Other Cosmic Mentions

With a growing list of remarkable exoplanets, it's hard to keep up with every finding. In 2011, astronomers found another exoplanet called Kepler-22b that might be more like Earth than other planets in our solar system. Or take planets b, c and d in the PSR B1257+12 system. These derelict exoplanets are the leftovers of a supernova from a star in the system, which now orbit a pulsar. Astronomers think the explosion from the dying star might have been enough to obliterate any possible life on the planets [source: Rodriguez].

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.