Just 20 years ago, astronomers had no direct evidence that planets orbited other stars. Now, researchers estimate the Milky Way galaxy contains a huge number of planets, with Earth-sized worlds vastly outnumbering the rest.

"We find that, on average, every star has a planet, and since there are at least 100 billion stars, there are at least 100 billion planets," said astronomer Kailash Sahu of the Space Telescope Science Institute in Baltimore, Maryland, who co-authored the new study, appearing Jan. 11 in Nature.

Sahu and his team calculated this number by searching for planets using a technique called gravitational microlensing. The method works because, according to Einstein's Theory of Relativity, a massive object bends the fabric of space-time.

Though a planet's mass is relatively small, it is enough to curve space-time and create a "lens." An exoplanet bends light as it passes in front of its parent star, causing a slight brightening of the star's light.

Microlensing allows astronomers to look at a much larger sample of stars for exoplanets. Unlike other detection methods, such as looking for the slight wobble a planet exerts on its parent star, microlensing can discover planets with many different masses and distances from their star.

The team looked at roughly 30 different microlensing events and found that extrasolar planets caused three of them. Because microlensing observations are known to miss a certain percentage of planets, the researchers could use statistical analysis to get the true number of exoplanets in the galaxy.

"We think about one-sixth of stars should have a Jupiter-like planet, half have a Neptune-sized planet, and two-thirds should have an Earth," said Sahu.

Image: Wikimedia/Mila