Establish which rocky exoplanets possess atmospheres

Determine which rocky exoplanets have atmospheres, focusing on the currently observed population accessible to JWST and HST, and identify the specific planets where an atmosphere is present versus absent to enable targeted follow-up studies.

Background

The report’s recommended Director’s Discretionary Time concept centers on secondary eclipse observations of rocky planets to infer dayside temperatures and heat redistribution, a robust proxy for the presence of an atmosphere. The authors emphasize that thermal emission measurements via JWST/MIRI 15 μm photometry avoid stellar contamination effects common in transmission spectroscopy and provide a more direct test of atmospheric existence.

This question is embedded within the broader scientific motivation to assess the prevalence and diversity of atmospheres on rocky M-dwarf planets and to delineate the "cosmic shoreline" demarcating atmospheric retention versus loss under varying stellar irradiation and planetary escape conditions.

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?

While thin atmospheres --- with or without clouds --- remain compatible with emission spectra, it is challenging to distinguish such scenarios from a bare-rock case within current observational uncertainties. A definitive distinction between a tenuous, optically thin atmospheric layer and a completely bare surface requires future joint retrieval analyses combining transmission and emission spectra over a broader wavelength range ($0.71$--$12~\mu$m). In addition, filling the current gap between $1.6$ and $2.7~\mu$m in transmission spectra, for example using JWST/NIRSpec G295H or PRISM modes, would significantly improve constraints.

— Atmospheric Retrieval of Combined JWST, HST, and Ground-Based Observations Reveals a Featureless Transmission Spectrum for GJ 1132 b  (2608.24803 - Senobar et al., 25 Aug 2026) in Sections 3.3, “Comparison with JWST Thermal Emission Spectra,” and 4, “Summary and Conclusions”