We all witness the presence of with gravity and how it pulls objects towards the earth’s surface. Despite being made up of virtually the same particles of rocks lying on its surface, what makes earth or any other planet as a whole so special, that supports the existence of gravity?
Turns out there is nothing special about planets, except for their sheer size and mass making their forces more pronounced. Every single particle in the universe exerts gravitational forces, but their magnitude depends on a very specific quantity – their mass. According to Newton’s law of gravity, gravitational forces are directly proportional to the product of masses and inversely proportional to the square of distance between them. Sounds too complicated?
In simple terms, the more the mass of an object, the more gravitational force it exerts. Objects we are used to dealing with (including the largest mountains ranges in the world) are nothing, when it comes to the masses of celestial bodies that weigh in billions and trillions of tonnes. Their gravitational forces aren’t enough to pull objects into its vicinity, and thus appear “gravitation-less” for all our practical purposes.
Apart from the obvious “pull” it has on objects in its vicinity, there are other interesting consequences of this mysterious force. It shapes itself. That’s right!
When celestial objects have radius larger than 200-300 kms(considering their densities to be constant average of celestial bodies), it is strong enough to pull its own surface onto itself! As a consequence, the default irregular shape transitions into regular spheres like the planets in our solar system. This radial threshold has an interesting name known as “potato radius” (PR), and is named after the irregular potato-like shapes of smaller celestial bodies such as asteroids and dwarfs.
Note how their shapes of moons change with increasing radius(Source)
But why a sphere, and not any other shape? If we were to exert enormous pressure on an object, it would either resist change due to its own structural strength, or break/deform depending on its material. But despite being so strong, since gravity acts uniformly from all directions, objects with radius higher than PR maintain this unique spherical shape, characteristic of all planets in our solar system and beyond. Objects in the universe are unique in their own design, shape, color and also countless other properties, but the fact that gravity, that holds everything together, is the universe’s way of conveying that we are a part of the same universal system.
