Thomas Mikal-Evans et al have released a new paper analysing the heated, dayside face of WASP-121b. Teams studying the atmospheres of exoplanets either look at the transit, when the planet’s atmosphere is projected against the host star, such that molecules produce absorption features in the spectrum, or they study the eclipse, when the heated face of the planet disappear and then reappears. In the latter, atmospheric molecules produce emission features in the spectrum.

Here is the spectrum of the heated face of WASP-121b, based on recording five eclipses using the WFC3 spectrograph on the Hubble Space Telescope. The orange line and yellow banding show the spectrum expected for a pure black body of the same temperature as the planet. The red lines then show model fits, which reveal emission features caused by H⁻ ions and water (H₂O) molecules.