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Mercury is the planet that orbits the closest to the Sun. It receives more sunlight per square meter than any other planet in the Solar System. So, is Mercury the hottest planet? You might be surprised to know that Mercury actually isn’t the hottest planet in the Solar System. The hottest planet is Venus.
If Mercury orbits closer to the Sun, and receives more energy from the Sun, why is Venus hotter? It all comes down to atmosphere. Venus has an atmosphere, and Mercury doesn’t.
The side of Mercury facing the Sun can get as hot as 700 Kelvin (426 degrees Celsius). But without an atmosphere to trap the heat, the side facing away from the Sun gets down to only 100 Kelvin (-173 degrees Celsius). Venus, on the other hand, has a very dense carbon dioxide atmosphere – 92 times denser than Earth. The atmosphere acts like a blanket, trapping the heat, and keeping the planet very hot.
The temperature of Venus is 735 Kelvin (461 degrees C) everywhere on the whole planet. And that makes Venus the hottest planet in the Solar System.
We have written many stories about Mercury here on Universe Today. Here’s an article about a the discovery that Mercury’s core is liquid. And how Mercury is actually less like the Moon than previously believed.
We have also recorded a whole episode of Astronomy Cast that’s just about planet Mercury. Listen to it here, Episode 49: Mercury.