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 than Neptune, which has an orbital semi-major axis of 30.1 astronomical units (au).
Pluto imaged by New Horizons
Euler diagram showing the types of bodies in the Solar System.
Looking back at Pluto, the largest visited KBO so far
According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term minor planet, but that year's meeting reclassified minor planets and comets into dwarf planets and small Solar System bodies (SSSBs). In contrast to the eight official planets of the Solar System, all minor planets fail to clear their orbital neighborhood.
Euler diagram showing the types of bodies in the Solar System according to the IAU
Image: Pluto in True Color High Res
Image: Ceres RC3 Haulani Crater (22381131691)
Image: Vesta in natural color