SEATTLENASA's venerable planet-hunter, the Kepler spacecraft, has shaken its one-thousandth planet from the sky. Eight new worlds beyond our solar system, announced Tuesday, boost the number of Kepler's confirmed planets to 1,004 (if you're keeping count), including two of the most Earthlike planets discovered so far.

Those eight new worlds are each less than 2.7 times the size of Earth, astronomers reported at the American Astronomical Society's annual winter meeting. But hiding in the wings, among a group of 554 newly announced planet candidates, is an even more tantalizing set of planets.

"These candidates represent the closest analogues to the Earth-sun system found to date, and this is what Kepler has been looking for. We are now closer than we have ever been to finding a twin for Earth around a star," says Fergal Mullally of the SETI Institute and NASA's Ames Research Center.

Kepler's eight newly confirmed planets are all relatively small, and they all orbit stars that are smaller and cooler than the sun. Depending which calculations scientists use, at least three of the planetsand perhaps all eightare in the habitable zones of their parent stars. This is the region where temperatures are just right for supporting liquid water on the planet's surface. (Learn more about habitable-zone planets in "Kepler Telescope Discovers Most Earth-Like Planet Yet.")

At least two of those planets, Kepler 438-b and Kepler 442-b, are likely to be rocky, like Earth.

"We have significantly increased the number of these verified, small, habitable-zone planets from Kepler," says Doug Caldwell of the SETI Institute and NASA's Ames Research Center.

"They really make up a special population that is of interest for understanding the prevalence of life in the universe. Yesterday we had five Kepler exoplanets in this special hall of fame, and today we have eight in this elite club."

Earth Twins

The new catalog of worlds from Kepler identifies an additional 554 planet candidates, bringing the mission's total number of candidatesobjects that might be exoplanetsto 4,175. Of those 554 new candidates, eight are small, less than twice the size of Earth, and in the habitable zones of their stars. (These candidates are in addition to the eight newly confirmed planets.)

And here's the really tantalizing bit: Six of those potential planets are orbiting sunlike stars and represent a class of planet that Kepler hasn't yet gotten a good look at: the real exo-Earths.

Read more:

NASA's Kepler Spacecraft Discovers New Batch of Earthlike Planets