NASA this week marked the end of the Kepler Space Telescope’s prime mission, which began in March 2009. Like other NASA equipment, the telescope is now beginning another, extended mission NASA says could last as long as four years.

Kepler’s prime mission was to determine what fraction of stars might have Earth-like planets in their orbit. So far, the telescope has identified over 2,300 planet candidates and hundreds of Earth-size planet candidates. There are also candidates that orbit in the habitable zone of their system, where liquid water can exist. Kepler has confirmed more than 100 planets so far.

“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,” said William Borucki, Kepler principal investigator at NASA’s Ames Research Center. “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.”

Over the three and a half years of its prime mission, Kepler’s discoveries have revealed much about planetary systems. Just this year, Kepler Astronomers have confirmed a planet in a two-star system (like Tatooine) and even one in a four-star system.

“Kepler’s bounty of new planet discoveries, many quite different from anything found previously, will continue to astound,” said Jack Lissauer, planetary scientist at Ames. “But to me, the most wonderful discovery of the mission has not been individual planets, but the systems of two, three, even six planets crowded close to their stars, and, like the planets orbiting about our sun, moving in nearly the same plane. Like people, planets interact with their neighbors and can be greatly affected by them. What are the neighborhoods of Earth-size exoplanets like? This is the question I most hope Kepler will answer in the years to come.”

Back in April, NASA extended Kepler’s mission, which might run through 2016. Astronomers will use the extra time to continue to search for Earth-sized planets in the habitable zone of their system.

(Image courtesy NASA/JPL-Caltech)