February 27, 2014

The number of known exoplanets in the Milky Way Galaxy has nearly doubled, thanks to a new method of confirming planet candidates discovered by the Kepler space telescope. Mission control officials at NASA‘s Ames Research Center in Moffett Field, California, announced that they had identified 715 new planets, which raises the total number of alien planets to some 1,700. The research has also significantly increased the number of smaller exoplanets. About 95 percent of the new planets are smaller than Neptune, which is about 17 times as massive as Earth. Many exoplanets found previously were at least as massive as Jupiter, which is 318 times as massive as Earth. All of the newly discovered planets orbit one of 305 stars in a multiplanet system like the solar system.

Although Kepler’s original planet-hunting days are over–the telescope has been nonoperational since May 2013–mission scientists are still on the job. The new exoplanets were found in data collected during the telescope’s first two years in orbit, from 2009 to 2011. Kepler’s main mission was to search one section of the Milky Way for Earth-like planets in the “habitable zone.” The habitable zone, also called the Goldilocks zone, is the region around a star that scientists believe is neither too hot nor too cold to support life as we know it. In this zone, the temperature is cool enough to let liquid water form and warm enough to prevent water from freezing. Earth orbits in the habitable zone of the solar system. Although Kepler searched over 150,000 stars for signs of orbiting planets, its search zone was actually quite small. Scientists estimate that the Milky Way alone has up to 200 billion stars.

All of the newly identified planets orbit a star in a multiplanet system, like the solar system. (NASA)

The new method of identifying planets is called validation by multiplicity. This method, which relies on the logic of probability, allows scientists to identify exoplanets in groups, rather than one by one. NASA officials compared the technique to the behavior of lions and lionesses living on the savannah. “In our imaginary savannah, the lions are the Kepler stars and the lionesses are the planet candidates,” the agency explained in a press release. “If you see two of the big cats it could be a lion (star) and a lioness (planet) or two lions. But if more than two large felines are gathered, then it is very likely to be a lion and his pride (star and its planets).”

Additional World Book articles:

• Binary star
• In Search of Other Worlds (a special report)
• Space exploration (2011) (a Back in Time article)