A Spectroscopic Survey of the TRAPPIST-1 System: Atmospheric Characterization of Seven Terrestrial Worlds

Located approximately 12.1 parsecs (40 light-years) from Earth, the TRAPPIST-1 star is a late M-type ultracool dwarf. Its seven known planets (designated b through h) are locked in a complex chain of orbital resonance, suggesting a calm and orderly formation history. The planets have sizes and masses consistent with a rocky composition. Notably, planets d, e, f, and g orbit within the star's habitable zone, the region where surface temperatures could potentially allow for liquid water, making this system a prime target in the search for life.

We utilized the JWST's Near-Infrared Spectrograph (NIRSpec) and Mid-Infrared Instrument (MIRI) to capture high-precision transit spectra of all seven planets. Data was collected over multiple transit events for each planet to build a sufficient signal-to-noise ratio (SNR) and to mitigate the effects of stellar activity. The resulting transmission spectra were analyzed using advanced atmospheric retrieval models to identify the presence and abundance of key molecules, such as H₂O, CO₂, CH₄, and CO, and to test for the presence of clouds or hazes.

Our analysis reveals that the two innermost planets, TRAPPIST-1b and c, likely possess no significant, hydrogen-dominated atmospheres. Their transmission spectra are flat, which is consistent with bare rock surfaces or, potentially, very thin, high-mean-molecular-weight atmospheres similar to Venus. The high levels of stellar irradiation these planets receive would likely have stripped away any primordial, lighter atmospheres over their lifetimes.

The planets within the habitable zone presented a diverse and complex set of results, challenging simplistic models of habitability.

The outermost planet, TRAPPIST-1h, also yielded a flat spectrum. Given its greater distance from the star, it is considered too cold for liquid water. Our findings are consistent with it being a frozen, airless ice world, though a thin atmosphere cannot be entirely ruled out at this time.

This systematic survey provides a crucial reality check in the search for life around M-dwarf stars. The results suggest a diversity of atmospheric outcomes even among sibling planets of similar size and mass. The lack of thick, hydrogen-rich atmospheres on the inner worlds is consistent with models of atmospheric stripping by stellar winds and high-energy radiation, a key concern for the habitability of planets orbiting active red dwarfs. The ambiguous results for the habitable zone planets highlight the immense challenge of characterizing these worlds and demonstrate that proximity to the habitable zone does not guarantee a hospitable, Earth-like atmosphere.

Our initial survey of the TRAPPIST-1 system has provided the first comparative atmospheric reconnaissance of a family of Earth-sized exoplanets. While no definitive biosignatures have been detected, we have successfully constrained the atmospheric possibilities, ruling out primordial hydrogen envelopes for the inner planets and identifying TRAPPIST-1e as a high-priority target for future, deeper investigation. The next phase of research will involve significantly longer-duration observations with JWST to build the SNR needed to detect more subtle atmospheric features and potentially constrain the presence of key biosignature gases.