US astronomers say the Hubble Space Telescope has allowed them to study for the first time the atmospheric structure of a planet orbiting another star. Hubble discovered a dense upper layer of hot hydrogen gas where the super-hot planet’s atmosphere is bleeding into space. The planet, designated HD 209458b, is unlike any world in our solar system. Researchers say it orbits so close to its star and becomes so hot that its gas is streaming into space, making the planet appear to have a comet-like tail. The new research reveals the layer in the planet’s upper atmosphere where the gas becomes so heated it escapes, like steam rising from a boiler.
https://hubblesite.org/contents/news-releases/2007/news-2007-07.html