Not to scale. All images are in the public domain. Source: NASA
Not to scale. All images are in the public domain. Source: NASA

The planets, from left to right: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus (possibly the most hilariously named planet of all) and Neptune. Also pictured is the dwarf planet Pluto.

Some information on the planets:

Planet:
Distance from Sun (AU):
Time for one orbit (years):
Diameter compared to Earth:
Surface Temperature (centigrade):
Moons
Mercury
0.4
0.2
0.4
350
0
Venus
0.7
0.6
0.9
470
0
Earth
1
1
1
15
1
Mars
1.5
1.9
0.5
-30
2
Jupiter
5.0
12
11
-150
63
Saturn
9.5
30
9
-180
60
Uranus
19
84
4
-210
27
Neptune
30
165
4
-220
13

The planets are held in almost-circular orbit by the gravitational pull of the Sun, but they are not the only things that orbit the sun.
Astronomical measurements are often done in Astronomical Units (AU) which is the distance from the Earth to the Sun.

Moons
Some planets have moons which orbit the planets orbiting the sun. Earth has one while Jupiter has 63! Moons are held in place by the gravitational pull of the planets.
The Earth's moon takes 29.5 days (called a Lunar Month) to orbit the Earth. The moon spins much more slowly on its axis than the Earth, it completes one turn in 29.5 days. Because the times match, we can only ever see one side of the Moon from the Earth. Not everything that orbits a planet is a moon: the general term for things that circle a planet are satellites. A moon is a natural satellite, but there are also artificial satellites like the ones used for telecommunication around Earth.

Comets
Comets also orbit the sun. They can be up to 30km in diameter and are made of dust and ice. They often have very elliptical orbits and so vary greatly in distance from the Sun.

Asteroids
Between Mars and Jupiter there are a large number of asteroids: minor 'planets' or rocks that orbit the sun. This is called the asteroid belt, but they are found throughout the solar system. Some are very small (just a few metres across) and some are large enough to be mistaken for planets. In fact, the first asteroid to be discovered was designated a planet; it was called Ceres.

Dwarf Planets
Dwarf planets is a category for things too small or odd to be planets but too large to be asteroids. Ceres was later re-classified as a dwarf planet, and it still keeps the name. Pluto was originally designated a planet, but because of its very elliptical orbit and small size it is now considered a dwarf planet, along with Sedna, which was discovered more recently.





Models of the Solar System

Originally it was believed that the Earth was at the centre of the solar system, with all the other planets and the sun orbiting around it. This was known as the Ptolemaic System, after the Egyptian astronomer Ptolemy.

It was not until 1530 that Copernicus put forward his idea of a heliocentric model, with the sun at the centre. Kepler added the idea that the orbits of the planets were elliptical not circular, which forms out modern model of the solar system. Of course, this was not well recieved by the religious community and it was a very controversial issue.