NASA's Kepler mission has discovered four new planets that are less than twice the size of Earth and which orbit in their star's habitable zone.
It's found 461 new planets in all between May 2009 and March 2011, bringing the total number of potential planets to 2,740, orbiting 2,036 stars. The biggest jump was in the number of Earth-size and super Earth-size candidates discovered, which grew by 43 and 21 percent respectively.
The finding means there could be as many as 17 billion Earth-sized planets in habitable zones in our galaxy.
"There is no better way to kick off the start of the Kepler extended mission than to discover more possible outposts on the frontier of potentially life bearing worlds," says Christopher Burke, Kepler scientist at the SETI Institute.
The number of stars discovered to have more than one planet candidate has now reached 467: 43 percent of Kepler's planet candidates have neighbor planets.
"The large number of multi-candidate systems being found by Kepler implies that a substantial fraction of exoplanets reside in flat multi-planet systems," says Jack Lissauer, planetary scientist at NASA's Ames Research Center. "This is consistent with what we know about our own planetary neighborhood."
The Kepler space telescope identifies planet candidates by repeatedly measuring stars' change in brightness as planets transit in front - and at least three transits are required to verify a signal as a potential planet.
There's then a period of follow-up observations and analyses before a planet can be confirmed. At the beginning of 2012, 33 planets had been through this process; today, the figure is 105.
"The analysis of increasingly longer time periods of Kepler data uncovers smaller planets in longer period orbits - orbital periods similar to Earth's," says Steve Howell, Kepler mission project scientist . "It is no longer a question of will we find a true Earth analogue, but a question of when."