Looking at any planet in our solar system, it's easy to see that they all have one thing in common: their shape. It doesn't matter if it's made of gas or rock, all planets are spherical in shape. Also, this property is not unique to our solar system. Of the 4,000 exoplanets we have discovered, they all have this feature in common. So why are all the planets round?
To understand, it is convenient to investigate what criteria make a body of the Solar System be defined as a planet. According to the International Astronomical Union, a planet is a celestial body that:
- is in orbit around the Sun,
- has enough mass for its self-gravity to overcome rigid-body forces so that it achieves hydrostatic equilibrium
- has cleared the vicinity of its orbit.
The second condition implies that, by definition, the planets have to be spherical. An astronomical body is in hydrostatic equilibrium when its own gravitational force is balanced by its internal pressure; so that the body neither expands nor contracts. A body in this state will assume a spherical shape. Small objects, such as a notebook or an apple, can resist this impulse because their self-gravitational force is relatively weak, allowing them to retain their shape unchanged. However, in truly massive bodies, the force is so strong that they cannot help but take this form.
We know that the magnitude of the gravitational force on a body is directly proportional to its mass. The smallest planet in our Solar System, Mercury, has a mass of 3.28 x 10^23 Kg. This is an incredibly large number! A planet's gravitational pull pulls all of its mass toward its center. This happens evenly in all directions, giving rise to the spherical shape. Smaller celestial bodies, like asteroids, have strange shapes because their gravitational pull isn't strong enough, just like objects on Earth.
The planets; however, they are not perfectly round, but rather oblate spheroids that bulge slightly along their equator. This happens because, as they spin on their axis, centrifugal force tries to flatten them at the same time that gravity pulls their mass toward its center. The Earth is 43 km wider at its equator than it is from north to south, a phenomenon known as the equatorial bulge. Also, our planet is not the only one with an equatorial bulge. Mars's is 50 kilometers, Jupiter's is 10,175 kilometers, and Saturn's is the largest, 11,808 kilometers. The reason is that as a planet's rotational speed increases, so does its centrifugal force, and Saturn is the planet that spin faster on its own axis.
A fascinating fact is that the bulge of Jupiter and Saturn is so large that, if you had the opportunity to observe the sky with a powerful enough telescope, you could see it yourself. You may realize that space has infinite curiosities and wonders to discover, so keep asking yourself the reason for the things that everyone sometimes takes for granted.