Four small worlds

About this video

The four planets closest to the Sun are Mercury, Venus, Earth and Mars. They are small worlds made of rock and metal, with solid ground you could stand on, which is why they are called the rocky planets.

Mercury is the smallest, only a little bigger than our Moon, and covered in craters. It has almost no air, and air works like a blanket: without one, the side facing the Sun bakes at about 430 °C while the night side freezes at about −180 °C.

Venus is almost as big as the Earth, but its air is so thick that standing on it would feel like being almost a kilometre deep in the sea. That air traps the Sun's heat like a closed car in summer, so Venus is about 465 °C, hotter than Mercury even though it is about twice as far from the Sun.

The Earth, third from the Sun, has liquid water, air we can breathe and life, the only life we know of anywhere.

Mars is about half as wide as the Earth. It is red because its dust is rusted, like an old iron nail. Its air is thin and it is cold. It has Olympus Mons, a volcano about two and a half times as tall as Mount Everest, and robot rovers drive across it today.

Mercury and Venus have no moons, the Earth has one, and Mars has two tiny ones shaped like potatoes. Shrink them onto a table: if the Earth were an orange 8 cm across, Venus would be a slightly smaller orange, Mars a ping-pong ball and Mercury a large grape.

Transcript

1. 0:00Closest to the Sun are four small worlds. Mercury, Venus, Earth and Mars. They are made of rock and metal, with ground you could stand on.
2. 0:20Mercury is the smallest, a little bigger than our Moon, and covered in craters.
3. 0:30It has almost no air, and air works like a blanket. So the day side bakes at about 430 degrees, and the night side freezes at about minus 180.
4. 0:44Venus is almost as big as the Earth. Its air is so thick, standing there would feel like being almost a kilometre deep in the sea.
5. 0:58That thick air traps the Sun's heat, like a closed car in summer. So Venus is about 465 degrees, hotter than Mercury, though it is twice as far from the Sun.
6. 1:12The Earth is the third. It has liquid water, air we can breathe, and life. The only life we know of, anywhere.
7. 1:30Mars is about half as wide as the Earth. It is red because its dust is rusted, like an old iron nail. Its air is thin, and it is cold.
8. 1:44Mars has Olympus Mons, a volcano about two and a half times as tall as Mount Everest. Robot rovers drive across its ground today.
9. 2:00Mercury and Venus have no moons. The Earth has one. Mars has two tiny ones, shaped like potatoes.
10. 2:18Now shrink them onto a table. If the Earth were an orange, Venus would be a slightly smaller orange, Mars a ping-pong ball, and Mercury a large grape.
11. 2:40Four small, rocky worlds close to the Sun. Only one of them is home.