What is the coldest?

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

Answer: (A)

Temperature on planets depends on both distance from the Sun and how much heat the planet retains from inside. The coldest outer-planet temperatures come from a combination of being far from the Sun and having very little internal heat warming the upper atmosphere. Uranus has almost no excess internal heat reaching its upper atmosphere, so its temperatures plunge to around -224°C, making it the coldest among the options. Neptune is also extremely cold and far away, but it radiates more internal heat than Uranus, which keeps its upper atmosphere slightly warmer. Mercury experiences extreme day–night swings because it has little atmosphere to trap heat, so while its surface gets very hot in the sun, its average conditions aren’t as cold as Uranus. Mars is cold compared with Earth, but its thinner atmosphere and closer proximity to the Sun mean its average temperatures aren’t as low as Uranus’s.