First exosolar planetary transit seen in x rays reveals evaporating planet

Ars Technica : If the orientation is right, a planet orbiting a star blocks some of the light emitted in the direction of an observer. Those transits are one of the most common ways to detect planets around other stars. Now, looking at a known planetary system, Katja Poppenhaeger of Harvard University and her colleagues have observed the first planetary transit in the x-ray spectrum. Previous visible-light observations revealed the planet to be a hot Jupiter—a gas giant orbiting close to the star. The x-ray observations were harder to interpret because x-ray emissions are not evenly distributed across the surface of the star, so the planet’s transit can be difficult to detect against the noisy background. Once the transit was detected, the researchers found that there was a difference between the amount of visible light and the amount of x-ray light that was blocked. They believe that the best explanation is that heat from the star has caused the planet’s atmosphere to expand and allowed a layer of hydrogen gas to form far out from the surface. That hydrogen would absorb significantly more x-ray emissions than visible-light emissions. However, the extended layer of hydrogen would experience less gravity and thus probably evaporates away at a rate that Poppenhaeger’s group calculated to be 10 ⁹ kg/s.