By Jason Palmer
Science and technology reporter, BBC News
Earthshine lights up the whole moon when sunlight reveals a crescent
Studies of earthshine - light reflected by the Earth on to the Moon - have revealed a means for searching out planets with liquid oceans and land.
In the first experiment of its kind, differences in earthshine from water and land were demonstrated.
Simply watching the dimming of the light skimming off exoplanets could give clues as to the amount of ocean they have or to their orbital period.
The study will be published in the journal Astrobiology.
The idea rests on a distant star's light skimming off an exoplanet's surface and toward the Earth in a specular, or mirror-like, reflection.
That the reflection would show evidence of water or land-masses was first put forward by Edwin Turner of Princeton University and colleagues in 2001.
Because liquid reflects the light more efficiently than land, the theory goes, the amount reaching Earth would change as land-masses spin into view.
Further theory has shown that light reflected by exoplanets would show signatures of plant life, since the photosynthetic machinery of plants absorbs light of characteristic colours.
But until now, no-one had done the experiment with the closest planet known to harbour life - the Earth.
In the red
Professor Turner worked on the new research with Sally Langford of the University of Melbourne in Australia (who is the paper's the lead author).
They measured the earthshine reflected back towards the Earth from a remote part of Australia using a small telescope and camera.
Three days a month - with the moon as a crescent - she measured the earthshine reflected from the Indian Ocean and then, as the Earth rotated, from the east coast of Africa.
What she found was a clear sign of dimming of the earthshine through the transition; the amount of light reflected was reduced by as much as 23% over an hour of observation.
What is more, the spectrum of the light changed with time; the mirror-like reflection of the ocean was "reddened" as Africa's land and plant life absorbed some of the incident light.
By observing the light skimming off of exoplanets, the team reasons - using future space-based telescope missions such as Nasa's Terrestrial Planet Finder (TPF) or the European Space Agency's Darwin - sudden dimming and reddening would indicate the presence of a land-mass.
"I call it looking for exobeaches, rather than exoplanets," Professor Turner told BBC News.
The method would only be useful if the observed exoplanet rotated more slowly than the telescope observing it needed to get a good fix, and would not provide much information on planets whose land was made up of small islands or archipelagos, Professor Turner said.
"Whether we could see this or not depends partly on luck - if the exoplanets are relatively nearby and are bright, if they rotate slowly - and different geometries are more helpful than others. There are many ways the luck factor could help or hurt, but you have to hope you're lucky.
"It's important nevertheless to have information like this because it drives the design of experiments like [TPF and Darwin]. We're just at the beginning of what we hope will be a major, long-term research programme."