Nasa said that its Kepler space telescope has collected enough data to start work on finding true Sun-Earth analogues – Earth-sized planets with a one-year orbit around Sun-like stars.
The agency added that Kepler has taught us that far from our own planet being unique, the galaxy is teeming with planetary systems. More importantly, many of those planets are in the habitable zone around a star where surface water could exist.
So far, scientists have used Kepler data to identify more than 2,300 planet candidates and confirm more than 100 planets. Hundreds of those candidates are Earth-size, although none is exactly like Earth. Kepler will now have its mission extended until 2016, Nasa said.
"The initial discoveries of the Kepler mission indicate at least a third of the stars have planets, and the number of planets in our galaxy must number in the billions," said William Borucki, Kepler principal investigator at Nasa's Ames Research Centre in California. "The planets of greatest interest are other Earths and these could already be in the data awaiting analysis. Kepler's most exciting results are yet to come."
Kepler searches for planet candidates, or exoplanets, by continuously measuring the brightness of more than 150,000 stars. When a planet candidate passes in front of the star from the spacecraft's point of view, light from the star is blocked. Different sized planets block different amounts of starlight, so the amount of light blocked reveals the planet's size relative to its star.
Kepler was launched on 6 March 2009 with the aim of of discovering what fraction of stars might harbour potentially habitable, Earth-sized planets. Within months, it had confirmed five exoplanets, known as hot Jupiters because of their enormous size and orbits close to their stars.
Among its subsequent discoveries are planets almost as small as Mars, Kepler-11, which has six planets larger than Earth, all orbiting closer to their star than Venus orbits our Sun, and the discovery of at least seven worlds that orbit double stars, similar to the one famously portrayed in 'Star Wars'.
More recently, members of the public taking part in Planet Hunters, a programme led by Yale University to comb through Kepler data, made their first planet discovery – a planet orbiting a double star that was in turn being orbited by a second distant pair of stars.
"Kepler's bounty of new planet discoveries, many quite different from anything found previously, will continue to astound," said Jack Lissauer, planetary scientist at Nasa Ames. "But to me, the most wonderful discovery of the mission has not been individual planets, but the systems of two, three, even six planets crowded close to their stars, and, like the planets orbiting about our Sun, moving in nearly the same plane."
"The Earth isn't unique, nor the centre of the universe," added Geoff Marcy, professor of astronomy at the University of California at Berkeley. "The diversity of other worlds is greater than depicted in all the science fiction novels and movies. Aristotle would be proud of us for answering some of the most profound philosophical questions about our place in the universe."