NASA's Planet-Hunter Telescope, Kepler, Runs Out of Fuel

The U.S. space agency's Kepler space telescope has run out of fuel and is being retired after nine years, having helped discover 2,600 planets, some of which may hold life, officials said Tuesday.

The unmanned space telescope, which launched in 2009, revealed that billions of hidden planets are in space and revolutionized humanity's understanding of the universe, experts said.

The space telescope helped astronomers measure potential planets by glimpsing transits, or moments when planets passed in front of their stars. 

Kepler showed that "20 to 50 percent of the stars visible in the night sky are likely to have small, possibly rocky, planets similar in size to Earth, and located within the habitable zone of their parent stars," NASA said in a statement. 

"That means they're located at distances from their parent stars where liquid water -– a vital ingredient to life as we know it -– might pool on the planet surface."

Kepler's demise was "not unexpected and this marks the end of spacecraft operations," said Paul Hertz, astrophysics division director at NASA, on a conference call with reporters.

Signals that fuel was nearly out were seen two weeks ago.

Scientists were able to get all the data from Kepler down to Earth before it completely ran dry.

NASA said it has decided to retire the spacecraft "within its current, safe orbit, away from Earth."

NASA's retired principal investigator for the Kepler mission, Bill Borucki, described it as an "enormous success."

"We have shown that there are more planets than stars in our galaxy," Borucki said.