4.5 billion 'alien Earths' may populate Milky Way, says new study

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Study author Courtney Dressing, of the Harvard-Smithsonian Center for Astrophysics (CfA), told SPACE.com that "according to our analysis, the closest Earth-like planet is likely within 13 light-years, which is right next door in terms of astronomical distances."

Dressing and her team analyzed data gathered on over 150,000 stars by NASA's Kepler space telescope. Kepler can detect alien planets by measuring the fluctuation in brightness when the planets transit, or cross in front of, their stars. Kepler has spotted 2,740 Earth-like candidates since its 2009 launch. Researchers have only confirmed 105 of these possibilities so far, but scientists estimate that more than 90 percent will eventually be confirmed.

In the new study, Dressing and the CfA team re-analyzed the red dwarfs in Kepler's data and found that nearly all are smaller and cooler than previously thought.

Astronomers also determined that about 6 percent of the Milky Way's red dwarfs should harbor roughly Earth-like planets in their habitable zones, also known as the "Goldilocks zone," where temperatures and mass are compatible with life as we know it. The new red dwarf analysis suggests that at least 4.5 billion such worlds should exist in the Milky Way.

Red dwarfs also live much longer than stars like our sun, meaning that if any exoplanets do harbor life, that life has possibly been around a lot longer than life on Earth, which first appeared roughly 3.8 billion years ago.

"We might find an Earth that’s 10 billion years old," CfA co-author Charbonneau said.

The new study will be published in The Astrophysical Journal.