Why Does Jupiter Have the Shortest Day of Any Planet?


My name's Eric Loberg, with the Taylor Planetarium at the Museum of the Rockies. I'm going to discuss why Jupiter has the fastest day of any other planets. It has the fastest rotational period. Earth rotates in about 24 hours, Jupiter takes only about 10, it's much quicker than any of the other planets. And that's because how all the planets formed was a similar pattern. They were all rotating in kind of a flat disk, the Sun was in the middle. Well, what would become the Sun, was in the middle. All this materials, it's mostly gas and dust, is slowly rotating around, kind of a flat disk like a Frisbee. And it's rotating on that Frisbee and starts to clump together. Most of it clumps together, most of it clumps together in the middle at the Sun. The Sun takes most of that material. But the planets, where they're at probably today, started to also grab some of that material and star to spin. And Jupiter's the biggest planet, it grabbed more of that material than any of the other planets. Jupiter has more mass in it than all the other planets combined. So, it became a pretty big disk of its own, started spinning together. And as that disk spins, the material starts to clump together in the middle and turn into kind of a spherical ball. Mostly still gas, but really starts to clump together and get really thick and hard in the middle of it. Why is it spinning fast? It's spinning fast due to angular momentum, a conservation of angular momentum. It's one of Isaac Newton's Laws, as he discovers gravity, figures out some of the rules of gravity. And one of it's rules is angular momentum has to be conserved. And we know that angular momentum is mass times velocity times radius. Radius is half of the diameter and so, the diameter is the width of our sphere, of our disk in this case. The disk is spinning and is very wide. We're going to conserve the mass here because as Jupiter starts to clump together, it clumps into that ball. And most of the mass is conserved. All that mass still is in that ball, it still has some moons and some other little red material. But most of the mass stays the same. But the radius went very, very far down, very quickly. Now, radius went down into the small clump instead of the spinning disk. Since we know angular momentum had to be conserved, the velocity had to go up, the mass remained about the same. And so, now instead of a slowly spinning wide disk, you have a really quick spinning ball out there in space. And this happened with all of our gas giants. The other gas giants also have quicker days than Earth. Saturn has the second quickest day, than Uranus, Neptune a little farther out. They're a little smaller but they're all spinning much quicker than Earth. You can see this in a real world example when you look at figure skaters. Figure skaters like to cross their arms and they'll spin really, really fast. Well, they're doing the same thing as Jupiter is. If they're out wide, they're spinning slower, and the want to conserve their momentum. They come in close, they spin and they spin faster. And that's the same thing that Jupiter is doing, all that matter came in nice and close and it's starting to spin faster. And this is why Jupiter has the shortest days of any of the planets. I'm Eric Loberg with the Taylor Planetarium at the Museum of the Rockies.

J. Eric Loberg is the manager, lecturer, and program planner at the Taylor Planetarium at the World, a renowned museum of the Rockies in Bozeman, Montana.