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Because they could not possibly count every planet in the Milky Way galaxy -- a pinwheel of stars more than 120 thousand light years across -- scientists analyzed selected data from observations of a small field of just 100 million distant stars, and then made an estimate of the number of planets in the entire galaxy.


The six-year project involved observations by 42 astronomers from around the world. A smaller group of scientists sifted through the data for evidence of microlensing, a kind of cosmic magnifying glass in which the light of a background star is bent, and brightened, by the gravitational field of another star passing directly in front of it.


When that foreground star is also circled by one or more planets with their own gravitational fields, the brightness of the microlensing shows a tell-tale fluctuation.


That can last hours or days, depending upon the size of the exoplanet and the duration of its orbit, according to Kailash Sahu of the Johns Hopkins University Space Telescope Institute in Maryland. Sahu helped develop the gravitational microlensing technique in the mid-1990s. The technique enables astronomers to detect very small and very distant planets, Sahu said.


“This is actually sensitive to finding planets far away from us, all the way to the outskirts of the galaxy. So we can get a real census of planets throughout the galaxy rather than a little box around the sun.”


Sahu said many of the stars astronomers analyzed have one or more planets. They found about 1500 of these alien solar systems within 50 light years of Earth. 


According to scientists, two-thirds of all the exoplanets they detected with microlensing are probably about five times the size of Earth, and about one-fifth were the size of the gas giant Jupiter. But it is the Earth-sized planets that have captured the imagination of astronomers like Sahu.


“The Holy Grail will be to find planets, really Earth-like planets, with some things that can actually support life," said Sahu. "And so now we find that, yes indeed, Earth-like planets are common. So, the next thing would be to try to find which are the planets that can hold water and sustain life.” 


So far, astronomers using a combination of ground-based and earth-orbiting space telescopes, including the U.S. space agency’s planet-hunting Kepler mission, have confirmed the existence of more than 700 planets outside our solar system. Another two-thousand candidate exoplanets are waiting to be verified. 


The survey estimating as many as 160 billion planets in the Milky Way Galaxy is described in the January 12th issue of the journal Nature.




Voice of America

Study: Milky Way Galaxy Has 160 Billion Planets