UCSB Science Line
Sponge Spicules Nerve Cells Galaxy Abalone Shell Nickel Succinate X-ray Lens Lupine
UCSB Science Line
How it Works
Ask a Question
Search Topics
Our Scientists
Science Links
Contact Information
Why are all the planets in the shape of a sphere? Why are orbits also round?
Question Date: 2019-04-26
Answer 1:

It is the force of gravity which makes planets into spheres. Everything gets pulled closer to the center of the planet, so the planet becomes nearly a perfect sphere, if it is big enough.

Pluto used to be a planet, but now scientists say it isn't a planet, because it's not round enough. Pluto isn't big enough to have enough gravity to pull it into a sphere. Asteroids are even smaller than Pluto, and they have shapes that aren't even as nearly spherical as Pluto.

Earth's moon looks like a sphere. Do you think the moon is larger or smaller than Pluto?

Gravity is not always easy to understand. You might want to read about gravity here.

Now, for your question of why are orbits also round: Actually orbits are not all round, but also elliptical, spiral, and hyperbolic. However for planets, the orbits are almost circular. In order for planets to have "orbits", we have to consider again the existence of gravity and the fact that two objects with mass will be attracted to each other, and this will affect their movement through space. We also have to consider that an object in motion will stay in motion unless something pushes or pulls on it.

Then, in order to analyze the reasons why an orbit is circular, or elliptical, let's consider the following cases:

1) If a planet moves too fast toward the sun, it will have too much speed to be pulled back around by gravity. Instead of orbiting, it will be flung out of the solar system.

2) If the planet moves too slowly toward the sun, it willn’t have enough speed to stop it from falling toward the Sun. The orbit will continue to get closer and closer to the sun until it fell in.

3) If the planet has just the right amount of speed and comes in at the right angle, it will move in a circular orbit, not moving closer or farther from the Sun.

4) If a planet has a speed and approach that is somewhere in between, it’s orbit will be elliptical (a circle that isn’t perfectly round). When it’s closer to the sun, the planet like Earth is going faster but the force pulling it closer is stronger. When it’s farther from the sun, it’s slower but the force is also slower. Over time, these forces will make the planet's elliptical orbit more and more round until it is a perfect circle, but there may be other forces (from the other planets) that will pull it out of this alignment.

Click Here to return to the search form.

University of California, Santa Barbara Materials Research Laboratory National Science Foundation
This program is co-sponsored by the National Science Foundation and UCSB School-University Partnerships
Copyright © 2020 The Regents of the University of California,
All Rights Reserved.
UCSB Terms of Use