Barry Eitel

Earlier this month, NASA’s Kepler Space Telescope, adrift in space, successfully completed its three-and-a-half year mission to catalog faraway planets. As a reward, NASA is extending Kepler’s mission for up to four years.

Since its launch in 2009, Kepler has helped scientists identify over 2,300 planet candidates and confirm at least 100. It proves that planets are quite common in the universe.

"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," says William Borucki, the Kepler principal investigator at NASA's Ames Research Center in Moffett Field, California.

Finding Earth-like planets that might support life has been much more difficult. NASA has identified hundreds of candidates in “habitable zones,” where they are the right distance from their star to have liquid water, but none is close to an Earth clone.

Borucki remains hopeful: "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."

The extended mission will focus on finding stars and exoplanets (planets that orbit around a star that isn’t the Sun) similar to the Earth and Sun. Key characteristics include distance, size and the existence of years and days.

"Kepler's bounty of new planet discoveries, many quite different from anything found previously, will continue to astound," said Jack Lissauer, a planetary scientist also 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."

How does Kepler spot a planet? It constantly monitors the brightness of over 150,000 stars. If there is a blip in the brightness, it could mean a planet has passed by, darkening the star from Kepler’s vantage point. Different sized planets block out different amounts of starlight.

Researchers use this data to hone in on the candidate and judge its planetary credentials.

Geoff Marcy of UC Berkeley sees a philosophical component to Kepler’s mission: "The Earth isn't unique, nor the center of the universe…The diversity of other worlds is greater than depicted in all the science fiction novels and movies. Aristotle would be proud of us for answering some of the most profound philosophical questions about our place in the universe."