Exploded diagram showing the Solar System and the Oort cloud of comets that surrounds it. The inner Solar System (top) extends out to the asteroid belt. Beyond this (middle) is the realm of the gas giant planets and then the Kuiper Belt a thick donut of icy bodies on the edge of the Solar System. Finally the Solar System is embedded within the Oort cloud (bottom). Its exact extent is not known, but it could stretch as far as 50000 astronomical units from the Sun almost one lightyear.Mark GarlickScience Photo Library