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For a very long period of time our solar system was believed to comprise nine planets: the terrestrial planets Mercury, Venus, Earth, Mars, followed by the gas giants Jupiter, Saturn, Uranus and Neptune. The exotic and tiny Pluto brought up the rear. Pluto was demoted from its planet status in 2006. Thus, only eight planets remain. In spite of this, the question remains what lies beyond the orbit of Neptune? Is it merely Pluto? Just plain, empty space? We have already come across the answer in the previous chapters. It is not empty space we can find beyond Neptune. A vast number of small bodies exist there. All of them are referred to as trans-Neptunian objects (TNO).