Pluto is the biggest trans-Neptunian object by volume that is known.
Pluto ( minor-planet designation : 134340 Pluto ) is a dwarf planet in the Kuiper belt , a ring of bodies beyond Neptune . Pluto is the largest and second-most-massive known dwarf planet in the Solar System and the ninth-largest and tenth-most-massive known object directly orbiting the Sun . A trans-Neptunian object ( TNO , also written transneptunian object ) is any minor planet in the Solar System that orbits the Sun at a greater average distance ( semi-major axis ) than Neptune , 30 astronomical units ( AU ) .