Which is the hottest planet?
Short answer: Venus.
Long answer: In our Solar System, the planet with the highest average surface temperature is Venus. Temperatures on Venus are fairly consistent with a minimum of around 462°C (735 K, 864°F).
Mercury is closer to the Sun than Venus, so you might think it would be hotter. It would be, except that Mercury has no atmosphere so it doesn't retain heat well. Venus, with its very thick atmosphere consisting largely of CO2, has a strong greenhouse effect. It retains heat very well indeed.
Farther out in the Milky Way galaxy, we see many planets orbiting close to their parent stars. These planets will generally be hot but it's hard to measure their exact temperature. We have no way of knowing how hot the hottest planet in the galaxy or Universe are.
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Author: Dave Owen