The artist concept depicts multiple-transiting planet systems, which are stars with more than one planet.
Nearly a year after a pointing system failure ended the main planet-hunting mission of NASA's Kepler spacecraft, planets continue to pour out of the sky.
Kepler astronomers still combing their vast data archive announced Wednesday in a news conference and a pair of journal papers that they had verified the existence of 715 more planets orbiting other stars.
It was the largest single-day haul of exoplanets, as such objects are called, since the first such planet was discovered in 1995, bringing the overall total to about 1,700.
"Today we almost doubled the number of planets known to humanity," said Jack J. Lissauer of NASA's Ames Research Center in Moffett Field, California.
He and Jason Rowe of the SETI Institute, in Mountain View, Calif., are each the lead author of a paper to be published in The Astrophysical Journal detailing the findings. They discussed their work in a telephone news conference hosted by NASA.
The new planets were culled from 3,601 candidates previously found by Kepler, using a new statistical technique known as verification by multiplicity. The method vastly reduces the need for outside telescopic observations to verify suspected planets in batches. It works only for multiple-planet systems, but as Lissauer and his colleagues pointed out, that includes about 40 percent of the Kepler candidates.
The result is a deluge of small planets that has tipped the cosmic balance from the giant Jupiter-size worlds that were the earliest discovered to smaller, friendlier worlds.
"Small planets from the size of Neptune to Earth make up the majority of the planets in the galaxy," said Douglas Hudgins, exoplanet program scientist at NASA headquarters.
The planets in the new set are divided among 305 stars.
About 95 percent, according to Rowe, are two to four times the size of Earth, a range conspicuously absent in our solar system. Because we have no local examples, astronomers do not know what planets of this size might be like - rocky like Earth or gassy like Neptune.
Four of the newfound planets orbit in the so-called habitable zones of small stars, where temperatures on a planet like our own would be moderate enough for water and thus the kind of life we know.
Sara Seager, a planetary expert at the Massachusetts Institute of Technology, who was not involved in the new analysis, said the data reinforced Kepler's major finding: The Milky Way is brimming with small planets that could be friendly to life.
"This is a boon for any future habitable-zone planet-finding mission," Seager said.
Kepler seeks planets by the blink method, looking for the dimming of stars' light caused by planets passing in front of them. While this method can tell the sizes and orbits of planets, it is prone to false positives because of confusion with background binary stars, among other things. It also cannot tell the masses and thus the densities and compositions of the worlds it detects, requiring outside observations with other telescopes to confirm their planethood.
The idea behind verification by multiplicity, which has been long in development, as Lissauer explained it, is that the false positives should be randomly distributed around the 160,000 stars that Kepler observes, resulting in the appearance of only a few, if any, multiple systems. The fact that they are instead stuck on 400 or so particular stars suggests that those systems are real. He put the odds that one of these was a mistake at about one part in a thousand.
There is more to come, the astronomers said. The present results are based on a statistical analysis of only the first two years of Kepler data. There are two more years' worth to go, and several hundred more planets are likely to be verified.
"Kepler is the gift that keeps on giving," Seager said.
© 2014, The New York Times News Service