Before 1991, scientists hadn't discovered a single exoplanet. Now, in addition to the hundreds of exoplanets discovered throughout the Milky Way, they've found six orbiting just one star: Kepler-11.
The Kepler-11 solar system is interesting not just because it has more planets than any we've observed outside our own, but also because it's extremely compact. For instance, five of the six planets orbit closer to their parent star than any planets in our solar system orbit the sun.
Kepler-11f stands out from the other planets in the system for its size, which is about 2.3 times the mass of Earth and makes it yet another super-Earth [source: NASA]. Like all of the planets in the system, Kepler-11f has a lower density than Earth, meaning its composition is likely quite different as well.