AUSTIN, Texas -- Using data collected by NASA's Kepler space telescope, researchers have discovered the three smallest planets so far found outside the solar system, scientists announced Wednesday.

Artist's concept of the KOI-961 system. Credit: NASA/JPL-Caltech
 
The smallest of the three discoveries is the size of Mars, and all of the planets are rocky and orbit scorchingly close to the same parent star.

"Astronomers are just beginning to confirm thousands of planet candidates uncovered by Kepler so far," said Doug Hudgins, Kepler program scientist at NASA Headquarters in Washington. "Finding one as small as Mars is amazing, and hints that there may be a bounty of rocky planets all around us."

The planets are too close to the star, named KOI-961, to be in the habitable zone, where temperatures are just right to support liquid water and the possibility of life.

KOI-961 is a red dwarf with a diameter about one-sixth the size of the sun. It is only 70 percent larger than Jupiter. The three planets each take less than two days to circle the star.

"This is the tiniest solar system found so far," said John Johnson, the principal investigator of the research from NASA's Exoplanet Science Institute at the California Institute of Technology in Pasadena, Calif. "It's actually more similar to Jupiter and its moons in scale than any other planetary system. The discovery is further proof of the diversity of planetary systems in our galaxy."

A team of astronomers led by researchers at the California Institute of Technology made the KOI-961 discovery with data publicly released by the Kepler mission. Follow-up observations from the Palomar Observatory in California and the W.M. Keck in Hawaii confirmed the findings, according to NASA.

Launched in March 2009, Kepler is stationed in an Earth-trailing solar orbit and aims its 3.1-foot telescope toward constellations Cygnus and Lyra, observing a 10-degree-wide field containing 4.5 million detectable stars. Kepler is focusing on approximately 156,000 stars for the purposes of its research.

Kepler's camera constantly monitors the brightness of each star in its target catalog, and the mission detects planets by measuring dips in the brightness of stars as bodies pass in front. By observing "transits" three times, scientists are close to confirming its a planet.

Comparison of the smallest planets discovered by the Kepler mission with Earth and Mars. Credit: NASA/JPL-Caltech
 
Red dwarfs, like KOI-961, are the most common type of stars in the Milky Way galaxy. Up to 80 percent of the stars in the galaxy could be red dwarfs, Johnson said.

"When you combine that with the fact that these are some of the most numerous stars in the galaxy, you realize this type of system could be common," said Philip Muirhead, a postdoctoral scholar and lead author of the study from Caltech. "There's no question that it's exciting."

Scientists say the discovery of the small planets among a tiny fraction of the galaxy's red dwarf stars offers intriguing evidence such planetary systems could be spread across the universe.

In December, scientists announced the discovery of the first planet inside a star's habitable zone and the first confirmed Earth-sized planets found by the Kepler telescope.