Mercury hugs the Sun, closer than any other planet, so obviously it is the hottest. Closest to the fire, hottest in the room. It is the kind of clean logic that gets a tick in a primary-school test and never gets questioned again.
Venus is the hottest planet, with a surface around 465 degrees Celsius, hot enough to melt lead, and it is the second planet out, not the first. Mercury peaks around 430 degrees on its day side but plunges to about minus 180 at night, because it has almost no atmosphere to hold heat. Venus is the opposite. It is wrapped in a thick blanket of carbon dioxide that traps heat in a runaway greenhouse effect, keeping the whole planet roasting, day and night, all over. So proximity to the Sun is only half the story. An atmosphere that traps heat beats raw distance every time, which is also a fairly pointed lesson about greenhouse gases sitting one planet over. Mercury is closer. Venus is the furnace.
Reception
- NASA Science: VenusPRIMARY
- NASA Science: MercuryPRIMARY