13.2 The Planets & the Greenhouse Effect
To see what makes Earth special, it helps to compare it with its neighbours. Our solar system has eight planets orbiting the Sun in near-circular paths.
Rocky and Gas Planets In order of distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. The four nearest — Mercury, Venus, Earth, Mars — are small rocky planets. The four farthest — Jupiter, Saturn, Uranus, Neptune — are huge planets made mostly of gas.
Activity 13.2 — A planet temperature table Collect each planet’s average temperature, size, and whether it has an atmosphere. You’d expect planets to cool down steadily as you move away from the Sun — and mostly they do. But there’s a striking exception: Venus, the second planet, is the hottest of all — hotter even than Mercury, which sits closer to the Sun. Why?
The greenhouse effect
Why Venus Is the Hottest Venus is hottest not because of its distance, but because of its thick atmosphere of carbon dioxide, which traps heat and won’t let it escape. This is the greenhouse effect. The same thing happens, more gently, on Earth: gases like carbon dioxide absorb the heat (radiation) the warmed Earth gives off, keeping some of it in. A mild greenhouse effect is actually vital — it keeps Earth at just the right temperature for liquid water and life.
A Step Further: Two Different “Greenhouses” The planetary greenhouse effect is not the same as a glass greenhouse for plants. A plant greenhouse traps warmed air simply because it’s a closed glass box — the hot air can’t get out. The planetary effect instead works by gases absorbing the heat radiation given off by the warmed surface. Both keep things warm, but in different ways — a common exam confusion worth getting straight.
Science Talk A mild greenhouse effect keeps Earth comfortably warm. But what would happen if we added a lot more carbon dioxide to the atmosphere — would the effect stay “just right,” or push Earth toward a hotter, more Venus-like state? Hold that thought; it returns as global warming in topic 13.6.