What is the Coldest Planet of Our Solar System?

by Fraser Cain on May 28, 2010

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What is the Coldest Planet of Our Solar System

Clouds of Neptune


As the planets in our Solar System get further away from the Sun, they generally get colder. So what is the coldest planet of our Solar System. That would be Neptune, the 8th and final planet in the Solar System. If Pluto was still a planet, it would be colder, but now it’s Neptune.

Neptune is located 4.5 billion km from the Sun, on average, and it receives a fraction of the sunlight that the other planets receive. In fact, even though it’s only a little further from the Sun than Uranus, it sees only 40% of the solar radiation received by Uranus.

The surface temperature of Neptune dips down to 72 kelvin, or -201° C. That’s cold. You might be surprised to know that the temperature on the surface of Uranus isn’t much warmer. It only gets as cold as 76 kelvin, or -197° C. You would expect Neptune to be much colder than Uranus, but astronomers think that it has a higher internal temperature, and so more of its internal heat reaches the surface and prevents it from being truly frigid.

Even though Pluto isn’t a planet any more, its average surface temperature is only 44 kelvin, or -229° C. Pluto doesn’t have the same internal temperature as the ice giant planets, so it’s able to cool down much further.

We’ve written many articles about Neptune for Universe Today. Here’s an article about the temperature of Neptune, and here’s an article about how far Neptune is from Earth.

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We’ve also recorded an entire episode of Astronomy Cast all about Neptune. Listen here, Episode 63: Neptune.

About 

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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