PI: Joanna Barstow

Project: Developing the NEMESIS retrieval code for studying exoplanets II

Of the exoplanets we have been able to study in detail to date, the majority are in very close orbits around their parent stars, meaning they experience extremes in radiation environment and temperature. Over the last few years, observations with the James Webb Space Telescope (JWST) have revealed that hot gas giant atmospheres have compositions that are driven out of thermochemical equilibrium by transport processes or photochemistry. Determining the origins of disequilibrium chemistry is critical for understanding the dynamics and radiation environment of these worlds.

Even before the arrival of JWST, observations with the Hubble Space Telescope had revealed hints of an unusual trend for planets orbiting F-type stars – stars slightly bluer and hotter than the Sun. A handful of these planets showed evidence in their spectra for absorption by the H^– (hydride) ion, even though their atmospheres are not hot enough for extensive thermal ionisation, implying a possible photochemical origin. We conducted a NEMESISPY analysis of the Hubble data for a similar planet, KELT-7b, using the DiAL3 data intensive service; comparing analyses performed with and without H^– absorption in the atmospheric model revealed the strongest evidence for its presence yet (Gascón+2025).

We also conducted a NEMESISPY analysis incorporating new JWST data for KELT-7b (Ahrer+2025). Whilst these new measurements do not cover the H^– absorption feature, they do cast some doubt on its presence. In order to simultaneously fit the Hubble and JWST data, assuming the presence of H^–, we must invoke a baseline offset between the two datasets of several hundred parts per million. Extrapolated models from fits to the JWST data alone, assuming equilibrium chemistry, do not agree with the Hubble measurements. Clearly, more data are required to solve the mystery of KELT-7b’s atmospheric chemistry; watch this space!

Figure E1 of Ahrer+2025, showing the disagreement between NEMESISPY model fits with JWST only and with JWST combined with Hubble. The H^– feature is the smooth curve followed by the steep drop off at 1.6 microns (purple model).