Solar System

The Solar System
The Solar System, drawn to scale, but not at the correct relative distances.
Credit:Wikimedia

Our Solar System contains the Sun, 8 planets, and lots of smaller objects. It formed 4,500 million years ago. It is on an outer spiral arm of the Milky Way galaxy.

The 4 planets closest to the Sun are the inner or terrestrial planets. They are small, warm, rocky worlds, with few (or no) moons and no rings. They are: Mercury, Venus, Earth, and Mars. Venus, Earth, and Mars all have atmospheres and weather.

The 4 outer planets are larger and cooler. They contain gases (plus ices). Together they contain 99% of the mass that orbits the Sun. They are often called giant planets. There are gas giants: Jupiter and Saturn. And ice giants: Uranus and Neptune. The giant planets have a lot of moons, over 200 between them! They also all have rings, although we only see Saturn’s rings from Earth.

Between the inner and outer planets is the asteroid belt. The asteroid belt contains millions of bits of rock. These pieces are leftovers from when the planets were forming. The asteroid belt also contains the dwarf planet, Ceres. If a piece of asteroid travels near to Earth, we call it a Near Earth Object.

The Sun is the centre of our Solar System. Every object in our Solar System orbits the Sun. But they orbit at various speeds. Why not use our Electric Orrery to explore the orbits of planets over time.

Everything in our Solar System is bound to the Sun by its gravity. The Sun's gravity extends out past the 8 planets. Including to the Kuiper Belt beyond Neptune. This is a huge ring of icy and rocky objects. The Kuiper Belt includes dwarf planets. Beyond the Kuiper Belt is the Oort Cloud. The Oort Cloud is too far away and dark for astronomers to observe it. They have used maths to predict its existence. Many comets come from the Oort Cloud.

Distances in the Solar System are huge. Too huge for kilometres or miles to be useful. Instead, we use astronomical unit (AU). One AU is the distance from the Earth to the Sun. It is equal to 150 million kilometres.