We used HST/WFC3 to obtain ultra-precise transmission and emission spectra for WASP-43b, one of the shortest-period hot Jupiters. These data reveal that the water abundance in the planet's atmosphere is consistent with expectations for a solar composition gas to within one order of magnitude. This is a more precise measurement of the water content than is available for the solar system giant planets, which have water locked up in icy clouds that are out of reach of remote observations. In addition, we also observed three full-orbit, spectroscopic phase curves of the planet and used these to map the thermal structure of the atmosphere as a function of longitude and altitude.
These results are published in two companion papers, available here (water abundance) and here (thermal structure). A general summary is available in the Space Telescope press release, or articles at space.com and National Geographic. I also made a YouTube video illustrating what we learned and how we learned it - see here.
We observed a record 15 transits of GJ1214b with HST/WFC3 to measure the planet's transmission spectrum to unprecedented precision. This is the first measurement sensitive to an Earth-like atmospheric composition on an exoplanet, though that is not what we found! The featureless spectrum we obtained conclusively rules out all plausible cloud-free, high mean molecular mass compositions. The best explanation for the data is that high altitude clouds must be present in the atmosphere. For more info, see our paper, or some of the associated media coverage at NASA, Nature, NPR, Time Magazine, the NYT, or space.com.