Surprise: Mercury isn’t the Solar System’s hottest planet Despite being the closest planet to the Sun, Mercury “only” reaches 800 °F at its hottest. Venus is always hotter, even at night. In the grand scheme of the Solar System, the greatest source of energy by far is the Sun. While radioactivity and gravitational contraction might supply…

https://medium.com/starts-with-a-bang/surprise-mercury-isnt-the-solar-system-s-hottest-planet-584909520e90