The four inner planets of the Solar System — Mercury, Venus, Earth, and Mars — are known as the terrestrial planets because they have solid, rocky surfaces. They are much smaller and denser than the gas giants and orbit closer to the Sun.

Mercury is the smallest planet and the closest to the Sun. It has almost no atmosphere, so it cannot retain heat, and its surface swings between extreme heat during the day and bitter cold at night. Heavily cratered, Mercury looks much like Earth's Moon and completes an orbit in just 88 days.

Venus is the second planet from the Sun and is often called Earth's twin because of their similar size. In reality it is the hottest planet in the Solar System, with surface temperatures around 465 degrees Celsius. A thick atmosphere of carbon dioxide traps heat in a runaway greenhouse effect, and the surface is hidden beneath dense clouds of sulphuric acid.

Mars, the red planet, owes its colour to iron oxide, or rust, in its soil. It is home to Olympus Mons, the tallest volcano in the Solar System, standing about three times the height of Mount Everest. Mars has a thin atmosphere, polar ice caps, and two small moons, Phobos and Deimos. Evidence of ancient riverbeds suggests it once had liquid water, which keeps it a leading target in the search for past life.
