Why Are Planets Round?

Why Are Planets Round?

You might have noticed that the Earth is round (it’s a spheroid, if you want to get technical). So are the other planets in our solar system. And so is the Sun. Are you starting to spot a pattern here? But is there a valid reason for this or does nature simply have a thing for spheres?

The simplest answer points to one reason – gravity. Each object has its own gravitational field – the larger the object, the stronger the field. Objects like stars and planets exhibit very strong gravitational forces. When they are being formed, all of the matter that goes into them is pulled towards the center with equal force and that results in a sphere. It might happen gradually at a slow speed but it still happens eventually. That’s why even if a planet would somehow have another shape like a pyramid or a square, gravity would pull all the corners towards the center of the planet until they eventually collapse.

Photo: NASA

Mercury, Venus, Earth, Mars and Ceres. Photo: NASA

Then what about moons and asteroids? They are celestial objects and they can come in weird shapes and sizes. Here is Mars’ moon Phobos and it’s not round at all. It all comes down to how strong their force of gravity is. Like we said, larger objects have stronger gravitational fields so it could simply be that these moons and asteroids don’t have the size required in order to generate a gravitational force strong enough to pull apart their matter and form it into spheres.

Maybe one day, Phobos, maybe one day. Photo: ESA

Maybe one day, Phobos, maybe one day. Photo: ESA

If you look at the largest moons in our solar system like Titan, Ganymede, Io and even our own Moon, you’ll notice that they are all spheres while smaller moons like Phobos or Hyperion haven’t bulked up to the required size yet. But what is the size where celestial objects start to shape themselves into spheres? Approximately one fifth the size of our planet.

And just to be clear, planets are not perfect spheres because there are other factors at play which influence their shape. Earth, for example, bulges at the equator because of its spin. The topology that naturally forms on planets leads to the development of geographical features like mountains, volcanoes etc. This means that they might look like perfect sphere from far away, but get close enough and you can see their surfaces are not smooth.

 

 

Featured image courtesy of Bluedharma via Flickr.