Here’s a plot from a new paper on WASP-107b by James Kirk et al. It shows data taken with a near-infra-red spectrograph on the 10-m Keck II telescope on Mauna Kea, and is focused on the Helium line at 10833 Å. The plot shows the spectra as a function of time (y-axis), though a transit. When the planet passes in front of its host star (white horizontal lines are times of ingress and egress) the helium line shows excess absorption. This helium is in the atmosphere of the planet and is absorbing some of the starlight. There is a slight change in the wavelength of the absorption owing to the orbital motion of the planet (denoted by the dashed white lines).

The paper shows, firstly, that ground-based telescopes such as Keck can do a fine job of discerning the compositions of exoplanet atmospheres. Secondly, the fact that the absorption extends beyond transit-egress indicates that the atmosphere is boiling off the surface of WASP-107b, under the fierce irradiation of the star, and is forming a comet-like tail.