline Piaulet-Ghorayeb and colleagues present new James Webb Space Telescope (JWST) observations of TRAPPIST-1 d, one of seven rocky planets orbiting the ultracool M dwarf star TRAPPIST-1. This system, located just 12 parsecs away, has long fascinated astronomers because several of its planets sit within or near the habitable zone, where liquid water might exist. TRAPPIST-1 d, about 0.8 times the size of Earth, lies right at the inner edge of this zone, making it a key candidate for testing whether small, rocky planets around low-mass stars can keep atmospheres.

TRAPPIST-1 d in Context

The authors begin by outlining TRAPPIST-1 d’s background. Earlier climate models suggested that while the planet is very close to its star, reflective dayside clouds might make surface liquid water possible. However, whether such water exists depends critically on whether the planet ever cooled enough for oceans to condense. Adding to the mystery, previous Hubble and Spitzer studies could only rule out large, hydrogen-rich atmospheres but could not test for heavier ones like carbon dioxide or water vapor. TRAPPIST-1 d also stands out because its density is lower than expected, hinting it may contain extra water or other volatiles.

JWST Observations and Data Analysis

To address these questions, the team observed two of TRAPPIST-1 d’s transits with JWST’s NIRSpec/PRISM instrument, which spreads starlight into a wide range of wavelengths from 0.6 to 5.2 microns. This technique, called transmission spectroscopy, looks for tiny dips in starlight as the planet passes in front of the star. Any atmosphere would imprint a spectral fingerprint of molecules like methane (CH₄), water vapor (H₂O), or carbon dioxide (CO₂). After carefully reducing the data and correcting for contamination caused by bright patches on the star’s surface, the authors found that the resulting transmission spectrum was essentially flat, showing no convincing molecular absorption features.

Atmospheric Constraints

With no evidence for thick atmospheres, the team turned to models. Their retrieval analysis showed that JWST’s sensitivity was high enough to rule out clear, hydrogen-dominated atmospheres at more than 3σ confidence. They also ruled out atmospheres dominated by methane, water vapor, or carbon dioxide at surface pressures greater than one bar, the same as Earth’s atmospheric pressure, with better than 95% confidence. Even scenarios resembling Titan, early Mars, or both cloud-free and haze-free Earths are inconsistent with the data. If TRAPPIST-1 d has an atmosphere at all, it must either be extremely thin, heavily clouded at high altitudes, or dominated by gases that leave no strong spectral fingerprints.

Implications for Planetary Evolution

The authors then considered broader implications. One possibility is that TRAPPIST-1 d has lost its atmosphere entirely, leaving behind a bare rock like Mercury. In that case, their evolutionary models suggest the planet and its neighbors must have formed with relatively little water, less than about four Earth oceans’ worth, otherwise it would have retained a thicker atmosphere. Another possibility is that clouds or aerosols are hiding the signal, with some models predicting water clouds could form at the boundary between day and night. Either way, TRAPPIST-1 d’s outcome does not rule out atmospheres on its cooler siblings, especially TRAPPIST-1 e, f, and g, which lie deeper in the habitable zone.

Conclusion

Piaulet-Ghorayeb and collaborators show that TRAPPIST-1 d almost certainly lacks a thick secondary atmosphere. This result makes the planet less promising for habitability but provides an important benchmark in understanding where rocky exoplanets can retain air. It also sharpens the search for the so-called “cosmic shoreline,” the dividing line between airless rocks and worlds with substantial atmospheres. With JWST continuing to observe the TRAPPIST-1 system, astronomers will soon learn whether its other planets fared any better at holding onto their skies.

Source: Piaulet-Ghorayeb