The average distance between the Earth and the Sun is about 149.6 million kilometers, which provides the perfect temperature for life. However, distance from the Sun is not the only thing that determines the temperature of the planet. Neptune, for example, is the farthest planet from the Sun (4.5 billion kilometers away). At the same time, Uranus, which is 1.7 billion kilometers closer to the Sun, is the coldest planet in the Solar System.
The minimum temperatures of Uranus and Neptune are -224°C and -214°C, respectively.
Why is that?
Planets form from collisions between rocks over millions of year. A newly forming planet heats up with each collision. If you clap your hands together for a long time they will start to feel hot, and the same thing happens with planets. Thus, the heat accumulates inside the planets.
Billions of years ago, something huge crashed into Uranus with so much force that it tipped the planet over onto its side. As a result, some of the internal heat evaporated. Neptune did not experience serious collisions, so it has been able to retain most of its heat.
Interestingly, the closest planet to the Sun, Mercury, is also extremely cold. The side of Mercury facing the Sun has a temperature of +400°C, while the other side is nearly -200°C. The reason for this temperature difference is that, unlike Earth, Mercury does not have an atmosphere that acts as a blanket, retaining heat and spreading it around.
How is the temperature of the planets measured?
We can send probes to study the atmosphere of nearby planets like Mars. However, this is impossible for distant worlds like Neptune and Uranus. Scientists determine their temperatures by analyzing the light from the planet, which contains data on the types of atoms and molecules in the planet’s atmosphere (‘temperature fingerprint’).
Although Neptune and Uranus are incredibly cold, there are even colder places in the universe. The protoplanetary Boomerang Nebula, a cloud of dust and gas located 5,000 light-years away from Earth, has a temperature of -272°C.
Nothing in the universe can be colder than -273°C. At this temperature, called absolute zero, the tiny particles and atoms that everything is made of basically stop moving, which excludes further cooling. This means that we are unlikely to ever find a colder region in the universe.