Exoplanets are complex astrophysical bodies but are difficult to study in detail. Despite the challenges, we are starting to solve the interrelated puzzles of what exoplanets are made of; how they evolve; and how their atmospheric dynamics work. Exoplanet atmospheres have particularly small measurable signatures, to which we must apply precise and innovative observations. We must choose case-study planets carefully, as time on the best telescopes is limited. One solution is to study extreme systems (e.g. the coldest, the hottest, the lowest-density planets), which are laboratories for testing our understanding of atmospheric physics at their limits. To that end, the three projects presented here are observations of extreme gas-giant exoplanets that transit their host stars. Firstly, using the Hubble Space Telescope (HST)’s Wide Field Camera 3 instrument (WFC3), we measured the 0.8 - 1.1 μm transmission spectrum of WASP-107b, which has a relatively cold equilibrium temperature of 700 K. With these observations we detected helium on an exoplanet for the first time, via the 10 830 ̊A line of metastable helium, and showed that WASP-107b has an extended and possibly escaping upper atmosphere. Secondly, we observed a near-infrared phase curve of the hot (2 100 K) exoplanet WASP-19b with HST’s WFC3, covering the 1.1 - 1.7μm wavelength range. We detected a large hotspot offset in its phase curve (60◦ in longitude), which means WASP-19b likely has strong equa- torial winds in its deep (1 bar) atmosphere. Thirdly, WASP-127b is one of the lowest-density planets known to science and an attractive target for atmospheric characterisation. We observed a near-ultraviolet to near-infrared transmission spec- trum, covering 0.3-5μm, of WASP-127b using HST and the Spitzer space telescope. On this planet, we detected sodium, potassium, water, carbon-bearing species, and some unknown hazes and clouds. In summary of our contributions to the puzzles mentioned above: we introduced a new method to observe exoplanet atmospheres and escape processes; we added to the growing sample of measurements of heat transport in exoplanet atmospheres; and we made a step towards determining the atmospheric composition of an ideal planet for study with the upcoming James Webb Space Telescope.Science and Technology Facilities Counci
