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 ( Jura wants to use Hubble to monitor
perhaps five nearby stars for this absorption. It's a gamble,
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