Which planet is Hottest?
It's thought that any atmosphere that Mercury might have had may have been stripped away by the solar wind. That caused it to lose its ability to retain heat. (Interesting factoid: Mercury is tidally locked with the sun in a 3:2 resonance, which means that it rotates on its axis exactly three times for every two revolutions around the sun it makes. This allows the night side of Mercury to get surprisingly cold for being so close to the sun, with nightly "low temperatures" at 100 K (−173 °C; −280 °F).)