AUSTIN, Texas -- More than 100 billion planets could populate the Milky Way galaxy, and many of the undiscovered worlds could be the size of Earth, according to a study released Wednesday.

This artist's illustration gives an impression of how common planets are around the stars in the Milky Way. Credit: NASA/ESA/M. Kornmesser (ESO)
 
Using a statistical analysis of data collected by a worldwide team of astronomers, the study shows smaller planets are more common than massive Jupiter-sized gas giants.

The results, which are published in the journal Nature, were released by the Space Telescope Science Institute in Baltimore, Md.

"This means, statistically, every star in the galaxy should have at least one planet, and probably more," said Kailash Sahu, a member of the international team reporting on the study.

The observations used a method called microlensing, in which one star passes in front of another, bending the light of the background star. The gravity of the foreground star amplifies light from the background object, and a planet circling the closer star makes the light even brighter, according to scientists.

This effect reveals the presence of a planet around the foreground star.

The amplitude of the microlensing event reveals the mass of the planet, but it provides no information on its composition.

Other popular exoplanet detection techniques, known as the transit and radial velocity methods, monitor stars for the shadow of a planet or the wobble induced by a planet's gravity field.

NASA's Kepler space telescope, which has discovered thousands of candidate planets outside the solar system, stares at a field of stars for evidence of exoplanet transits.

Microlensing events are more rare, but they are more sensitive to smaller planets further away from parent stars.

Based on the statistical analysis, one in six stars hosts a Jupiter-mass planet, half of the galaxy's stars have Neptune-mass planets, and two-thirds of stars harbor planets the mass of Earth.

"Results from the three main techniques of planet detection are rapidly converging to a common result: Not only are planets common in the galaxy, but there are more small planets than large ones," said Stephen Kane, a co-author from NASA's Exoplanet Science Institute at the California Institute of Technology in Pasadena, Calif. "This is encouraging news for investigations into habitable planets."