Characterize the chemical composition of atmospheres on rocky exoplanets

Determine the chemical composition of atmospheres on rocky exoplanets by identifying their molecular constituents and abundances, so that detected atmospheres can be interpreted in terms of thermochemical and photochemical processes.

Background

The authors note that, to date, transmission spectra of several rocky planets have not shown definitive spectral features, underscoring the need for observations capable of constraining atmospheric composition. They highlight that JWST offers access to key molecular bands (e.g., CO2 near 15 μm) and that complementary HST UV data are crucial for modeling photochemistry and atmospheric loss.

Establishing composition is central to assessing habitability-related parameters (e.g., redox state, presence of CO2 and CH4), and is an essential next step once atmospheres are detected on rocky worlds.

References

It is not known which planets have atmospheres, or what the chemical composition may be of any atmospheres that are present.

— Report of the Working Group on Strategic Exoplanet Initiatives with HST and JWST  (2404.02932 - Redfield et al., 2024) in Section 7, Frequently Asked Questions: Why not transits?

Some of the more pressing open questions in this field are: What are exoplanet atmospheric compositions and dynamics? How do atmospheric escape rates depend on various stellar and planetary properties? How does atmospheric escape shape exoplanet populations? How do atmospheres evolve over time? How do we detect hydrogen exospheres and metal escape in dozens of exoplanet systems, down to Earth-sizes? How does this influence exoplanet habitability?

— The future of high-resolution UV spectroscopy: Science with a UV Échelle spectrograph on the Habitable Worlds Observatory, or a dedicated mission  (2609.09329 - Cia et al., 8 Sep 2026) in Section 3, subsection “Exoplanet atmospheres,” and subsection “Atmospheric escape”

Whether TOI-561\,b is ultimately a super-Venus (CO$_2$-rich) or a super-Io (SO$_2$-rich) remains a key open question. Answering it would make this old molten super-Earth a decisive benchmark for how volatile-rich rocky worlds evolve on ultra-short-period orbits.

— Evolutionary pathways toward survival of a thick CO2- or SO2-rich atmosphere on the lava world TOI-561 b  (2609.03144 - Postolec et al., 2 Sep 2026) in Section 6, Conclusions

Indeed, the apparent presence of NH$_3$ alongside the non-detection of SO$_2$ and CS$_2$ raises the question of how nitrogen chemistry respond to the specific irradiation environment of HD~106315.

— JWST-SUPER I: New insights into irradiated warm Neptunes atmospheres from MIRI observations of HD 106315 c  (2609.09893 - Dyrek et al., 9 Sep 2026) in Section 5.4, “Ammonia as a tracer of cool atmospheric chemistry”