The latest Hubble Space Telescope spectrum of a WASP exoplanet has just been published by Thomas Evans et al. The spectrum of WASP-121b extends from near-UV wavelengths through the optical to the infra-red, combining data from three different gratings (shown in different colours in the figure):

Of particular interest is the rapid rise in the data in the near-UV (the extreme left of the plot), which is clearly out of line with the fitted model (purple lines). The rise is too rapid to be attributed to Rayleigh scattering in a clear atmosphere.

Instead, the authors suggest that it is due to sulfanyl, a molecule consisting of one sulfur and one hydrogen. Evans et al conclude that the near-UV absorber “likely captures a significant amount of incident stellar radiation at low pressures, thus playing a significant role in the overall energy budget, thermal structure, and circulation of the atmosphere”.

The work points to the ongoing importance of the Hubble Space Telescope, even after the James Webb Space Telescope is launched, since the JWST is designed for infrared astronomy, and can’t see the near-UV wavelengths that can be observed with Hubble.

Update: One of the authors, Jo Barstow, has tweeted the following thread on the @astrotweeps account: