The article was published at The Conversation.
The Director of the E.A. Milne Centre for Astrophysics is Brad Gibson.
Sejal, aged seven, Bangalore, India, asked, "What is the coldest planet in the Solar System?"
The planets are heated by the sun. The perfect temperature for life is provided by the 100 million miles away from the sun.
Neptune is the furthest away from the sun's warmth in the solar system. Three billion miles from the sun is Neptune.
Even though Neptune is a billion miles closer to the sun than Uranus, the planet is the most frigid. According to NASA, the solar system's record low temperature is minus 372 degrees Fahrenheit. The temperature on Neptune is usually minus 353 degrees F, but it's better than that on Uranus.
Our solar system has a photo tour of the planets.
A Hubble Space Telescope view shows the planet with its 4 major rings and 17 satellites. The image is from NASA/CXO/University College London.
It's not the distance from the sun that makes Uranus so cold. Something big crashed into Uranus and tipped it over onto its side, billions of years ago. The sun is on its side. The impact of the crash let some heat escape from Uranus.
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The heat is left over from when planets were formed. Smaller chunks of rock smash together to form the full planet piece by piece over millions of years. The planet gets a little hotter when these rocks smash together. The same thing happens with planets if you clap your hands together for a long time.
Neptune has been able to hold on to more of its heat because it wasn't hit by a huge asteroid.
Mercury, the closest planet to the sun, can be very cold. The side facing away from the sun is minus 327 degrees F, which is 200 degrees C.
Mercury does not have an atmosphere like Earth. Our atmosphere acts like a blanket, holding heat in and spreading it around. The front and back sides of Mercury can have different temperatures because they don't have a blanket.
The temperatures are measured in space.
We can send probes to study the atmosphere of nearby planets like Mars. We haven't been able to do this for Neptune and Uranus.
We have to figure out how cold they are by measuring their temperature here on Earth. The light from the planet can tell us the types of atoms and Molecules which make up the planet's atmosphere. The atoms and molecules act as a kind of temperature "fingerprint" for the planet, and this information lets us know exactly what the temperature is.
The planets in our solar system are very cold, but there are even more cold places in the universe. There is a cloud of dust and gas 30 billion miles away from us. The temperature is minus 478 degrees F.
It's impossible for the universe to be colder than minus 459 degrees F, because the tiny particles and atoms that make up the universe stop moving when that temperature is reached. The temperature is known as absolute zero. It is not likely that we will find anywhere in the Universe colder than the Boomerang Nebula.
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