Leicester University Physics Department

The Solar System

The Solar System montage

picture : The Solar System taken from Los Alamos National Laboratory

Solar System Index

The Sun

Mercury

Venus

Earth

The Moon

Mars

Jupiter

Saturn

Uranus

Neptune

Pluto

The Kuiper Belt

Introduction

Our world, Earth, is one of nine planets orbiting the Sun. A great ball of fire, the Sun provides light and heat to the entire Solar System. If it wasn't for the Sun, our world would just be a cold rock, falling through space.

The picture above is called a montage : it shows the relative sizes of all the planets. The fiery edge at the left of the picture is the Sun. As you can see, the Sun is far larger than any of the worlds orbiting it. Well over half a million Earths (the Earth is the third planet out from the sun) could fit inside it! This is a similar size difference to that between a beach ball and a ball bearing.

The term 'planet' has recently been redefined by the astronomers of the International Astronomical Union. The definition is outlined below.

A planet must:

Orbit the sun
Be able to maintain a spherical shape under its own gravity
Have cleared its orbit of debris

This definition is however subject to some controversy. It can be argued that any planets Jupiter being one, that shares its orbit with groups of asteroids has not cleared its orbit, and therefore cannot be classified as a planet.

Orbits of the Planets

Planets and moons orbit due to gravity. Just like we are pulled towards the ground, the Moon is attracted to the Earth too, as are all the planets to our Sun. The orbits of the planets about the sun are almost, but not quite, circular. They are in fact elliptical, which is like a slightly squashed circle.

picture : The Solar System taken from The Solar System page at Arizona State University

The four inner planets are sometimes known as the Terrestrial Planets : Mercury, Venus, Earth and Mars. Unlike the gas giants, they all have a solid surface.

The four largest planets are known as the Gas Giants or Jovian Planets: Jupiter, Saturn, Uranus and Neptune. They are massive balls of fluids without a solid surface that you could stand on. If you were able to send a spacecraft into the atmosphere of one of them, it would soon be crushed by the huge pressures.

Pluto used to be known as the furthest planet from the Sun. It has now been reclassified by the International Astronomical Union as a dwarf planet, because although it is spherical and orbits the sun, it shares its orbit with many smaller bodies. (All interior diagrams courtesy of NASA).