Scientists with NASA's Kepler mission announced Wednesday that they had identified more than 1,200 likely new planets in the past year - a revolutionary development in the quest to understand what lies beyond our solar system.
This celestial bonanza represents more than a tripling of the number of known distant planets. It also includes a remarkable single system with at least six planets orbiting its sun - the most populated solar system found so far outside our own.
The discovery of a system with so many planets could be a sign that other habitable solar systems may be common, according to an exuberant William Borucki, head of the Kepler mission, the small satellite launched two years ago to survey some far reaches of the Milky Way.
Borucki and others likened the importance of Wednesday's announcement to the discovery of the first planet outside the solar system some 15 years ago.
Among the new and yet-to-be-fully-confirmed planets are 68 near the size of Earth and 54 in what is deemed the temperate, or habitable, zone of their solar systems.
"If Earth-sized planets are common, then it's likely that life is common on the planets around their stars," Borucki said. "This is really our first step in man's exploration of surrounding galaxies in terms of life and the extent of life that might be there."
The scientists made it clear that the planets they were finding were almost certainly not capable of supporting life - even the five Earth-size planets in comfortably habitable zones. It will take another year or more to locate planets orbiting far enough from the kinds of stars believed most likely to support life.
But at NASA headquarters, the Kepler scientists described their results with evident excitement. The announcement coincided with publication of a paper in the journal Nature that reported on the six-planet system, named K-11.
Those planets - called exoplanets because they are outside the Earth's solar system - are believed to be largely gaseous rather than rocky and are too close to their sun to support life as we understand it. But the discovery of a system with so many planets orbiting in a manner similar to those in our own system raised hopes that other habitable solar systems may yet be found.
"This is a remarkable system and a very exciting sign of what else is to come," said Jonathan Fortney, a member of the Kepler science team from the University of California at Santa Cruz.
"Given the information Kepler is sending back, we're not only able to identify the planets, but we can tell a lot about how big they are, how close they are to their suns and to some extent what they're made of," he said.
Of potentially great importance as well, the scientists said that some 170 of the stars found to have an exoplanet turned out to have more than one of them. That also signaled that our solar system, with its eight aligned planets, may be far from unique.
Yale University astronomer Debra Fischer, who wasn't part of the Kepler team but serves as an outside expert for NASA, said in the news conference that the new information "gives us a much firmer footing" in the search for life beyond Earth.
The Kepler results, she said, have "blown the lid off of everything we know about extrasolar planets."
The unprecedented job of the Kepler space telescope is to observe a small section of deep space to determine how many Earth-size planets exist there, how many suns like our own have planets, and how many of those planets might be in habitable zones. The ultimate goal is to assess whether some planets can - and maybe do - support life.
The telescope detects planets that pass in front of their stars (or "transit") and thereby cause a tiny periodic dip in the brightness of the star. The amount of reduced brightness lets scientists know how large the planet is while the time between transits reveals the speed of its orbit.
The latest findings are based on four months of observations from 2009. The region Kepler is investigating includes some 156,000 stars in what amounts to only one-four-hundredth of the sky. While the 1,235 new exoplanets are for now deemed only "candidates," Borucki and others said they expect 80 to 90 percent of them will ultimately be confirmed as true planets.
Because of the technique used by Kepler to find the exoplanets, those identified so far are usually closer to their suns than Mercury is to our sun, with orbits of a few months to a few days. As Kepler collects information over a longer time, planets further from their suns will be identified and those are more likely rocky, watery and potentially more hospitable to life.
Nonetheless, the early Kepler results have astronomers buzzing. Not only does the six-planet solar system, called K-11, contain a surprisingly large clutch of planets in a relatively small area, but the planets are lined up as if on a flat disc - one that's even flatter than our own solar system. Many other solar systems identified so far have planets that move around their suns at sharp angles to one another, a dynamic that reduces the chances that they could support life.
"We've come to expect the unexpected," said Sara Seager, a Massachusetts Institute of Technology exoplanet specialist on the Kepler science team. "This new system is one of the most interesting ever discovered because of what it says is possible."
Five of the planets in K-11 are close to one another, while the sixth is considerably farther from its sun.
The K-11 solar system is some 2,000 light-years away (the distance light would travel in 2,000 years), and so is unlikely to be visited from Earth or definitively understood. That will be true of all the Kepler-discovered planets, simply because the telescope is looking so far into space - leaving exoplanet experts such as Seager to wonder whether the public will accept them as fully real.
While Kepler is canvassing deep space, ground-based astronomers and planet hunters are using increasingly sophisticated telescopes and spectrometers to discover and describe exoplanets much closer to our solar system. So far, these scientists have discovered more than 500 planets distinct from the 1,200 candidates identified by Kepler.
One of those planets, called Gliese G, was identified last year as existing within a habitable zone in relation to its relatively small sun about 20 light-years away. The discovery, announced by Paul Butler of the Carnegie Institution of Washington and Steve Vogt of the University of California at Santa Cruz, remains to be confirmed.
Last month, NASA announced Kepler's discovery of the first confirmed "rocky" planet, about 1.4 times the size of Earth. But that planet, some 560 light-years away, is definitely not habitable; it's close enough to its sun to orbit every 20 hours and is likely to record surface temperatures as high as 2,500 degrees.