To the list of planets orbiting distant stars, add another 715. That’s the number of planets, strewn among 305 planetary systems, popping out of the observational data from NASA’s Kepler space telescope.

“We’ve almost doubled today the number of planets known to humanity,” said Jack Lissauer, a NASA planetary scientist, announcing the discovery during a teleconference Wednesday with reporters. The findings will be published in March in two scientific papers in the Astrophysical Journal.

These new planets are all in multi-planet systems and are relatively modest in size — most of them smaller than Neptune. Four of the new planets are about twice the size of Earth and are in orbits that put them in what is considered the habitable zone of their stars, at a distance that could allow water to be in a liquid state at the surface.

There are surely more habitable-zone planets out there, scientists said in the teleconference. Small planets in very tight orbits are the ones most likely to be detected by Kepler, which looks for the dimming of starlight as a planet passes, or “transits,” the disk of the star as seen from the telescope. The star, planet and telescope have to be aligned, a matter of pure chance: Planets that are orbiting at a great distance from the parent star are less likely to be lined up propitiously.

The new studies looked at only the first two years of Kepler’s data collection. To find a planet, Kepler needs to make several observations of transits, and the more-distant planets have longer orbital periods and are less likely to be verified in just two years.

The discovery nearly doubles the number of known planets outside our solar system, according to two new reports. Video courtesy of NASA's ScienceCasts. (NASA's ScienceCasts)

“Although we’ve gotten the big numbers this time, when we have a full four years of Kepler data, that will have more planets in the habitable zone,” Lissauer said. “We need more transits.”

The discovery that small planets are common is good news for astronomers hoping that NASA and other space agencies will build more advanced planet-hunting telescopes capable of directly imaging an Earth-like planet.

“Nature wants to make small planets,” said Sara Seager, an MIT astrophysicist and planetary scientist who was not among the authors of the new Kepler papers.

In an interview, Seager said the multi-planet systems seen so far don’t tend to look like our solar system. Here, Earth and the other planets are spaced relatively far apart, at significant distances from the sun. But Kepler keeps finding compact multi-planet systems with the planets crammed together very close to the star.

For example, Kepler has found a system in which five of the six planets are closer to the star than Mercury is to our sun. It’s unclear whether the planets formed in those tight orbits or migrated there over time.

The Kepler team has previously announced several thousand possible exoplanets outside our solar system, but most have yet to be confirmed by subsequent observation. This new batch of planets has been verified — or “validated to substantially better than the 99 percent confidence level,” as one of the new papers puts it — through a technique that calculates the number of “false positives” in multi-planet systems. The scientists concluded that the “candidate” planets in those systems are almost always bona fide planets.

The search for exoplanets is intrinsically difficult because they’re very far away — many tens of trillions of miles at least — and the light reflecting from the planet is overwhelmed by the brilliance of the parent star. Someday, a new generation of space telescopes may be able to occlude the starlight and see only the planet.

For now, planet hunters look for quirks in the starlight that might reveal a planetary companion. If the starlight wobbles, that can be the signature of the gravitational tug of a planet. If the starlight dims in a regular pattern, that can be from a planet passing in front of the star as seen from the telescope.

The latter technique is the one that Kepler employs. No instrument has been as adept at finding large batches of exoplanets as the $600 million Kepler telescope, which is about to celebrate the fifth anniversary of its launch on March 7, 2009.

The Kepler telescope suffered a malfunction in its pointing system last year, hampering its observational abilities. But scientists continue to examine the massive amount of data accumulated over its first four years of operation.

In November, scientists working with Kepler’s data announced that, extrapolating from what has been seen in one small patch of the sky, they infer that one out of five sunlike stars has a roughly Earth-size planet in a “habitable zone” orbit. That would suggest there are tens of billions of potentially habitable, roughly Earth-size worlds in this galaxy alone.

Whether there is actually life out there, in any shape or form, is unknown.