Why are planets formed in a spherical shape?

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Updated - May 28, 2024 05:36 pm IST

Published - May 28, 2024 04:37 pm IST

An artist’s illustration of a protoplanetary disc around a star. Planets form when gravity causes parts of the disc to clump together and coalesce.

An artist’s illustration of a protoplanetary disc around a star. Planets form when gravity causes parts of the disc to clump together and coalesce. | Photo Credit: NASA

Q: Why are planets formed in a spherical shape?

- Hemant Sardesai

A: The short answer is gravity. This ‘force’, by virtue of the large masses of planets and stars, forces them into a spherical shape.

Part of the answer is also geometry: a sphere is the most compact three-dimensional shape. To be more accurate, for a given volume, a sphere is the shape with the lowest surface area.

If stars and planets had any other shape, gravity would force them to become spherical.

Bodies that are less massive also experience less of a ‘force’ due to gravity and thus will be less compelled to enter into a spherical shape. The electromagnetic forces between the atoms in these bodies will be able to better resist gravity’s attempts to sculpt it.

This is why comets, most asteroids, and even us humans aren’t spherical.

But let’s get even more accurate: nothing in the universe is truly spherical. Stars and planets are actually oblate spheroids. Since they both rotate around a central axis, they create a centrifugal force that pushes mass outwards. So the oblate spheroid shape is a sphere that appears to bulge at the equator.

As a result, gravity is weakest at the equator and strongest at the poles. This is why, on the earth, objects fall to the ground down a teeny-tiny bit faster at the poles than at the equator.

Karthik Vinod is interning with The Hindu.

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