Formation signatures in exoplanet atmospheric composition

Determine whether the composition of exoplanet upper atmospheres preserves information related to formation location—such as carbon-to-oxygen partitioning set by disk temperature and radius—and assess the impact of confounding formation and evolutionary processes on such inferences.

Background

The author suggests that the partitioning of volatile elements between condensed and gaseous phases varies with disk temperature, potentially enabling formation-radius diagnostics via atmospheric compositions from transmission spectroscopy. However, multiple formation and evolutionary processes could obscure or erase these signals, making empirical validation essential.

References

Open questions and problems that I would like to see resolved include, in no particular order: Does the composition of the upper levels of exoplanet atmospheres retain formation information? The division of abundant elements such as carbon and oxygen between condensed and gaseous phases is expected to vary with temperature, and thus radius, in the disk. This opens up the possibility of using atmospheric composition measurements from transmission spectroscopy as a new probe of planet formation. Multiple confounding factors, in both planet formation and atmospheric evolution, might however frustrate such hopes.

— Planet formation theory: an overview  (2412.11064 - Armitage, 2024) in Section “Some open questions”, Item 7

In this context, it would be important to determine whether gas giants formed in the outer disc with relatively small cores can be observationally distinguished from their inner-disc counterparts with larger cores, for example through measurements of their bulk heavy-element content or atmospheric metallicities with facilities such as JWST and Ariel.

— Beyond Pebble Isolation: Diverse Pathways to Giant Planet Formation Across Stellar and Orbital Scales  (2609.09382 - Peerani et al., 8 Sep 2026) in Section 7, “Limitations and Future Work”

As we do not know the specific formation pathways for each planet, we make necessarily arbitrary assumptions regarding which set of plaNETic priors to use (water-poor or water-rich).

— No statistically significant evidence for a correlation between stellar and planetary composition  (2609.17159 - Turner et al., 15 Sep 2026) in Section 5.5, “Overcoming current limitations”

Our stellar value of $0.65_{-0.20}{+0.28}$ serves as a reference for these retrievals, but the present atmospheric data do not yet firmly establish whether the planet is genuinely super-stellar in C/O.

— Elemental Abundances of Cool Stars: A Spectroscopic Framework Applied to Five Planet-Host Stars  (2609.24970 - Gromek et al., 21 Sep 2026) in Section 5.4.2, Volatile-Rich Planets

If sulphur is indeed depleted, the origin of this depletion remains unclear.

— 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 5.3, “Where is the sulphur? Constraints from C/O and S/H”