For the first time in history, two planets have been discovered that share the same orbit.

The pair of planets are part of a four-planet solar system called KOI-730. They orbit a sun-like star, one 60 degrees ahead of the other, with a full revolution taking 9.8 days. The reason this phenomenon is possible is because there are two gravitational "sweet spots" called Lagrange points along a planetary body's orbit where another body can share the same orbit. These points are located 60 degrees ahead of and 60 degrees behind the orbiting object.
This even occurs within our solar system - group of asteroids called Trojans inhabit the Lagrange points of Jupiter's orbit! The existence of this pair of planets also supports an existing theory that the moon was formed by the collision of the Earth and a Mars-sized planet that once shared its orbit.