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 a substantial amount of energy to the cores of massive planets, the light and heat emitted from our parent star is overwhelmingly responsible for a planet’s surface temperature. To an excellent approximation, the Sun keeps not only Earth, but all the planets at a temperature well above what they’d be without it: just a few Kelvin. (Without an external heat source, most planetary temperatures would equilibrate at -270 °C / -455 °F.)
During the day, the planets absorb energy from the Sun, but during both the day and the night, they radiate energy back into space. This is why temperatures heat up during the day and cool off during the night, something that’s pretty much true for every planet that has both a day side and a night side. We also expect seasons — cool times and warm times — based on both how elliptical a planet’s orbit is and on its axial tilt.
