By Jonathan Amos
BBC News science reporter, in Baltimore
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.
Nasa would have to find hundreds of millions of dollars to fund the TPF mission
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.
In the glare
Traub is hopeful Nasa will approve the funds necessary to launch a Terrestrial Planet Finder (TPF) mission some time in the next decade.
It will comprise two space-borne observatories which will hunt down and study Earth-sized planets orbiting stars at distances where liquid water could exist and sustain life.
Europe has a similar, ambitious mission under consideration known as Darwin.
Essential to these observatories' success will be a new generation of instrumentation capable of seeing past the blinding glare of the parent star to pick out only the faint light reflected off the distant world's surface.
Sunlight is reflected off the Earth, hits the Moon and bounces back to Earth
Earthshine is seen in the faint glow our world gives to dark areas of the Moon
The light carries information about Earth's atmosphere and surface properties
Scientists see details in the light that betray different gases, even vegetation
The knowledge can be applied to the search for distant worlds
It is a hard task - the parent star is likely to be a billion to 10 billion times brighter than its tiny companion - but recent experiments at Nasa's Jet Propulsion Laboratory suggest the technologies are getting close to the sensitivities required.
The "template" for the information that a TPF or Darwin mission would target is based around what we know about the Earth's reflected light.
As the Sun's rays hit our world much of it bounces back out into space. We can see this light in the faint illumination it gives to the dark segment of a crescent moon.
Scientists have long established that this light carries information about the Earth's atmosphere and surface properties.
Pilar Montanes-Rodriguez, from the New Jersey Institute of Technology, reported to the meeting how she had been able to see in this earthshine a clear marker for chlorophyll, the pigment in plants that plays a critical role in photosynthesis.
Clearly, to be able to detect such a signature in the light from a world tens of light-years away would be astonishing; but Montanes-Rodriguez cautioned: "For a typical day the signal of the vegetation is very weak for the Earth because it is obscured by the bright clouds.
"We used models and satellite cloud data to simulate the Earth reflectance for a whole year. We then applied what we'd learnt from Earth to an extrasolar planet.
"As this planet revolves around a parent star, there will be times when the signal will be prominent and can be unambiguously detected. Unfortunately, at these times the angle of distance between the planet and the star will be small and it will be difficult to rule out the light of the star."
Current techniques are finding more and more distant worlds
She warned the instrumentation required might require a level of precision beyond what had previously been thought necessary.
Traub's own modelling has tried to work out what the Earth's planet shine would have looked like at various stages in geologic history - to get a set of "profiles" planet hunters could use to gauge what stage in the evolution of life a newly discovered world might have reached.
"If you look at the [current] earthshine spectrum and you calculate what that would have looked like at a point before any life arose on Earth, you see a lot of carbon dioxide (CO2) and methane," he informed the meeting.
"You wouldn't see any oxygen and you wouldn't see any ozone line, because the ozone comes from oxygen. As an Earth evolves, you would see the CO2 disappearing, you would see the oxygen increasing and at some late point you see the [infrared] of vegetation coming on."