Is Mercury the Hottest Planet?

Is Mercury the hottest planet in the Solar System?  At first glance it makes sense that it should be.  It's the closest planet to the Sun, and we all know that the closer you are to an energy source, say a campfire for example, the hotter you feel.  So the same should be true for planets, right?  Unfortunately common sense is wrong in this case.  There's another factor that determines a planet's temperature and that's the planet's atmosphere or lack thereof.

NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington / Public domain

Mercury has no atmosphere (well, maybe a very very thin one).  As a result, there's nothing to hold in the heat that escapes from the surface.  Mercury's surface can reach very high temperatures.  Due to Mercury's rotation and revolution about the Sun, a day on Mercury is equivalent to 176 Earth Days.  Therefore the Sun is up for several Earth months heating the surface.  As a result, Mercury's high temp can reach 750 degrees F (670 degrees K).  But this also means that Mercury's surface spends many Earth months with the Sun below the surface.  As a result, Mercury's low temp can reach -330 degrees F (70 degrees K).  That's a huge range of temperatures.

So if Mercury isn't hottest planet, what is?  The winner of the Solar System's hottest planet contest is Venus.  Although Venus is farther away from the Sun than Mercury (second planet from the Sun), it reaches higher temperatures due to a very thick atmosphere of greenhouse gasses that retain and prevent heat from escaping.  Venus has a very stable surface temperature across the entire planet, day or night, of 860 degrees F (733 degrees K).

Atmospheres play a very important role in the surface temperature of a planet.