Unlocking the hidden secrets of exoplanet atmospheres in the UV with Hubble’s UVIS grism

The 200-400 nm wavelength range holds critical spectral diagnostics of the chemistry and physics at work in planetary atmospheres, both in the solar system and beyond. To date, exoplanet atmospheric characterization studies have relied on a combination of modes on Hubble’s STIS instrument to access this wavelength regime. Recently, we successfully developed the use of Hubble’s WFC3 UVIS G280 grism mode for exoplanet time-series studies, to probe an exoplanet atmosphere from 200 to 800 nm with a single observation. UVIS data is impacted less by systematics, contains both spectral orders increasing the measurable flux by 60%, and has higher throughput than standard modes, making it more reliable, efficient, and precise over a wider wavelength range. The first measurements published by Wakeford et al (2020) and explored by Lewis et al. (2020) revealed evidence of H^- in a cooler atmosphere than predictions suggest. Lothringer & Sing et al. (2022) applied the UVIS grism to an ultra-hot Jupiter atmosphere which revealed absorption by SiO, suggesting likely trends in aerosol formation with temperature.

Here we show that Hubble’s UVIS spectroscopic grism is a powerful new tool to obtain the UV spectra of exoplanet atmospheres over a wide wavelength range, which will add to the UV legacy of Hubble and greatly complement future observations with JWST.