Curious Kids: What is the coldest planet in the solar system?


This article originally appeared on The conversation. The post contributed the article to Space.com’s Expert voices: Op-Ed & Insights.

Brad Gibson, Director of the EA Milne Center for Astrophysics and Head of the Department of Physics and Mathematics, University of Hull

What is the coldest planet in the solar system? – Seven-year-old Sejal, Bangalore, India

The planets in our solar system are heated by the sun. Here on Earth, we are approximately 100 million kilometers from the sun, a distance that provides the ideal temperature for life.

You might then think that the coldest planet in the solar system would be Neptune, because it is furthest from the heat of the sun. Neptune is an incredible five billion kilometers from the sun.

However, the coldest planet is not Neptune, but Uranus – even though Uranus is a billion kilometers closer to the sun than Neptune. Uranus holds the record for the coldest temperature ever measured in the solar system: a very cold temperature of minus 371 degrees Fahrenheit (minus 224 degrees Celsius), according to NASA. The temperature on Neptune is still very cold, of course – usually around minus 353 degrees F (minus 214 degrees C) – but Uranus beats that.

Related: Our solar system: a photo tour of the planets

A composite image of Uranus shows both X-ray and infrared emissions compared to an optical view of the planet.

A view from the Hubble Space Telescope showing Uranus surrounded by its 4 main rings and 10 of its 17 known satellites. (Image credit: X-rays: NASA / CXO / University College London / W. Dunn et al; Optical (HRC): WM Keck Observatory); Optical (VLT / HRC): ESO / VLT / Kirill Feigelman)

Hit on the side

The reason Uranus is so cold has nothing to do with its distance from the sun. Billions of years ago, something big crashed into Uranus with such force that it tipped the planet aside. Uranus still rolls around the sun on its side today. The impact of the crash also allowed some of the heat trapped inside Uranus to escape.

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The heat inside the planets comes from their formation. Planets are created when smaller pieces of rock shatter, building the entire planet piece by piece over millions of years. Each time these rocks collide, the planet gains a little more heat. If you clap your hands for a long time, they will start to get hot – the same happens with planets.

Neptune was not hit by a huge asteroid like Uranus, so it was able to retain more of its heat.

You might also be surprised to learn that the planet closest to the sun, Mercury, can also be extremely cold. While the side of Mercury facing the sun is over 750 degrees F (400 degrees C), the side opposite the sun is near minus 328 degrees F (minus 200 degrees C).

The reason is that Mercury has no atmosphere, unlike Earth. An atmosphere like ours acts like a blanket, retaining heat and diffusing it all around. Because it does not have this cover, 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 surface of the planet. However, we haven’t been able to do this for distant planets such as Neptune and Uranus.

Instead, we need to determine 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 what types of atoms and molecules make up the planet’s atmosphere. This information allows us to know exactly what the temperature of the planet is: 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 cooler places in the universe. The coldest of all is the Boomerang Nebula, a cloud of dust and gas 30 million billion kilometers from us. There, the temperature reaches minus 457 degrees F (minus 272 degrees C).

Nothing in the universe can be colder than minus 459 degrees F (minus 273 degrees C), because at that temperature the tiny particles and atoms that make everything up stop moving, and once that happens it is impossible to get colder. This temperature is called absolute zero. This means that we are unlikely to find a place in the Universe colder than the Boomerang Nebula.

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