Jupiter and Saturn are the largest planets in the solar system. Jupiter is about 11 times the diameter of Earth, while Saturn is almost 10 times Earth’s diameter. Despite their size, though, they spin faster than any other planets — 10 hours for Jupiter, a bit longer for Saturn. That’s compared to 24 hours for Earth.

You might wonder why these giants turn so fast. But scientists wonder why they aren’t faster. They should have spun a lot faster when they were born, but they slowed down.

A study a couple of years ago proposed a solution.

Jupiter and Saturn grew so big by gobbling up huge amounts of gas from a disk around the newborn Sun. As the gas fell onto the planets, it caused them to spin faster. Eventually, they should have been spun once every few hours — so fast that they were about to break apart.

But some of the gas was directed back outward, forming a disk around each planet. The planets’ magnetic fields grabbed onto the gas. That acted like a brake, slowing each planet to a much slower rate than we see today. Their own gravity quickly caused the planets to shrink, which made them spin faster — the same way a skater spins faster by pulling in its arms. That set up the rates we see today — fast or slow.

Jupiter and Saturn form a wide triangle with the Moon tonight. As they climb into view in late evening, bright Jupiter is to the left of the Moon, with fainter Saturn to the upper right.
 

Script by Damond Benningfield