The planet with the highest average surface temperature is Venus, because of its thick atmosphere and its proximity to the sun. The average temperature on the surface of Venus is 860 degrees Fahrenheit.
Venus is the second closest planet to the sun; the closest planet to the sun is Mercury. The atmosphere of Venus is made up of carbon dioxide and sulphur dioxide, which creates a greenhouse effect that traps in heat.
Mercury's lack of an atmosphere and its slow spin cause the surface temperature of the planet to vary widely, which does not average out to the highest surface temperature. Because of this, Mercury has the second highest average surface temperature.