NASA astronomers nearly doubled the number of alien worlds known to humanity on Wednesday, reporting the discovery of 715 planets located in nearby solar systems.

The discoveries bring the total number of known planets outside our solar system—so-called exoplanets—to roughly 1,700.

Launched in 2009, NASA's $591 million Kepler Space Telescope has now discovered most of the planets orbiting nearby stars.

"We've hit the motherlode; we've got a veritable exoplanet bonanza," says Kepler co-leader Jack Lissauer of NASA's Ames Research Center in Moffett Field, California.

The newly announced exoplanets reinforce the view that most solar systems around sunlike stars have smaller-size planets.

Most of those planets range in width from Earth-size (on the smaller side) to Neptune-size (on the larger). That's quite a change from the Jupiter-size planets that were often spotted orbiting nearby stars during the early planet searches that started in 1995.

"Nature likes to make small planets," says the Massachusetts Institute of Technology's Sara Seager, who was not part of the discovery team but commented on the findings at a Wednesday NASA briefing.

Four of the newly discovered planets orbit around their stars in "habitable zones"—regions where temperatures are just right for oceans, which bring with them the possibility of life.

But the four planets are all a little more than twice the width of Earth, which may make their atmospheres unfriendly to life as we know it. (See: "Earth-Size Planets Come in Two Flavors.")

Worlds Upon Worlds

The newly discovered 715 planets orbit in solar systems around 305 stars, mostly ones the size of the sun or smaller.

Many of the planets orbit in what is beginning to be seen as a more typical solar system, in which the largest planet is Neptune-size and a bevy of smaller Earth-size planets orbit close-in to their star and close to one another.

"These new Kepler results are very helpful in filling out the statistics of solar systems," says Princeton's Adam Burrows, who was not part of the discovery team. "The goal is to see how typical is our own solar system, and ones unlike it."

Multiplicity Simplicity

Kepler detects planets orbiting nearby stars by recording the telltale dips in light they cause as they eclipse their stars. These "transits" can be spotted only for planets traveling in orbits that are viewable edge-on to Earth, which happens roughly 10 percent of the time.

Of the roughly 1,000 planets known to be orbiting nearby stars prior to the Kepler team's Wednesday announcement, fewer than 100 multiple-planet solar systems had been found by the space telescope's investigations since 2009.

The floodgates of planetary findings opened when Kepler scientists realized that they could more confidently detect planets orbiting stars that showed evidence of having multiple planets.

With a "validation through multiplicity" technique, described in a forthcoming Astrophysical Journal paper, scientists could use the gravitational interactions of planets in multiple-world solar systems to rule out false observations of worlds.

The team realized it was seeing more multiple-planet solar systems than random chance would allow, Lissauer said, giving them statistical confidence in the technique.

Kepler Continues

The Kepler mission suffered a setback last year when a steering gyroscope aboard the spacecraft failed. The failure ended the spacecraft's main mission, but a reduced effort is still under way.

All told, Kepler has discovered about 3,500 "candidate" planets orbiting stars within about 3,000 light-years of Earth.

One of the light-detector modules aboard the spacecraft also failed in the last month, NASA announced late Tuesday. That leaves 19 detectors still functional, and the mishap will not stop the mission, according to the space agency.

At NASA, a new Kepler 2 mission has emerged that promises about 100 new planet discoveries every year, said the Kepler team's Jason Rowe of the SETI Institute in Mountain View, California.

Says MIT's Seager: "Kepler is the gift that keeps on giving."

Astronomers suspect that our Milky Way galaxy is home to perhaps 17 billion rocky worlds similar to Earth, along with hundreds of billions of larger ones.