The evaporating planet


Osiris (officially designated HD 209458b) is a large planet around two and a half times the size of Jupiter and 220 Earth masses, orbiting a yellow dwarf star 150 light years from Earth. It’s one of the first extrasolar planets discovered that has an atmosphere with hydrogen, carbon and oxygen – an indication that it has water vapour too. It might seem like a good place to look for a habitable environment, but for one thing: Osiris’s atmosphere is evaporating.

The planet orbits its parent star at a radius of one-eighth that of Mercury’s, resulting in a blistering hot surface temperature of around 1,000 degrees Celsius (1,832 degrees Fahrenheit). As a result, its atmosphere is boiling away, potentially at a rate of 10,000 tons per second, leaving a massive tail 200,000 kilometres (125,000 miles) long in its wake.

Evidence of water vapour is based on observations that were first made by the Hubble Space Telescope in 2007, and in 2009 it became the first planet outside the Solar System to show hard proof of the life-sustaining substance, in addition to carbon monoxide.