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Astronomers have discovered the three smallest exoplanets yet, each tinier than the Earth orbiting a small red dwarf star (known as KOI-961) 130 light years away. Of the three new planets, the smallest is only slightly larger than Mars and just 0.57 times the radius of the Earth.

The latest discovery comes from a team of astronomers led by Philip Muirhead at the California Institute of Technology in Pasadena. They performed follow-up observations on publically released data from the Kepler space telescope using the Palomar Observatory located in San Diego County, California and the W.M. Keck Observatory near the summit of Mauna Kea in Hawaii.

An artist's impression of the KOI-961 planetary system (NASA/JPL-Caltech)

Like Earth and Mars, all three planets are thought to be rocky but orbit much closer to their star, requiring only days to complete each orbit. Therefore even though KOI-961 is a red dwarf star, hence cooler and smaller than the Sun, the three planets are so close to their parent star that they lie outside the habitable zone, the region where liquid water could exist. The surface temperatures of the planets range from a hot 178 degrees Celsius (450K) to a blistering 448 degrees Celsius (720K).

John Johnson, the principal investigator of the research from NASA's Exoplanet Science Institute at the California Institute of Technology said, "This is the tiniest solar system found so far. 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."

The KOI-961 planetary system compared to Jupiter and the largest four of its many moons (NASA/JPL-Caltech)

Since Red dwarfs are the most common kind of star in the Milky Way, the discovery hints that our galaxy may be teeming with rocky planets like Mercury, Venus, Earth and Mars. Philip Muirhead added "These types of systems could be ubiquitous in the universe. This is a really exciting time for planet hunters."

The findings were announced on Wednesday 11th January 2012 at a meeting of the American Astronomical Society in Austin, Texas. The Kepler space telescope was launched in March 2009 and detects planets and planet candidates by measuring dips in the brightness of more than 150,000 stars.

See also

Kepler space telescope spots first Earth sized planets
Astronomers announce discovery of 50 new exoplanets
18 Scorpii - The nearest Solar twin
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