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)

Jan. 13, 2012

Planets orbiting stars other than the sun may be the rule rather the exception, an international team of scientists has reported. In fact, the Milky Way alone–an average-sized galaxy among the billions of galaxies in the universe–probably contains at least 160 billion planets. And even that, the scientists said, is a conservative estimate.

The scientists reached that number after making a statistical analysis of planets found using gravitational lensing, one of the main methods for hunting for extrasolar planets. They calculated that each star in the Milky Way has an average of 1.6 planets. Then they multiplied this number by the 100 billion stars estimated to belong to the Milky Way. Gravitational lensing, however, is not useful for finding exoplanets less than five times larger than Earth. Nor is it especially useful for finding planets that orbit closer to their star than Venus or farther from their star than Saturn. So the total number of alien planets is certainly much higher.

Three rocky planets smaller than Earth--the first alien planets that small found outside the solar system--have been found orbiting a red dwarf about 120 light-years from Earth. The smallest of the three planets (foreground), which is about the size of Mars, actually orbits the farthest from the star. NASA/JPL-Caltech

Evidence for that conclusion was the discovery of the first exoplanets smaller than Earth outside the solar system. Astronomers studying data imaged by the Kepler space telescope found the three rocky planets about 120 light-years from Earth. One of the planets is about the size of Mars, about half the size of Earth. All three orbit so close to their star, a red dwarf, that their sizzling surface temperatures rule out the existence of life as we know it. The Kepler scientists said the galaxy might have many more small planets than it has large planets. Astronomer John Johnson, one of the authors of the study, said that small exoplanets were “kind of like cockroaches. If you see one, then there are dozens hiding.”

Additional World Book articles:

• Back in Time (Astronomy 1996)
• Back in Time (Astronomy 1999)
• COROT
• In Search of Other Worlds (A Special Report)
• Kepler, Johannes
• Transit