Why Are Planets Round

Have you ever wondered why all the planets are mostly round? The spherical shape of all the planets is caused by 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. 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 of the wheel formed as a result is a circle. In the case of a three-dimensional object, it becomes a sphere.

Planets are not actually perfectly round, because they also 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." None of this has anything to do with getting older and developing our own human 'equatorial bulge' around the middle.

Gravity in Space

Contrary to common opinion, gravity is everywhere, even in space. Anything with mass creates gravity. The effect of gravity decreases as distance increases. At extreme distances, the gravity exerted on a particular object might be almost zero, but is never completely absent.

At the orbit of the International Space Station, Earth's gravitational pull is about 90% of what it is at Earth's surface.

Astronauts on spaceships in outer space are affected by gravity in the same way that their spaceships are. They are both orbiting Earth, which means they are falling sideways (in orbit) at the same time they are falling toward Earth.


On Earth, astronauts feel the force of gravity as weight, because the ground prevents them from falling. In outer space there is no ground to push against astronauts. As they orbit and fall toward Earth at the same rate as their spaceship, astronauts feel weightless, as if there were no gravity.

Gravity Myth

Many believe there is no gravity in space. Astronauts appear weightless in space, so there is an assumption that space is a place with zero gravity. That assumption carries no weight.

There is less gravity in space, but the idea that there is none is factually incorrect. There is gravity on Earth, on the moon, and on the sun. It all around us. The reason why space allows humans to be weightless is because they are gravitating toward the Earth at the same rate as their ships.

Space Myth, Weightless

People assume that being on a space station or spaceship means that you are totally weightless. This is a common misconception, because even space has something called microgravity. This minute version of gravity is the pull you feel between two objects while you are in space. For example, even though you are not on the Earth’s surface, there is still a gravitational pull coming from the Earth that is extremely strong. There would also be gravitational forces from the Sun and the Moon, among others acting on you.

What this means is that even on a space station, you actually do not weigh that much less than you would on Earth. The reason people float on a space station is because of the way the station orbits the Earth. Technically, the people onboard are actually in a form of constant free fall, and the way the station curves around the Earth during its orbit keeps them floating. This effect can be replicated with certain airplanes in our own atmosphere. These planes what they use to train astronauts.