It’s not who you think it is.
First of all, do you know all the planets in our solar system?
You would not be wrong to think that Mercury would be the hottest. But it isnt. Venus is. But why?
Every planet in our solar system gets its heat from the sun. Without the sun, most planetary temperatures would equilibrate at -270°C. The further the planet is away from the sun, the cooler it is. This is true for most planets except for the second one.
Mercury, the planet closest to the sun, is hot. It’s 700 Kelvin or 427°C. But Venus, the second planet, is twice the distance from the sun as Mercury is but is a whopping 735 Kelvin or 462°C. Why?
The smart people ate NASA compared these two worlds and noticed:
- Mercury is much smaller than Venus,
- Mercury is about twice as close to the Sun as Venus,
- Mercury is much less reflective than Venus, and
- Mercury has no atmosphere, while Venus has a very thick atmosphere.
To summarise their excellent article, Mercury and Venus don’t just absorb light from the Sun; each planet then re-radiates that energy as heat back into space. For airless Mercury, all of that heat goes immediately back into space. But on Venus, the story is different. Each quantum of infrared radiation — the re-radiated heat — has got to get through that thick, thick atmosphere (Venus possess an atmosphere many times the thickness of Earth’s and its shrouded in terrifically thick layers of highly reflective cloud), which is difficult.
And that is why the hottest planet in our solar system is not the planet closest to our majestic sun.
As a side note, the image below is the temperature of planets in our solar system, courtesy of NASA.