Ice World

StarDate: September 17, 2009

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If you like the cold -- really cold -- then forget mountaintops, Siberian tundra, and even the South Pole. The place for you is the planet Uranus -- the coldest planet in the solar system. Temperatures near the top of its thick atmosphere are around 350 degrees below zero Fahrenheit. That's even colder than Neptune, which is a billion miles farther from the Sun.

Uranus is so cold on the outside because it's cold on the inside.

The three other giant planets -- Neptune, Jupiter, and Saturn -- all radiate a lot more energy into space than they receive from the Sun. Some of that energy is the heat left over from their birth. And as their powerful gravity squeezes them, it heats their interiors to tens of thousands of degrees. Much of that heat makes its way into space, too.

But Uranus radiates only a little more energy into space than it gets from the Sun. In part, that's because it may already have cooled off and stopped contracting.

Even so, most models suggest that Uranus should produce more heat than it does, and scientists are still a little perplexed about why it doesn't. Perhaps some unknown process has stopped its gravitational contraction. We're not likely to know for sure until a spacecraft can orbit Uranus and map its gravitational and magnetic fields in detail -- readings that will allow scientists to probe the interior of this chilly planet.

We'll have more about Uranus tomorrow.

Script by Damond Benningfield, Copyright 2009

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