NASA's Kepler spacecraft has discovered its 1,000th alien planet, further cementing the prolific exoplanet-hunting mission's status as a space-science legend.

Kepler reached the milestone today (Jan. 6) with the announcement of eight newly confirmed exoplanets, bringing the mission's current alien world tally to 1,004. Kepler has found more than half of all knownexoplanetsto date, and the numbers will keep rolling in: The telescope has also spotted 3,200 additional planet candidates, and about 90 percent of them should end up being confirmed, mission scientists say.

Furthermore, a number of these future finds are likely to be small, rocky worlds with temperate, relatively hospitable surface conditions in other worlds, planets a lot like Earth. (In fact, at least two of the newly confirmed eight Kepler planets which were announced in Seattle today during the annual winter meeting of the American Astronomical Society appear to meet that description, mission team members said.) [Gallery: A World of Kepler Planets]

"Kepler was designed to find these Earth analogues, and we always knew that the most interesting results would come at the end,"Kepler missionscientist Natalie Batalha, of NASA's Ames Research Center in Moffett Field, California, told Space.com last month.

"So we're just kind of ramping up toward those most interesting results," she added. "There's still a lot of good science to come out of Kepler."

Exoplanet science is a young field. The first world beyond our solar system wasn't confirmed until 1992, and astronomers first found alien planets around a sunlike star in 1995. [7 Ways to Discover Alien Planets]

TheKepler spacecrafthas therefore been a revelation, and has helped lead a revolution. The $600 million mission launched in March 2009, with the aim of determining how frequently Earth-like planets occur around theMilky Way galaxy.

The telescope spots alien planets using the "transit method," watching for the telltale brightness dips caused when an orbiting planet crosses the face of its host star from Kepler's perspective.

The instrument generally needs to observe multiple transits to flag a planet candidate, which is part of the reason why the most intriguing finds are expected to come relatively late in the mission. (Several transits of a huge, close-orbiting "hot Jupiter," which has no potential to host life, can be observed relatively quickly, while it may take years to gather the required data for a more distantly orbiting, possibly Earth-like world.)

"Before, we were just kind of plucking the low-hanging fruit, and now we're getting down into the weeds, and things are getting a little harder," Batalha said. "But that's a challenge we knew we would have."

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NASA's Kepler Space Telescope finds 1,000th alien planet