NASA's Kepler Space Telescope has now identified 1,235 candidates for planethood, including several entire solar systems and some potentially habitable planets.
The discovery should keep astronomers on Earth and telescopes in space busy for decades.
"This just revolutionizes planetary studies," said Steve Howell, an astronomer in Tucson with the National Optical Astronomy Observatory and a member of the Kepler science team.
The candidates were found through observation of 156,000 sunlike stars in one small portion of the sky in the Milky Way galaxy around the constellation Cygnus - about 1/400th of the visible sky.
The results announced by NASA Wednesday, if verified by further observation, will triple the number of known extrasolar planets or "exoplanets" orbiting stars other than our own sun.
The big news from NASA's televised news conference was that Kepler's science team had winnowed the candidates to unveil 54 in the "habitable zone" of their suns where temperatures approximate those found on Earth and liquid water might exist. Five of those planets are near the size of Earth.
Howell said the Kepler finds are proof enough for him that the conditions that would support life - and life itself - exist abundantly in the universe.
"It's so clear to me," Howell said. "Planets are everywhere. How could there not be life?"
Among the planets already verified by follow-up observation is a system of six planets orbiting a sunlike star 2,000 light-years away. A light-year, expressed as a distance, is about 6 trillion miles.
Kepler, launched in March 2009, is a photometer, or light meter, positioned in orbit to stare at the same place for three years and record slight diminutions of light in its 156,000 target stars. The dips in the light curves indicate that a planet has passed or "transited" between the sun and Kepler's position.
Most of the planet candidates found in the first four months of Kepler's operation orbit close to their stars. The ones farther out at, say, Earth distance, pass by more infrequently and would take years of observation to discover.
The six planets orbiting a star the team named Kepler 11 are on "very near the same plane," with five orbiting closer to their sun than Mercury does to ours, said planetary scientist Jack Lissauer, of NASA Ames Research Center, a co-investigator for the mission. Their orbits take 10 to 47 days to complete.
The sixth planet is farther out, at a distance between Mercury and Venus, and has an orbital period of 118 days, Lissauer said at the news conference.
It is the largest solar system, other than our own, yet found and "the most compact system of planets ever discovered by any technique anywhere," Lissauer said.
The system is very far away. Light from Kepler 11 "left the star around the time Caesar was making his conquests," Lissauer noted.
Needless to say, we're not going to any of these planets anytime soon. Howell said our best bet for finding, confirming and eventually exploring habitable planets might lie closer to us.
There were practical reasons for not starting the exploration nearby, Howell said.
Stars are about eight light-years apart on average. Nearby stars are few and far between. It was more productive to look farther off at more stars. Now that Kepler has demonstrated that planets are so numerous, astronomers might be more inclined to stake out some of the 200 or so nearby stars for continuous observation, he said.
The Kepler team has not fully formed its follow-up plan, said Howell, but he expects it will concentrate on the stars it found in the habitable zone, even as it searches for more targets.
Follow-up is done from a variety of ground telescopes including four in Southern Arizona, on Mount Hopkins and Kitt Peak, where Howell uses the WIYN Telescope's spectrograph to make sure Kepler's candidates are planets and not binary stars.
William Borucki, the mission's principal investigator, compared the quest for finding life in space to building a cathedral. We're at the foundation stage, he said. It will be our grandchildren's generation that might make a decision to mount space missions to the best candidates.
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