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.

Mercury, Solar System's evolution

Mercury is the closest planet to the Sun and the smallest in our Solar System. Named after the Roman messenger god, the planet is not much larger than the Earth's Moon and superficially resembles it with a surface pock-marked by enormous craters.

These craters were caused by meteorites smashing into the planet's surface in the early stages of the Solar System's evolution, some four billion years ago. Although it's only a third the size of Earth, Mercury is almost as dense. Scientists put down to a massive iron core, which is also responsible for Mercury's magnetic field.