Our Universe, Solar System

The Solar System we live in contains the Sun, its eight orbiting planets and any other astronomical bodies that are under its gravitational pull such as comets and asteroids.

Comets originate from the Oort Cloud and Kuiper Belt, beyond Neptune, while most asteroids orbit in a region between Mars and Jupiter.

Mercury, Venus, Earth and Mars - the four planets closest to the Sun - are called terrestrial planets because they have solid rocky surfaces. Jupiter, Saturn, Uranus and Neptune are known as gas giants. Pluto, a dwarf planet, has a solid surface but is much icier than the terrestrial planets.

Our Solar System is just one star system among many within the Milky Way galaxy. There are 300 billion stars in the Milky Way and the nearest, Alpha Centauri, is 4.3 light years away. One light year is approximately 9,500 billion km, the distance travelled by light in one year.

There are around 100 billion galaxies in our Universe. So far, no one has detected life outside our home planet.

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.