Three new Earth-like worlds have been discovered by Nasa's planet-hunting space telescope Kepler.
Kepler-438b, Kepler-442b and Kepler-440b all obit their respective stars' habitable zones – the distance at which liquid water can exist, which is essential for life to flourish.
Of the three, two (Kepler-438b and Kepler-442b) are also made of rock, like Earth.
The latest additions make up a haul of 1,000 new verified planets, all recently discovered by Nasa – bringing the total number of known exo-worlds to 4,000.
John Grunsfeld, associate administrator of NASA's Science Mission Directorate said: "Each result from the planet-hunting Kepler mission's treasure trove of data takes us another step closer to answering the question of whether we are alone in the Universe.
"The Kepler team and its science community continue to produce impressive results with the data from this venerable explorer."
To determine whether a planet is made of rock, water or gas, scientists must know its size and mass. When its mass can't be directly determined, scientists can infer what the planet is made of based on its size.
Two of the newly validated planets, Kepler-438b and Kepler-442b, are less than 1.5 times the diameter of Earth. Kepler-438b, 475 light-years away, is 12 percent bigger than Earth and orbits its star once every 35.2 days. Kepler-442b, 1,100 light-years away, is 33 percent bigger than Earth and orbits its star once every 112 days.
Both Kepler-438b and Kepler-442b orbit stars smaller and cooler than our sun, making the habitable zone closer to their parent star, in the direction of the constellation Lyra. The research paper reporting this finding has been accepted for publication in The Astrophysical Journal.
"With each new discovery of these small, possibly rocky worlds, our confidence strengthens in the determination of the true frequency of planets like Earth," said co-author Doug Caldwell, SETI Institute Kepler scientist at Nasa's Ames Research Centere at Moffett Field, California. "The day is on the horizon when we'll know how common temperate, rocky planets like Earth are."
