Jan 17, 2012; 8:11 AM ET
A drawing of NASA’s Kepler mission in space
Astronomers using data from NASA’s Kepler mission have discovered the three smallest planets yet outside of our solar system. The Kepler mission from NASA latest discovery comes from a team led by astronomers at the California Institute of Technology in Pasadena.
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, which is the region where liquid water could exist. Of the more than 700 planets confirmed to orbit other stars, called exoplanets, only a handful are known to be rocky.
The three planets 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 percent bigger than Jupiter.
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.
The discovery follows a string of recent milestones for the Kepler mission. In December 2011, scientists announced the mission’s first confirmed planet in the habitable zone of a Sun-like star: a planet 2.4 times the size of Earth called Kepler-22b. Later in the month, the team announced the discovery of the first Earth-size planets orbiting a Sun-like star outside our solar system, called Kepler-20e and Kepler-20f.