11 Why Are Planets Round?

Look at anything in space and what do you see? All the planets are shaped like balls. So are the sun and all the other stars in the universe. Planets circle the sun in round orbits, and the sun follows a circular path around the Milky Way Galaxy. Everything seems to be round. And there’s one force out there making it work that way: gravity.

Everyone on Earth feels the pull of gravity. It doesn’t matter where you are—you feel a pull toward the ground, and you think you’re at the “top” of the planet. In fact, there is no single up or down; each one of us has our own personal sense of where down is, and all of us are correct! Down is simply toward the center of the ball.

Look at a globe of the Earth and find your home country—say, Canada. If you turn the Earth over, you’ll find Australia on the other side. That means at this very moment there are people under your feet, on the other side of the world, who are calling “down” what you call “up.”

Everything on the planet is pulled toward the middle of the Earth. The only shape that allows every part to be as close to the center as possible is a ball. Take a piece of Silly Putty, or if it is winter, make a snowball, and try to make it as small and compact as possible. Squeeze everything toward the center as best you can. What do you end up with? A ball.

Gravity works the same way, always pulling toward the center of an object or a group of objects, gathering everything into the smallest possible shape. That’s why large objects in space, no matter what they’re made of, are round.

The bigger an object is, the more gravity it possesses. By the way, we don’t know where gravity comes from. We just know that the more mass you have, the more gravity you get. As gravity continues to pull objects in toward the center, we get a larger and larger object… and the larger it grows, the more gravity it has, which pulls even more stuff to it.

Our whole solar system works this way—small objects gravitationally circling big ones. The sun is the biggest of all, thousands of times bigger than all the planets put together, so it has the most gravity. All the planets—which are little by comparison—are caught in the endless circle of the sun’s gravity. At the same time, moons are smaller than planets, so they loop around the planets as the planets circle around the sun. Everything is swinging around something in a giant circular swing dance in space.

All of those forces have an effect on the way the stars and planets move around the galaxy, which is also spinning around its center. But, although all planets are shaped like balls, they’re not always perfectly round. Sometimes they can have a little bulge in the middle.

The biggest planets in our solar system have the biggest bulges. Jupiter, Saturn, Uranus, and Neptune are called gas giant planets because they’re giant compared to the Earth and they’re made mostly of gas and liquids.

Imagine a liquid ball floating in space. Gravity is pulling it together into a sphere, but big planets rotate very quickly, twice as fast as the Earth. And when a ball of gas or liquid spins around, the equator is moving faster than the poles, the same way that the tire of a bicycle moves faster than the hub. That causes the planet to bulge out at the center because its material is being flung outward by the spin.

The Earth also bulges at the equator—when you stand on the equator, you are forty-two kilometers farther away from the center of the Earth than you would be if you stood at the North Pole. But that’s nothing compared to Jupiter, which is made of gas. Measured across its middle, Jupiter is 143,884 kilometers. But when you measure the planet from north to south, it’s 133,709 kilometers. That means that Jupiter’s equator sticks out an extra 10,175 kilometers farther away from its center. If it didn’t spin at all, Jupiter would be perfectly round.

Really small masses—say, less than one hundred kilometers across—come in crazy shapes. Space masses that small don’t possess enough gravity to pull everything toward the center and form a ball. Phobos, a moon of Mars that is only twenty-two kilometers across, looks like a potato. And an asteroid named Eros, which is about the same size, is shaped like a peanut. There’s even a comet that looks like a dog bone! These odd-shaped little moons and asteroids could be pieces that were chipped off larger bodies, or stuff that just never formed into a big moon or planet.

Recently, a robot spacecraft called Rosetta caught up with a comet that looked like a rubber duck. Philae, a small lander that was dropped off by Rosetta, was sent to the surface of the duck comet to try to take a sample. Philae had a harpoon in its belly and claws on its feet that were supposed to dig into the comet so the lander would not bounce off in the low gravity.

But there was a bit of a mishap. The harpoon didn’t fire, and the claws didn’t hang on, so the lander hit the ground and bounced back up. With almost no gravity at all on the comet, Philae drifted back up a kilometer high above the comet, then slowly floated back down and ricocheted up a second time. Each bounce took about a minute to happen. Finally, Philae ended up tilted at a crazy angle against a cliff.

If astronauts ever visit a comet or asteroid, hopefully they won’t bounce like Philae, but they will be able to take giant, slow-motion leaps and fly just like superheroes. Wouldn’t that be fun?

For now, we know how gravity works—it pulls you down, tries to make you round, and keeps you stuck here on the ground. But we still don’t know what it is beyond a mysterious, invisible force. If we understood it better, maybe we’d be able to turn it off, or turn the volume up and down like on a TV. Imagine that! Maybe one of you will become the scientist that figures out gravity and makes it work by remote control!