Gas giant exoplanet WASP-121b. Illustration showing the planet being stretched into an egg shape by the tidal pull of its host star (left). The top of the planet's atmosphere is heated to 2500 degrees Celsius. This is the first planet outside our Solar System where astronomers have found the strongest evidence yet for a stratosphere, a layer of atmosphere in which temperature increases with higher altitudes. The exoplanet WASP-121b is about 900 light-years away from Earth.