Why are planets spherical?

Answer 1

In fact, the Earth is not a perfect sphere. It's poles are flattened and its equator bulges outwards. This is due to centripetal force as the planet spins on its axis.

However, setting this fact aside for a moment, Earth is spherical in shape because its gravity is pulling everything towards the center of the planet. This pull is spread equally across its entire surface. Because all of this mass cannot share the same space, gravity forces it into the most efficient configuration, which is a sphere. Any object of sufficient mass will over time become spherical in shape.

Formation of Earth

When the leftover elements from the sun's creation scattered throughout what is now our solar system, individual particles started attracting each other together with gravity. As more and more particles gathered, their gravity compressed them into a shape which allowed them to occupy as small of a space as possible. In essence, a sphere.

Another way of saying this is that the Earth's mass reached a point where its internal gravitational forces overcame any other rigid forces that might have kept the Earth in a non-spherical shape.

Answer 2

Because of gravity.

Gravity tends to pull everything to the center. The average distance of every particle from the center is minimized when the particles are arranged in the shape of a sphere.

Small irregularities... mountains, for example... can exist, as long as they're not too tall.

Answer 3

The Earth is actually an ellipsoid technically known as an oblate spheroid as the Earth is slightly flattened at the poles and bulges out at the equator due to the rotation of the Earth on it's axis. However a sphere could be considered a reasonable representation of the shape of the Earth!

The technical term for the shape of the Earth is "geoid" and an idealised shape based on the "geoid" is known as the reference elipsoid. The study of the Earth's true shape is known as "Geodesy" or "Geodetics".

For more information on these admittedly complex topics please see the related links.

Answer 4

Planets are spherical due to gravity. Gravity pulls material toward the center of the planet, causing it to compress and form a round shape. Additionally, as a planet spins, the centrifugal force also contributes to its spherical shape by flattening the poles and bulging at the equator.