Trans-Neptunian objects

Trans-Neptunian objects (TNOs) are astronomical bodies orbiting the Sun beyond Neptune.

Since Pluto's downgrading to a dwarf planet, many scientists now consider Pluto and its moons TNOs.

The regions of space beyond Neptune include the:

• Kuiper Belt
• Scattered disc objects
• Oort Cloud

Kuiper Belt

The Kuiper Belt lies between 4.3 and 6.4 billion km from the Sun and contains millions of small icy bodies or TNOs, including Pluto. It is named after Gerard Kuiper, the astronomer who proposed its existence in 1951.

Kuiper Belt objects tend to orbit the Solar System on roughly the same plane as planets and have almost circular orbits. Some of the largest TNOs in the Kuiper Belt include Quaoar (around 1,250 km in diameter) and the dwarf planet Eris, which is larger than Pluto.