David A. Aguilar (CfA)

17 percent of the stars surveyed by NASA's Kepler space telescope have Earth-sized worlds in orbit, underlining the preponderance of small exoplanets in our galaxy.David A. Aguilar (CfA)

Four years ago, NASA launched the Kepler space telescope in an attempt to learn just how common Earth-sized worlds are in the Milky Way galaxy.

On Monday, scientists unveiled part of the answer: About 17 percent -- one in six -- of Kepler's target stars have Earth-sized worlds orbiting closer to their parent stars than where Mercury orbits the sun.

With about 100 billions stars in the Milky Way galaxy, that means there's approximately 17 billion Earth-sized worlds, conclude astronomers who presented the research at the American Astronomical Society conference in Long Beach, Calif.

More work is needed to determine how many extrasolar 'Earths' orbit farther away from their parent stars in so-called "habitable zones" where temperatures are suitable for water to exist in a liquid state on the planet's surface. Water is necessary for life as we know it.

Overall, scientists determined that 17 percent of Kepler's target stars have a planet 0.8 to -1.25 times the size of Earth in an orbit of 85 days or less. In comparison, Mercury orbits the sun in 88 days. Earth is positioned at a 365-day orbit.

About 25 percent of Kepler's stars have a super-Earth that is 1.25 to twice as big as Earth in orbits of 150 days or less. The same percentage of stars have a mini-Neptune that is twice to four times as big as Earth in orbits up to 250 days long, reports astronomer Francois Fressin, with the Harvard-Smithsonian Center for Astrophysics.

Bigger planets are much more rare. Three percent have large Neptune-class planets four to six times as big as Earth and five percent have gas giants six to 22 times Earth's mass in orbits of 400 days or less.

Fressin and colleagues also determined that except for gas giants, planets form around all types of stars, including red dwarfs, the most common type of star in the galaxy.

"When you get into the subject of habitability, which is a very sticky subject, then it might matter that you're around a red dwarf instead of a sun-like star," California Institute of Technology astronomer John Johnson told Discovery News.

"It could be that the 20 or 30 factors that go into habitability that we know of on the Earth might not be met on other planets at all, he said. For example, it might be very rare to have a planet like Earth that is only partially covered with water, instead of completely submerged or with no water at all.

"That's something that we can't even begin to address right now," Johnson said.

The Kepler telescope works by looking for a slight dimming in light coming from 160,000 target stars that is due to a planet passing by, or transiting, relative to the telescope's point of view.

Planets closer to their parent stars are easier to find because they transit more frequently. In time, astronomers intend to determine how many Earth-sized worlds are in Earth-like orbits.

Also Monday, astronomers said they have added another 461 candidate planets to Kepler's roster, boosting the cumulative total to 2,740 potential extrasolar planets.

One of the newly found candidate planets is less than twice the size of Earth and is located in the habitable zone of a sun-like star. Three more similarly sized planets are in habitable zones of cooler stars.

To date, 105 of Kepler's candidate planets have been confirmed.