Astronomers using data from NASA's Kepler mission have discovered a tiny solar system consisting of a single red dwarf star, known as KOI-961 and three planets which are 0.78, 0.73 and 0.57 times the radius of Earth.

"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. "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."

Comparison of KOI-961 and its planets to Jupiter and its moons (Image: NASA)

To find the size of the planets, data on a well-studied twin star to KOI-961 (known as Barnard's Star) was used. Once they were able to estimate the size of KOI-961 - or Kepler Object of Interest 961 - astronomers were able to determine how big the planets must be to have caused the observed dips in starlight. In addition to the Kepler observations and ground-based telescope measurements, the team used modeling techniques to confirm the discoveries.

The smallest of the three planets is approximately the size of Mars with a surface temperature of around 400-degrees Celsius (750-degrees Fahrenheit).

The planets are thought to be rocky - like Earth - but orbit much closer to their star making them too hot to hold liquid water, making them uninhabitable ... at least for us.

Although over 700 planets have been discovered and confirmed to orbit other stars - known as exoplanets - only a handful are known to be rocky.

With the planets orbiting a red dwarf, the most common type of star in the Milky Way, the finding suggests that our galaxy may be teaming with other rocky planets, some falling within the habitable region around those stars meaning the temperature may be just right for producing liquid water.

"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 Kepler spacecraft is an American space observatory that serves as the space-based portion of NASA's Kepler Mission. Kepler searches for planets by continuously monitoring the position of more than 150,000 stars and looking for changes in their luminosity levels caused by transiting planets. The crossing, or transiting phenomenon is known as "astronomical transit" and describes the process of one celestial body passing in front of another, partially obscuring a small part of it. The occurrence is not dissimilar to a solar eclipse, although that phenomenon is known as "solar transit."

Discovering KOI-961 and its orbiting planets are the latest amongst the Kepler mission's many accomplishments. In December 2011, scientists announced the mission's first confirmed planet in the habitable zone. The planet, which is 2.4 times the size of earth, is now called Kepler-22b. Later that same month, the team discovered the first extrasolar Earth-size planets. These planets are known as Kepler-20e and Kepler 20-f.

Source: NASA