The planets in the Solar System are split into two groups. There are four small inner planets and four outer planets which are much larger and made up mainly of gas. In between
these two groups are the minor planets. The outermost planet is Pluto. It is so far away that
astronomers know little about it.
The closest planet to the Sun is Mercury. It is difficult to see from Earth as it always appears very close to the Sun. It can be seen either low in the east before sunrise or low in the west after sun set. It wasn't until an unmanned spacecraft flew past the planet and sent back photographs that we saw what Mercury was really like. It has a large number of craters. There are some mountains and one or two flat areas like those on the Moon. Mercury is very hot and has no atmosphere or water. There is no life on Mercury.