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Galaxy has at least 100 billion planets, says new estimate

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Three new astronomical findings this week are upending the conventional view of the cosmos: Planets (and planets that orbit two stars) are more common than we think.

If you've ever been high in the mountains, or anywhere away from city lights, you've seen the nighttime sky glitter with an endless array of stars.

Well it turns out there are just as many, if not more, planets out there.

Scientists reported in Nature on Wednesday that each of the 100 billion stars in the Milky Way galaxy holds at least one planet in its orbit, representing a huge shift away from the previous notion that our solar system was unique.

As recently as April 1994, we did not know of any other solar systems, but the Kepler space telescope, whose mission is to search for other planets, now finds them regularly.

In fact, two other findings announced this week demonstrate just how little we knew about planets in space.

Nature also reported this week that Kepler discovered two new planets that orbit two stars. The first such "circumbinary" planet was discovered in September; until then the only such "known" circumbinary planet was the science-fiction planet Tattooine, Luke Skywalker's home planet in Star Wars.

A third astronomy-related finding of the three smallest planets yet was reported at the American Astronomical Society. These three planets are 0.78, 0.73 and 0.57 times the diameter of Earth, with the smallest being the size of Mars.

The study of how many planets the galaxy has, conducted by 42 scientists across the world led by Arnaud Cassan at the Institute of Astrophysics in Paris, used gravitational microlensing to estimate the number of planets in the galaxy.

The BBC reports that gravitational microlensing uses "the gravity of a far-flung star to amplify the light from even more distant stars that have planets" whereas the Kepler telescope finds planets by looking for the slight dimming in the light of their host stars as the planets pass in front of them.

Kepler's method is better at finding large planets that swing to their host stars whereas gravitational microlensing is better at spotting planets of all sizes orbiting a range of distances from their hosts.

According to the Wall Street Journal, the Cassan study estimates that "Almost two-thirds of the stars likely host a planet measuring about five times Earth's mass, and half of them harbor a planet about the mass of Neptune, which is 17 times the mass of Earth. About one-fifth of them are home to a gas giant like Jupiter or a still more massive planet."

"One can point at almost any random star and say there are planets orbiting that star," astronomer Uffe Grae Jorgensen, of the University of Copenhagen in Denmark and a member of Dr. Cassan's team, told the Journal.

Related on SmartPlanet:

photos: Top: The Milky Way above the dome of the Danish 1.54-metre telescope at ESO's La Silla Observatory in Chile. (ESO/Z. Bardon/ProjectSoft)

Middle: This artist’s impression shows how common planets are around the stars in the Milky Way. The planets, their orbits and their host stars are all vastly magnified compared to their real separations. (ESO/M. Kornmesser)

Bottom: Artistic rendition of Kepler-35, where a Saturn-size planet orbits a pair of Sun-size stars. The discovery of Kepler-34 and Kepler-35 establishes a new class of "circumbinary planets", and suggests many millions of such systems exist in our galaxy. (Illustration by Lynette Cook)

via: The Wall Street Journal, Associated Press, The New York Times, MSNBC

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Laura Shin

Features Editor

Laura Shin has been published in The New York Times, The Wall Street Journal and The Los Angeles Times, and is currently a contributor at Forbes. Previously, she worked at Newsweek, the New York Times, Wall Street Journal and LearnVest. She holds degrees from Stanford University and Columbia University's Graduate School of Journalism. Follow her on Twitter. Disclosure