"We have shown that there are more planets than stars in our galaxy", Borucki said.
And now after a mammoth nine-and-a-half-year mission in search of potentially life-sustaining planets, NASA's Kepler Space Telescope has officially run out of fuel. "We're confident that TESS is going to find thousands more planets, just like Kepler did".
During its mission, Kepler found 2,681 confirmed exoplanets - the term for planets outside our solar system - and another 2,899 candidates, bringing its tally to 5,580.
Thanks to Kepler's data, which was all safely beamed back to Earth before the end of the mission, we now know that planets are, in fact, exceedingly common.
Continuing with Kepler's work is NASA's Transitting Exoplanet Survey Satellite (TESS), which launched in April to survey an area of the sky 400 times larger than that observed by its predecessor. Kepler was created to survey more than 100,000 stars in our galaxy to determine the number of sun-like stars that have Earth-size and larger planets, including those that lie in a star's 'habitable zone, ' a region where liquid water, and perhaps life, could exist.
The unmanned space telescope, which launched in 2009, revealed that there are billions of hidden planets in space and revolutionized humanity's understanding of the universe.
Now orbiting some 156 million kilometres from Earth, the spacecraft will drift further from our planet after its retirement, the United States space agency said. It is a possible "water world" the size of Earth perhaps covered with oceans and with a water-based atmosphere. It found inferno-like gas giants, rocky planets, planets orbiting binary stars, Earth-size planets, planets in the habitable zone capable of supporting liquid water on the surface, planets twice the size of Earth, the strangely flickering Tabby's Star, new details about the TRAPPIST-1 planets and, in December, an eight-planet system.
"We know the spacecraft's retirement isn't the end of Kepler's discoveries", Jessie Dotson, Kepler's project scientist at NASA's Ames Research Center, said in a statement.
Four years into the mission, after the primary mission objectives had been met, mechanical failures temporarily halted observations.
Mission planners reworked Kepler's mode of operation to point at other parts of the sky, expanding its list of targets to 500,000 stars.
The end of spacecraft operations means the end of new Kepler data, but project scientists said that the observations Kepler collected since its March 2009 launch will continue to be analyzed by astronomers for years to come, yielding new discoveries. This is the first star system found to have multiple transiting planets. It's also illuminated the tantalizing mysteries and possibilities among the stars. Launched in April, TESS will build on Kepler's planet-hunting legacy by searching for exoplanets around almost 200,000 of the brightest and nearest stars to Earth.
Kepler hands off the baton to TESS now, NASA said.
"That's the path Kepler has put us on", said Paul Hertz, astrophysics division director at NASA Headquarters in Washington.
Borucki, who dreamed up the mission decades ago, said one of his favourite discoveries was Kepler 22b, a water planet bigger than Earth but where it is not too warm and not too cold - the type "that could lead to life".
NASA's Ames Research Center manages the Kepler and K2 missions for NASA's Science Mission Directorate.