Uranus II 
Like the other outer planets in our solar system, Uranus is a giant. Compared to the other three giants, though, it's an oddball. For one thing, it orbits the Sun sideways, so both the north and south poles see more than 40 years of sunlight followed by 40 years of darkness. And for another, Uranus is the only giant that doesn't produce more heat than it receives from the Sun.
The planet's orientation is probably the result of a giant impact -- another planet-sized body probably smacked into Uranus, knocking it sideways.
The lack of an internal heat source is harder to explain, though. The other giants -- Jupiter, Saturn, and Neptune -- all radiate more energy into space than they receive from the Sun. Some of that heat is left over from their formation, while some is created as their gravity squeezes them tightly. But scientists still aren't sure why this isn't also happening with Uranus.
They hope to find some clues as the planet changes seasons, and the Sun warms a different hemisphere. In 2007, Uranus experienced an equinox, so the Sun stood above its equator. That provided sunlight to the entire planet for the first time in more than 40 years. The way the atmosphere responds to the extra solar energy should tell scientists whether Uranus is producing any heat on its own.
The observations may also help them understand why the top of the planet's atmosphere is much hotter than expected. More about that tomorrow.
Script by Damond Benningfield, Copyright 2010