The heat inside planets is left over from when they were formed. Planets are made when smaller chunks of rock smash together, building the full planet piece by piece over many millions of years. Every time these rocks smash together, the planet gains a little more heat. If you clap your hands together for a long time they will start to feel hot – the same thing happens with planets.
Neptune wasn’t hit by a huge asteroid like Uranus was, so it has been able to hold on to more of its heat.
You might also be surprised to learn that the closest planet to the Sun, Mercury, can also be extremely cold. While the side of Mercury facing the Sun is more than 400℃, the side facing away from the Sun is nearly -200℃.
The reason for this is that Mercury does not have any atmosphere, unlike Earth. An atmosphere like ours acts like a blanket, holding heat in and spreading it all around. Because it does not have this blanket, the front side and the back side of Mercury can have very different temperatures.
Measuring temperatures in space
For some nearby planets like Mars, we can send probes to study the atmosphere directly from the planet’s surface. However, we haven’t been able to do this for distant planets such as Neptune and Uranus.
Instead, we have to work out how cold they are by measuring their temperature from here on Earth. We do this by studying the light from the planet, which can tell us the types of atoms and molecules which make up the planet’s atmosphere. This information lets us know exactly what the temperature of the planet is: the atoms and molecules act as a kind of temperature “fingerprint” for the planet.
While these planets in our Solar System are incredibly cold, there are even chillier places in the universe. The coldest of all is the Boomerang Nebula, a cloud of dust and gas 30 million billion miles away from us. There, the temperature reaches -272℃.
Nothing in the Universe can be colder than -273℃, because at that temperature the tiny particles and atoms that everything is made of basically stop moving, and once that happens it’s impossible to go colder. This temperature is known as absolute zero. This means it is unlikely that we will ever find anywhere in the Universe colder than the Boomerang Nebula.
Article written by Professor Brad Gibson
Curious Kids is a series by The Conversation that gives children the chance to have their questions about the world answered by experts. If you have a question you’d like an expert to answer, send it to firstname.lastname@example.org. We won’t be able to answer every question, but we’ll do our very best.
This article is republished from The Conversation under a Creative Commons license. Read the original article.