Artist's concept of Kepler-186f, the first Earth-size planet found orbiting in the habitable zone of its parent star. The exoplanet circles a red dwarf about 490 light-years from Earth.

The first true alien Earth may not elude planet hunters for much longer.

This week, astronomers announced that NASA's Kepler space telescope had discovered eight more relatively small planets that may be capable of hosting life as we know it, describing two of the new finds as the most Earth-like alien worlds known.

Mission scientists also announced 554 new unconfirmed Kepler "planet candidates" on Tuesday (Jan. 6); six of these potential worlds orbit sunlike stars, are close to Earth-size and are possibly habitable. [10 Exoplanets That Could Host Alien Life]

"These candidates represent the closest analogues to the Earth-sun system found to date," Fergal Mullally of the Kepler Science Office said Tuesday in Seattle during a news conference at the annual winter meeting of the American Astronomical Society (AAS). "This is what Kepler has been looking for. We are now closer than we have ever been to finding a twin for the Earth around another star."

Elusive alien Earths

The $600 million Kepler mission launched in March 2009, with the primary goal of determining how commonly Earth-like planets occur throughout the Milky Way galaxy. Kepler hunts for alien worlds by searching for "transits," noting the tiny brightness dips caused when a planet crosses the face of its host star from the observatory's perspective.

The spacecraft carried out its original planet hunt until May 2013, when the second of its four reaction wheels failed, robbing Kepler of its precision pointing ability. The observatory began a new mission called K2 last year, during which it has continued to search for exoplanets, albeit in a more limited fashion, and has studied other cosmic objects as well. (Kepler has detected an exoplanet during the K2 mission, but all of the planets and candidates discussed in this story were spotted during the spacecraft's first four years of operation.)

The observatory has been remarkably successful. Kepler has discovered 1,004 alien planets to date, along with nearly 3,200 other candidates, the vast majority of which will likely be confirmed eventually. But Kepler has not yet found an Earth twin, and neither has any other telescope. [Gallery: A World of Kepler Planets]

The known exoplanets most similar to Earth may be Kepler-438b and Kepler-442b, two of the eight newfound worlds announced Tuesday. Kepler-438b is just 12 percent larger than Earth, and Kepler-442b is 33 percent wider than our home planet. Both exoplanets are probably rocky, and both apparently orbit in their host stars' "habitable zone" — the range of distances that could support liquid water on a world's surface.

But Kepler-438b and Kepler-442b circle a red dwarf and an orange dwarf, respectively — stars smaller and dimmer than the sun — so they cannot be true Earth twins.

Indeed, many of Kepler's confirmed habitable-zone planets orbit red dwarfs. This makes sense, and not just because about 70 percent of the Milky Way's stars are red dwarfs: Potentially habitable red dwarf planets transit more frequently than do their counterparts that circle sunlike stars, because the dimmer red dwarfs' habitable zones lie closer in. For example, Earth completes one orbit every 365 days, while the orbital period of Kepler-438b is just 35 days.

Probing sunlike stars

But scientists have now analyzed four years of Kepler data, giving them the ability to start seeing bona fide Earth twins at last. (The 554 newly announced candidates were pulled from observations the spacecraft made between May 2009 and April 2013.)

"We've finally become senstive to small planets in one-year orbits similar to our own Earth," Mullally said.

Two newfound candidates — known as KOI (Kepler Object of Interest) 5737.01 and KOI 2194.03 — are particularly intriguing, he added. Both circle sunlike stars, and appear to lie in the habitable zone. KOI 5737.01 is about 30 percent wider than Earth and completes one orbit every 376 days. KOI 2194.03 has an orbital period of 445 days, and researchers think it's about 40 percent larger than Earth.

"They are our best candidates for [Earth] twins," Mullally said, stressing that both potential worlds still need to be confirmed as bona fide planets.

And researchers aren't done poring over the observations Kepler made during its four-year prime mission.

"We anticipate two more [planet candidate] catalogs," Mullally said. "We don't have that much more data to analyze, but we still feel we can refine and improve the way that we search for these candidates ... and hopefully dig out a few more."

So the discovery of the first alien Earth may be just around the corner. And the quest to notch this milestone continues to enlighten scientists about the nature and variety of worlds beyond our solar system.

"By continuing to fill in the neighborhood around Earth-like conditions, we're not only closing in on an Earth twin, but we're better understanding the diversity in this special neighborhood of planets," Douglas Caldwell, of the SETI Institute in Mountain View, California, said Tuesday during the AAS press conference. Caldwell co-authored the study that announced the discovery of the eight new potentially habitable Kepler planets.

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