The extrasolar planet Osiris orbits the star HD 209458 and is only 150 light years from Earth. It is fortuitously aligned with respect to us, so that the planet transits in front of the star, allowing direct measurements of the planet's radius, mass, and atmospheric composition. Osiris completes an orbit in less than 4 days, and because it is so close to its parent star, hydrogen boils off the hot upper atmosphere; Osiris belongs to a planetary class called "hot Jupiters" because of its heated atmosphere.
Vidal-Madjar et al. observed four transits with the STIS spectrograph on the Hubble Space Telescope. They detected an evaporating envelope of oxygen and carbon around Osiris. The oxygen and carbon must be escaping from the lower atmosphere, but the typical mechanism for escape, eddy diffusion, that works for Jovian planets does not match their observations. Instead, oxygen and carbon are being hydrodynamically dragged along with the much more abundant hydrogen, and all three species appear to be escaping at a ratio consistent with their composition in the lower atmosphere. Amazingly, in less than a decade, astronomers have progressed from detecting the presence of extrasolar planets to probing their atmospheres from top to bottom. -- LR
Astrophys. J. 604, L69 (2004).
