The Kepler team found that planets which are about 1.75 times the size of Earth and smaller tend to be rocky, while those two- to 3.5 times the size of Earth become gas-shrouded worlds like Neptune.
"The Kepler data set is unique, as it is the only one containing a population of these near Earth-analogs - planets with roughly the same size and orbit as Earth", Dr. Mario Perez, a scientist on NASA's Kepler program, said in the press statement.More news: Shot US Congressman Scalise undergoes surgery
For Kepler's first four years, it stared at a patch of sky in the Cygnus constellation. The 1,284 newly discovered exoplanets were only announced as such after being identified with 99% confidence, but as NASA admits, that leaves out 1,327 other likely exoplanets that couldn't reach that 99% certainty. Among these hundreds of planets, about 550 of them appear to be rocky planets similar to Earth, and a precious 9 orbit their stars at just the right distance to have liquid water on their surface-meaning that they might support life.
"This carefully-measured catalog is the foundation for directly answering one of astronomy's most compelling questions - how many planets like our Earth are in the galaxy?" said Susan Thompson, Kepler research scientist for the SETI Institute.
Of the new candidates, 10 are near in size to Earth and sit in the habitable zone of their stars-the range of orbits in which liquid water could exist on their surfaces. If it is a planet, that Kepler data can be used to determine its mass, size, and orbital period, or how long it takes to go around the star. Both results have significant implications for the search for life. Such planets are harder to spot because they might have made only a few transits across their star during Kepler's 4-year watch.More news: Dozens killed, wounded in attack on Afghanistan police headquarters
Below is a plot showing only the small subset of relatively Earth-sized planet candidates.
Since the Kepler mission began in 2009, the telescope has helped researchers identify thousands of exoplanet candidates, and more than 2,000 of those have turned out to be confirmed exoplanets.
With the final catalog of planetary candidates from Kepler's original mission released, NASA will now focus on the "K2" mission, which began in 2014. Scientists with the mission expect that Kepler's K2 mission will continue until sometime in 2018.More news: Scalise upgraded from 'critical' to 'serious' condition
"We like to think of this study as classifying planets in the same way that biologists identify new species of animals", said Benjamin Fulton, doctoral candidate at the University of Hawaii in Manoa, and lead author of the second study. The size of the dimming tells scientists how the big the planet passing in front of the star is. He likened the discovery to realizing that mammals and reptiles are on separate branches of the evolutionary tree.