Which is the coldest planet in the solar system? Where is the coldest place in the universe?

Curious children ask: What is the coldest planet in the solar system?

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What is the coldest planet in the solar system? — Asked by Sejal, 7 years old, from Bangalore, India

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

You might think, then, that the coldest planet in the solar system would be Neptune, since it is farthest from the heat of the Sun. Neptune is an incredible 3 billion miles from the Sun.

However, the coldest planet is not Neptune, but Uranus - even though Uranus is a billion miles closer to the Sun than Neptune. Uranus holds the record for the coldest temperature ever recorded in the Solar System: a very chilly -224 degrees Celsius. Of course, Neptune's temperatures are still very cold, usually around -214 degrees Celsius, but Uranus's coldness exceeds Neptune's.

Tilt caused by collision

The reason Uranus is so cold has nothing to do with its distance from the sun. Billions of years ago, a massive object hit Uranus with considerable force, causing it to tilt on its side. Uranus still orbits the sun today. In addition to tilting Uranus, the impact also allowed some of the heat in the planet's interior to escape.

The Hubble Space Telescope has shown that Uranus is surrounded by its four major rings and 10 of its 17 known satellites.

National Aeronautics and Space Administration, CC BY-NC-ND

The heat inside planets is a leftover from their formation. Planets are formed from small pieces of rock that collided together, piece by piece over millions of years. Every time these rocks collided, the planet gained a little more heat. Just like when you clap your hands for too long, they start to feel hot - the same thing happens with planets.

Neptune has not been hit by giant asteroids like Uranus, so it is able to retain more heat.

You may also be surprised to learn that Mercury, the planet closest to the sun, can be extremely cold. The temperature on the side of Mercury facing the sun is over 400°C, while the side facing away from the sun is close to -200°C.

The reason is that Mercury has no atmosphere, while Earth does. Earth's atmosphere is like a blanket, keeping the heat inside and spreading it around. Because Mercury does not have this "blanket", there is a big difference in temperature between its front and back sides.

Measuring temperature in space

For planets close to Earth, like Mars, we can send probes directly from the planet's surface to study the atmosphere. However, for distant planets, such as Neptune and Uranus, we have not yet been able to do this.

Instead, we calculate how cold they are by measuring their temperatures here on Earth. We do this by studying the light coming from the planet, which can tell us about the types of atoms and molecules that make up the planet's atmosphere. This information lets us know exactly what the planet's temperature is: the atoms and molecules are a kind of temperature "fingerprint" for the planet.

Although these planets in our solar system are very cold, there are even colder places in the universe. The coldest is the Boomerang Nebula, which is composed of a cloud of dust and gas 300 billion miles away from us. The temperature there reaches -272 degrees Celsius.

Nothing in the universe can be colder than -273°C because at this temperature, the tiny particles and atoms that make up all matter essentially stop moving, and once this happens, it's impossible to cool it down any further. This temperature is called "absolute zero." This means that it's unlikely that we'll find a place in the universe that's colder than the Boomerang Nebula.

Related knowledge

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BY: Brad Gibson

FY: Tessa

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