The planets, and why Venus is the hottest

You met the solar system in 'Beyond Earth' in Curiosity, Grade 6. It has eight planets going round the Sun in nearly circular orbits. In order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

Activity 13.2 asks you to collect, from your school library, trusted websites or your teachers, each planet's average temperature, its size, and whether it has an atmosphere, and fill in Table 13.2. Three rows are given:

All the planets get their energy from the Sun. So you'd expect the closest planet to be hottest, and the planets to get colder as you go outwards. Is that what your table shows?

Mostly, yes, with one big exception. Venus, the second planet from the Sun, has the highest average temperature. It is the hottest planet, hotter even than Mercury, which is closer to the Sun.

Venus is hottest not because of its distance but because of its thick atmosphere, which traps heat. It happens in three steps:

• sunlight warms Venus;
• warmed Venus gives off heat;
• the air on Venus, almost all carbon dioxide, catches that heat and won't let it escape.

This is the greenhouse effect. Mercury has no atmosphere at all, so its heat simply escapes into space.

The same thing happens on Earth, but gently. Gases such as carbon dioxide in our air absorb the heat the Earth gives off after the Sun has warmed it. So the greenhouse effect helps keep the Earth at just the right temperature. (A glasshouse for growing plants works differently: it keeps the warmed air inside simply because it is a closed glass space, so it stays warm even at night. Both keep things warm, but in different ways.)