Despite being the closest planet to the Sun, often orbiting less than 60 million kilometres away from the star, temperatures on Mercury can drop below —180°C (-290°F). This is because Mercury is too hot and too small to be able to hold on to much gas. With no clouds to stop heat from escaping into space at night, temperatures on Mercury plummet.
Mercury is the planet in our solar system that sits closest to the sun. The distance between Mercury and the sun ranges from 46 million kilometers to 69.8 million kilometers. The earth sits at a comfy 150 million kilometers. This is one reason why it gets so hot on Mercury during the day.
The other reason is that Mercury has a very thin and unstable atmosphere. At a size about a third of the earth and with a mass (what we on earth see as ‘weight’) that is 0.05 times as much as the earth, Mercury just doesn’t have the gravity to keep gases trapped around it, creating an atmosphere. Due to the high temperature, solar winds, and the low gravity (about a third of earth’s gravity), gases keep escaping the planet, quite literally just blowing away.
Atmospheres can trap heat, that’s why it can still be nice and warm at night here on earth. Mercury’s atmosphere is too thin, unstable and close to the sun to make any notable difference in the temperature.
Space is cold. Space is very cold. So cold in fact, that it can almost reach absolute zero, the point where molecules stop moving (and they always move). In space, the coldest temperature you can get is 2.7 Kelvin, about -270 degrees Celsius.
Sunlight reflected from other planets and moons, gases that move through space, the very thin atmosphere and the surface of Mercury itself are the main reasons that temperatures on Mercury don’t get lower than about -180 °C at night.