You may look out on a starry night and get a lonely feeling, but astronomers now say our Milky Way galaxy may be thick with planets much like Earth -- perhaps 4.5 billion of them, according to the Harvard-Smithsonian Center for Astrophysics.
Astronomers looked at data from NASA's Kepler space telescope in orbit, and conclude that 6 percent of the red dwarf stars in the Milky Way probably have Earth-like, habitable planets. That's a lot by space standards, and since red dwarfs are very common -- they make up three out of four stars in our part of the galaxy -- we may have a lot more neighbors than we thought.
The nearest of them, astronomers said today, could be 13 light-years away -- not exactly commuting distance, since a light-year is six trillion miles, but a lot closer than most yellow stars like Earth's sun.
"We thought we would have to search vast distances to find an Earth-like planet. Now we realize another Earth is probably in our own backyard, waiting to be spotted," said Courtney Dressing, an astronomer at the Harvard-Smithsonian Center, in announcing the findings today. The results will be published in The Astrophysical Journal.
David Charbonneau, a co-author, said, "We now know the rate of occurrence of habitable planets around the most common stars in our galaxy. That rate implies that it will be significantly easier to search for life beyond the solar system than we previously thought."
Red dwarfs are older, smaller and dimmer than our sun, but a planet orbiting close to one could be sufficiently warmed to have liquid water. Dressing and her colleagues cited three possible planets that were spotted by Kepler, which was launched in 2009. One is 90 percent as large as Earth, and orbits its red sun in just 20 of our days.
There is no saying what such a world would actually be like; the Kepler probe can only show whether distant stars have objects periodically passing in front of them. But based on that, scientists can do some math and estimate the mass and orbit of these possible planets. So far, Kepler has spotted more than 2,700 of them in the small patch of sky it has been watching.
There are estimated to be 200 to 400 billion stars in the Milky Way -- which is probably a pretty average galaxy. So the new estimate implies a universe with tremendous numbers of Earth-like planets, far beyond our ability to count.
Could they be friendly to life? There's no way to know yet, but space scientists say that if you have the right ingredients -- a planet the right size, temperatures that allow for liquid water, organic molecules and so forth -- and the chances may be good, even on a planet that is very different from ours.
"You don't need an Earth clone to have life," said Dressing.