With an average surface temperature of about 737K, Venus is the hottest planet. You might think that Mercury would be hotter since it is closer to the Sun; however, being so close to the Sun has actually decimated Mercury's atmosphere meaning it has to ability to retain heat. The side of Mercury facing the Sun is very, very hot, but the side facing away is much colder and thus the average surface temperature is lower. Venus, on the other hand, has an atmosphere and thus retains heat. The high level of volcanic activity has additionally increased the Greenhouse Effect on Venus, which traps heat on the planet's surface.