Kepler-11: a strange and crowded planetary system

Kepler-11 is an amazing, newly-discovered system of exoplanets. About 2000 light years from Earth, six planets orbit a star like our Sun. Each planet is bigger and more massive than the Earth. This whole planetary system is squeezed into a region slightly larger than Mercury’s orbit.

Image of Kepler-11

Kepler-11 and its family as imagined by by a NASA artist (Image credit: NASA)


This bizarrely shrunken planetary system was found by NASA’s Kepler mission, hence the parent star has been named Kepler-11. All of the planets orbiting Kepler-11 are larger than Earth, with the largest ones being similar in size to Uranus or Neptune. Planetary scientist and Kepler science team member Jack Lissauer said “These six planets are mixtures of rock and gases, possibly including water.”  None of these strange new worlds are likely to be home to any form of life; their surface conditions are simply too extreme.

This is a very peculiar family of worlds: one large planet is out in an orbit around that would place it between Mercury and Venus if it circled our Sun, but the inner five planets are closer to their star than any planet in our Solar System. In them, the equivalent of ten times the mass of Earth is packed inside a radius smaller than the orbit of Mercury.  So tightly do they huddle together that they all interact gravitationally! As they whirl around their star in mere days, these planets pull on each other, thus slightly altering their orbits. It sounds like a recipe for chaos but this system has presumably been stable for millions if not billions of years. In fact, observing these continuously changing orbits enabled astronomers to precisely calculate the planets’ masses. Explaining how this system came to be is difficult with our current knowledge of planetary formation.

Image of Kepler-11 compared to Solar System

A very peculiar place: the Solar and Kepler-11 systems compared. The SF fan in me wonders if this is what a Ringworld under construction might look like! (Image credit: NASA)


Progress in exoplanetary research has been extraordinary; twelve months ago finding a single planet like those in the Kepler-11 system would have made the headlines. Now we are discovering them by the half dozen! But there will be many, many more to come. The press conference announcing this system’s discovery has also revealed that the Kepler satellite has identified 1235 potential planets. Of these, 68 are approximately Earth-size; 288 are super-Earth-size; 662 are Neptune-size; 165 are the size of Jupiter and 19 are larger than Jupiter. Five of the Earth-sized planets are in their star’s habitable zone.

These findings are based on the results of just four months of observations of more than 156 000 stars in the Kepler satellite’s field of view, which covers approximately 1/400 of the sky. This led William Borucki, the mission’s science principal investigator, to say “The fact that we’ve found so many planet candidates in such a tiny fraction of the sky suggests there are countless planets orbiting sun-like stars in our galaxy.”