NASA’s Kepler spacecraft has given its first results: among more than 1,200 planet candidates, it spotted six planets orbiting a single star, as well as dozens of potential Earth-sized planets.

Extraordinary new planetary system

The team using NASA’s Kepler space telescope announced the largest collection of planets yet spotted orbiting a single star. The star, known as Kepler-11, is similar to our Sun and is located approximately 2,000 light-years away from Earth.

Most of the planets orbiting Kepler-11 are closer to it than Mercury is to our Sun (only one of them is at a further distance, but still fairly close); they are made of a mix of rocks and gases, possibly including water. All of them are all bigger than Earth and the largest ones are comparable in size to Uranus and Neptune. All this makes Kepler-11 the star with the fullest and most compact planetary system yet discovered beyond our own. Another important characteristic of the system is that it is amazingly flat: five of the planets have an inclination within a single degree of each other, and the sixth is less than half a degree off.

© NASA/Wendy Stenzel

Dozens of Earth-sized planets

The press briefing given by NASA to talk about Kepler results was also the occasion to provide a status report on the entire mission. The question of other planets with a size similar to that of Earth has been of great interest since the first exoplanets were discovered. Before the Kepler mission, there were not any of such planet candidates; today, there are 68 Earth-sized planet candidates, including 54 in the habitable zone (a region where liquid water could exist on a planet’s surface). “Some candidates could even have moons with liquid water,” said William Borucki of NASA’s Ames Research Center, Moffett Field, California, and the Kepler Mission’s science principal investigator. Five of these planet candidates are both near Earth size and orbit in the habitable zone of their stars. The remaining 49 habitable zone candidates range from super-Earth size – up to twice the size of Earth – to larger than Jupiter.

© NASA/Wendy Stenzel

Of course, all these planet candidates require follow-up ground-based observations to verify they are actual planets.

Kepler has given only four months of data (it will conduct science operations until at least November 2012), and it is looking at only 1/400 of the sky. Extrapolating the number of candidates based on all these limitations, you get a very large number of planets (today, the total number of discovered exoplanets is “only” 525). “In the coming years, Kepler’s capabilities will allow us to find Earth-size planets in the habitable zone of other stars,” Borucki said; you can safely assume that the next data release will probably include a lot of planetary candidates in the habitable zone of their parent stars.