Public data released by NASA's Kepler mission allowed a team of researchers at the California Institute of Technology in Pasadena to identify three of the smallest planets ever detected that orbit a star beyond the Earth's sun.

Researchers at the California Institute of Technology used the data--combined with follow-up observations from the Palomar Observatory near San Diego and the W.M. Keck Observatory atop Mauna Kea in Hawaii--to discover three planets that are 0.78, 0.73 and 0.57 times the radius of the Earth orbiting a single star called KOI-961.

The smallest of the three--called "exoplanets" because they orbit stars--is about the size of Mars, according to NASA.

Kepler is a NASA mission aimed at finding planets the size of Earth that are located in what is referred to as the "habitable zone," or area in a planetary system where water can exist as a liquid on an orbiting planet. The mission helps researchers understand the Earth's place in the larger galaxy and possibly discover planets capable of sustaining life.

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NASA maintains a public archive of data from the mission and updates it regularly. Prior to the new discovery, only six other planets had been confirmed using Kepler public data, according to NASA.

The newly discovered planets take less than two Earth days to orbit around their star, a red dwarf with a diameter one-sixth of the Sun, making it 70% larger than Jupiter.

Researchers believe all of the planets are rocky like Earth, but orbit closer to KOI-961 than the Earth does to the Sun. This makes them too hot to be considered habitable.

Still, their size and the fact that the planets are rocky like Earth makes the finding significant, according to NASA. Of the more than 700 planets that have been discovered orbiting other stars, only a handful are rocky.

"Finding one as small as Mars is amazing, and hints that there may be a bounty of rocky planets all around us," said Doug Hudgins, Kepler program scientist at NASA Headquarters, in an agency press statement.

The Kepler spacecraft searches for planets by monitoring more than 150,000 stars continuously for dips in their brightness caused by planets that might be crossing them in orbit. The mission requires at least three of these transits around a star to verify a planet, which must be confirmed by follow-up observations from telescopes on the ground.

The verification of the three planets follows several notable discoveries for Kepler in the last two months. In December, the mission identified the first planet in the habitable zone of a star like the Sun, Kepler-22b, which is 2.4 times the size of the Earth.

Later that month, the Kepler team also unveiled the discovery of the first planets the size of Earth orbiting a sun-like star outside our solar system--Kepler-20e and Kepler-20f.

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