In comparison to the outer planets, the four planets nearest the Sun are remarkably similar.
The inner planets are all basically made of dense rock; they are all relatively small; and they are all situated fairly close together.
Astronomers believe that the cores of the inner planets are made out of metals such as iron and nickel.
They have very few or no moons, and have no ring systems like Saturn, for example.
The surfaces temperatures of the inner planets are relatively hot, ranging from an average of 427°C (806°F) to –23°C (-9°F).
Their orbits are relatively quick, they take between 88 days and 687 days to revolve around the Sun.