Scientists believe there are billions of planets in our Milky Way galaxy, based on findings from the 3-1/2 year prime mission of the Kepler space telescope which has come to an end.
Kepler had been used to identify more than 2300 planet candidates and confirm more than 100 planets, NASA said.
Hundreds of Earth-size planet candidates had been found, as well as some candidates that orbit in the habitable zone - the region in a planetary system where liquid water might exist on the surface of a planet.
None of the candidates was exactly like Earth.
The data collected by Kepler was being analysed for true sun-Earth analogues - Earth-size planets with a one-year orbit around stars similar to the sun.
"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," William Borucki, Kepler principal investigator at NASA's Ames Research Center, said.
"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 was launched in March 2009 and has been pointed at the same portion of the galaxy for 3-1/2 years, continuously measuring the brightness of more than 150,000 stars.
It looks for transits - when planet candidates pass in front of a star. Once a planet is detected, the length of time it takes to orbit its star is measured, while its size can be found from how much the brightness of the star drops.
With the completion of the prime mission, Kepler will continue working through to as late as 2016, giving it more time to continue to look for Earth-type worlds which are neither too far away from, or to close to, their sun.
Highlights of the mission:
- In August 2010, scientists confirmed the discovery of the first planetary system with more than one planet transiting the same star.
- In January 2011, the Kepler team announced the discovery of the first unquestionably rocky planet outside the solar system. Kepler-10b, measuring 1.4 times the size of Earth, is the smallest confirmed planet with both a radius and mass measurement. Kepler has continued to uncover smaller and smaller planets, some almost as small as Mars, indicating small rocky worlds may be common in the galaxy.
- In February 2011, scientists announced Kepler had found a very crowded and compact planetary system - a star with multiple transiting planets. Kepler-11 has six planets larger than Earth, all orbiting closer to their star than Venus orbits our sun.
- In September 2011, Kepler data confirmed the existence of a world with a double sunset. Since then, the discoveries of six additional worlds orbiting double stars further demonstrated planets can form and persist in the environs of a double-star system.
- In December 2011, NASA announced Kepler's discovery of the mission's first planet in a habitable zone. Kepler-22b, about 2.4 times the size of Earth, is the smallest-radius planet yet found to orbit a sun-like star in the habitable zone.
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