What are the inner planets?

The four planets that are nearest to the Sun are called the inner planets. In order from the Sun, they are Mercury, Venus, Earth and Mars. The inner planets are different from the outer planets, which are farther away from the Sun, because they are made of rock. The outer planets are mostly composed of gas. Each of the inner planets has an atmosphere. However, apart from the Earth's atmosphere, the atmospheres of inner planets are very thin and would be poisonous to humans.