Blown Away: HD 209458b, a.k.a. Osiris
Left: As this "hot Jupiter" cruises close to its sunlike star, its atmosphere slowly boils away. Subject to a surface temperature of about 1,800 degrees Fahrenheit, oxygen and carbon are ripped away from the planet by lighter hydrogen gas and carried in a streaming halo behind it. Observing this "blow-off" effect has led astronomers to propose a whole new category of planets: solid remnant cores of former gas giants.