Today’s Wonder of the Day was inspired by Jordan from chesterton, IN. Jordan Wonders, “Why do planets have to be round to be a planet?” Thanks for WONDERing with us, Jordan!

You're walking in the park one day, minding your own business, when you discover an old fountain off the beaten path. You've never noticed it before. It yields a trickle of water and the bottom is filled with shiny coins.

You reach into your pocket and find a quarter. With a flick of your thumb, you launch the quarter into the fountain. Then you close your eyes and make a wish. Will it come true? You doubt it. It was one of those wishes that would probably only come true in a dream.

As you drift off to sleep later that evening, you think about that old fountain and your wish again. Something feels different, but you can't quite put your finger on it before you close your eyes and begin to dream.

When you jerk awake sometime later, you realize you're not in your bed. Instead, you're strapped into the cockpit of some kind of spacecraft.

Could your wish really be coming true? You reach for the controls in front of you and your spaceship lifts off. You hit the accelerator and suddenly you're flying through the solar system. Next stop: Mars!

If such a dream of space travel could really come true, it would be a lot of fun to explore the solar system to see all the planets up close. While all the planets vary in size, color, and composition, there's one similarity you notice: they're all spherical!

Why is that? In the WONDERful solar system we call home, why don't we see planets shaped like pyramids, cubes, or even flat discs?

The spherical shape of all the planets can be explained by one force that we're all familiar with: gravity. When the planets in our solar system began to form, space was filled with billions and billions of pieces of dust and gas.

As these pieces bumped into each other, they began to form clumps that slowly grew larger and larger. Eventually these clumps of material grew large enough to develop their own gravitational fields.

As the forming planets continued to grow, the force of the collisions with additional matter caused them to become hot and molten, acting much like a liquid. Each planet's gravitational force is centered at its core. It pulls equally in every direction from the core, thus pulling all the molten material into a spherical shape. Scientists call this process "isostatic adjustment."

Think of a bicycle wheel. The center of the wheel is like the core of a planet, where the planet's center of gravity is. The gravity pulls equally in every direction, like the spokes of the wheel. The natural shape formed as a result is a circle or, in the case of a three-dimensional object, a sphere.

However, planets aren't actually perfectly round. Why? Because they spin! When planets spin, the forces created by spinning work against gravity, causing planets to bulge out around their equators. Scientists call this extra width the "equatorial bulge."

Wonder What's Next?

You might find tomorrow’s Wonder of the Day hanging out underneath a fairy tale bridge!