Which planet is the coldest?

• A. Uranus
• B. Neptune
• C. Mercury
• D. Mars

Answer: (A)

The key idea here is that a planet’s surface and atmosphere get their temperature mainly from two sources: how far it is from the Sun and how much internal heat it retains. Among the outer planets, the cloud-top temperatures are extremely cold, and Uranus has the coldest of these measured temperatures, around minus two hundred twenty-something degrees Celsius. Neptune is also very cold, but it emits more internal heat than Uranus, which warms its upper atmosphere slightly, so its cloud tops aren’t as cold. Mercury is much closer to the Sun and experiences extreme heating, while Mars, though cold, isn’t as frigid as the gas giants’ cloud tops. So the coldest planet, by the coldest cloud-top temperatures, is Uranus.