THE Hubble Space Telescope may soon be able to see other Earths – or at least those on their last legs – by detecting their evaporating oceans.
Almost all the extrasolar planets we know about are Jupiter-sized gas giants, Earth-sized ones being too small for existing methods to detect. But Michael Jura of the University of California in Los Angeles points out that any planet like Earth will eventually warm up as its star gets older and brighter. The oceans will start to evaporate, sending water vapour into the upper atmosphere, where ultraviolet light will split it into hydrogen and oxygen. Jura says that over billions of years, the hydrogen should form a tenuous cloud perhaps 5 million kilometres across (). That would absorb a characteristic wavelength of light from the planet’s star.
Jura wants to use Hubble to monitor perhaps five nearby stars for this absorption. It’s a gamble, because even if such a planet exists, the cloud will only be visible if the planet’s orbit carries it between us and the star. Even if Earth-like planets are common, Jura thinks his chances of seeing one are probably no better than 1 in 5. As no more servicing missions are to be sent to Hubble, it may be a race against time to spot a dying planet before the telescope dies too.
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