Four years ago, NASA launched the Kepler space telescope to find out how many stars like the sun host Earth-sized planets suitably positioned for liquid water, a key ingredient for life.

On Monday, a team of scientists announced an answer: about 10 billion -- enough for one planet for every person in the world, with 3 billion to spare.

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“We didn’t know what to expect,” astronomer Geoffrey Marcy, with the University of California at Berkeley, told Discovery News.

“It was only 18 years ago that most of us, myself included, thought we might never discover any planets of any sort around other stars. It was thought to be impossible. To have gone from where we were in the 1990s with nothing, to now finding Earth-sized planets in their habitable zones really boggles my mind,” Marcy said.

Before a positioning system problem sidelined the telescope in May, Kepler focused on a patch of the sky in the constellation Cygnus and dutifully assembled a digital picture every 30 minutes to send back to Earth.

Astronomers analyzed the images to find slight changes in the amount of light coming from about 150,000 target stars.

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Some variations were due to star flares and other stellar phenomena, but others provided telltale clues that an orbiting planet had passed across the face of its parent star, relative to Kepler’s point of view, blocking a smidgen of light in the process.

The timing of repeat light dips indicated the planet’s distance from its host star, information scientists could then use to estimate the planet’s surface temperature. Of particular interest are planets suitably positioned for liquid surface water, as water is believed to be necessary for life.

Finally, by measuring how much starlight was blocked during a transit, scientists could calculate a candidate planet’s diameter. Smaller planets like Earth block less light than giant Jupiter-sized worlds.

Artist's impression of the five-planet Kepler-62 system, with two worlds in the habitable zone -- the distance from their star at which they receive enough light and warmth for liquid water to theoretically exist on their surfaces. David A. Aguilar (CfA)

A new study based on 34 months of Kepler images found 10 Earth-size planets circling in the so-called "habitable zones" of sun-like stars.

Accounting for all the other viewing angles not observed by Kepler and extrapolating from the data, the study finds that 22 percent of sun-like stars in the Milky Way have planets one- to two times the size of Earth orbiting in their habitable zones.

With about 50 billion sun-like stars in the galaxy, that means about one out of every five stars, or roughly 10 billion, have an Earth-sized planet in its habitable zone.

“This is the first time that a team has offered such a number for stars like the sun based on a thorough detection analysis,” said Kepler mission science Natalie Batalha, with NASA’s Ames Research Center in Moffett Field, Calif., told reporters on a conference call.

Earlier this year, another team of scientists reported that about 15 percent of cooler, red dwarf stars have potentially habitable planets.

“This team is pushing that to hotter stars, stars more like our sun,” Batalha said.

The research is published in this week’s Proceedings of the National Academy of Sciences.