"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 in Mountain View, California, and lead author of the catalog study. The presence of liquid water means that a planet could potentially develop life, or one day host a human colony.
The exoplanets represented in this newly updated catalogue include all of those detected - candidates and confirmed planets - while Kepler was aimed at the constellation Cygnus, and stared down the length of the Orion Spur, the small arm of the Milky Way Galaxy where Earth's solar system resides.
The Kepler telescope has added several hundred new candidate exoplanets to its stable of 4034, including 10 that are near-Earth-size, in orbits that would allow liquid water at the surface.
"Are we alone? Maybe Kepler today has told us indirectly, although we need confirmation, that we are probably not alone", Kepler scientist Mario Perez said in a Monday news conference.
For more information about Kepler Exoplanet Week, go to nasa.gov/kepler/exoplanetweek. 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. That happens during a so-called "transit" - when a planet passes in front of its star. Researchers are now using the Hubble Space Telescope to determine if these planets had atmosphere.
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.
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"Understanding their frequency in the galaxy will help inform the design of future Nasa missions to directly image another Earth".
According to the scientists, over 2300 planets spotted during the Kepler & K2 missions have been confirmed so far, including 50 terrestrial-sized planets that lie in the "Goldilocks Zone" of their star.
This is the eighth release of the Kepler candidate catalog, gathered by reprocessing the entire set of data from Kepler's observations during the first four years of its primary mission. Instead of directly spotting individual planets and identifying them with something like high-powered optics, Kepler watches for "transits".
Scientists were even able to estimate the size and density of the planets. Earth-sized planets are of particular interest because they can teach us about how our own planet formed, and because there's a small chance they could harbor life.
One of Kepler's other big surprises was a profusion of planets intermediate in size between Earth and Neptune.
These other missions - such as TESS, the Transiting Exoplanet Survey Satellite, in 2018 and the James Webb Space Telescope later on - will continue the search for life beyond Earth.