Kepler mission finds three smallest exoplanets

Astronomers using data from NASA’s Kepler mission have discovered the three smallest planets yet detected orbiting a star beyond our sun. The planets orbit a single star, called KOI-961, and are 0.78, 0.73 and 0.57 times the radius of Earth. The smallest is about the size of Mars.

All three planets are thought to be rocky like Earth but orbit close to their star, making them too hot to be in the habitable zone. Of the more than 700 planets confirmed to orbit other stars, called exoplanets, only a handful are known to be rocky. But finding one as small as Mars hints that there may be a bounty of rocky planets all around us.

The three planets (called KOI-961.01, KOI-961.02 and KOI-961.03) are very close to their star, taking less than two days to orbit around it. The KOI-961 star is a red dwarf with a diameter one-sixth that of our sun, making it just 70% bigger than Jupiter. 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. Red dwarfs are the most common kind of star in our Milky Way galaxy. The discovery of three rocky planets around one red dwarf suggests that the galaxy could be teeming with similar rocky planets.

Above: (1) Artist’s concept of the KOI-961 system. (2) Comparison of the KOI-961 planetary system to Jupiter and the largest four of its many moons.