The hot Jupiter WASP-121b, discovered recently by Laetitia Delrez et al, is a very good opportunity for learning what the atmosphere of an exoplanet is made of. Being in a close, 1.27-day orbit around a hot star makes the atmosphere hot, while being a bloated planet of 1.9 Jupiter radii makes the atmosphere puffy. That means one can observe the planet in transit, projected against its star, and readily observe spectral features caused by the atmosphere absorbing star light.
Thomas Evans et al have pointed the Hubble Space Telescope at WASP-121b. To model the resulting spectrum they find they need an atmosphere containing titanium oxide, vanadium oxide, and iron hydride. In the plot below, models with these molecules are plotted red and yellow, and fit the observations, while models without, plotted in green and purple, do not.
The model also shows that WASP-121b has clear skies, rich in water vapour. It looks as though WASP-121b will become one of the most important exoplanets for such atmospheric characterisation work.