The worlds that surround our planet are all made of rock, but there the similarity ends. Some have a beating geological heart, others are frozen in time.

Brian Cox travels to the tallest mountain on Earth, the volcano Mauna Kea on Hawaii, to show how something as basic as a planet’s size can make the difference between life and death. Even on the summit of this volcano, he would stand in the shade of the tallest mountain in the solar system, an extinct volcano on Mars called Olympus Mons, which rises up 27 km.