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By Jonathan Amos
BBC News science reporter, in Baltimore
Nasa would have to find hundreds of millions of dollars to fund the TPF mission
Earth-like planets around distant stars may be too far away to be reached by spacecraft but scientists could still investigate whether they harbour life.
Telescope technologies are being developed that will probe the very faint light from these objects for tell-tale signs of biology.
These are the same "life markers" known to be present in light reflected off the Earth - so-called "earthshine".
They include signatures for water, and gases such as oxygen and methane.
"This gives you some information on habitability," said Wesley Traub, chief scientist on the US space agency's (Nasa) Navigator Program which specialises in the search for far-off worlds.
"These are only signs of life; they are only indicators. You can't actually detect the life itself crawling or sliming around on the surface of the planet," he told the American Geophysical Union Joint Assembly here in Baltimore, US.