The Kepler space telescope has spotted the most tightly-packed exoplanet system yet, with five planets orbiting around the star KOI-500 within a fraction of the distance between Mercury and our Sun. The planets orbit their star in (going from innermost to outermost) 1.0, 3.1, 4.6, 7.1, and 9.5 days each, respectively, and each planet is between 1.3 and 2.6 times the size of the Earth. The outer four planets exist in a kind of orbital resonance, which sees them return to a set formation every 191 days -- that seems to keep them from being knocked out of orbit by each others' gravitaties and hurled either further out into the system or into the star to burn up.
The system was discovered by Darin Ragozzine, a planetary scientist at the University of Florida at Gainesville, and his team. It's roughly 1,100 light years from us, in the direction of the constellation Lyre. Its five planets are each slightly larger than the Earth, but their orbits are remarkably close to KOI-500 -- 150 times smaller than the orbit of the Earth. That's even less than the orbital distance of Mercury. Yet despite flying around so fast that it's only a manner of Earth days for each "year", they exist in an orbital resonance that keeps them from crashing into each other or falling into the star.
Via Dr. Stefan Gruenwald