SEATTLE—NASA'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 planets—and perhaps all eight—are 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."
The new catalog of worlds from Kepler identifies an additional 554 planet candidates, bringing the mission's total number of candidates—objects that might be exoplanets—to 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.
"I'm over the moon," says Natalie Batalha of NASA's Ames Research Center. "We now have a sizable bunch of small planet candidates orbiting in the habitable zone of [sunlike] stars. This is tremendously good news for Kepler's census and for the search for life beyond Earth."
Kepler's mission is to determine how common Earthlike planets are in the galaxy. So far, astronomers' best guesses suggest that roughly 20 percent of sunlike stars host Earth-size planets in their habitable zones.
Kepler the Planet-Hunter
Launched in 2009, the Kepler spacecraft spent four years staring at a patch of northern sky studded with 150,000 stars. It looked for periodic twinkles caused by planets marching across the faces of those stars, a journey that temporarily dims the star's light.
Newly found planets came rolling out of Kepler's sky like cosmic marbles shaken loose. At first, the planetary marbles were big, but as Kepler continued to shake those stars, smaller planets followed, and finally the Earth-size planets (and smaller) came rolling out.
Then Kepler was struck by a mechanical problem in 2013, losing two of the four reaction wheels needed to aim the craft and keep it staring fixedly at its star field. But scientists breathed new life into the spacecraft with a different mission, called K2.
The spacecraft now uses the force of sunlight hitting its solar panels as essentially a third reaction wheel and is free to look anywhere it wants. Already, scientists have found one new planet: HIP 116454b, announced in December.
One of the K2 mission's potential planetary systems is stuffed with three super-Earth-size planets on 10-, 24-, and 44-day orbits, Andrew Vanderburg of the Harvard-Smithsonian Center for Astrophysics reported Monday at the AAS meeting.
"If real, these planets are orbiting a relatively cool, nearby star," Vanderburg says. "What's special about these is that they would be orbiting a bright star, with the potential for more sophisticated follow-up observations."
That so many worlds have been found in just four years with one dedicated instrument is heartening for astronomers who struggled for decades to find the first exoplanet orbiting a main sequence star. That planet, a large, hot, Jupiter-like gas giant called 51 Pegasi b, was discovered 20 years ago, in October 1994.
"Just before that time, I remember astronomers feeling very worried. Scientists had been looking for a long time for exoplanets—for decades—and hadn't found any," says veteran planet-hunter Debra Fischer of Yale University. "And we really had to step back and say, 'We have to entertain the possibility that maybe the Star Trek picture of the universe isn't right. Maybe there are no other planets around other stars, or they're there rarely.'"
Now, with roughly 1,800 exoplanets discovered by Kepler and through other observations, Fischer is glad to put that desolate possibility to rest and get to work tackling the next big question.
"We now know the universe is teeming with planets," she says. "The next step is to figure out if the universe is teeming with life."