It is something we take for granted. Roses are red and planets are spherical. That’s the way it is, right? But have you ever wondered why planets look like this? Why are they basically spherical and not, say, cylindrical or cubic?
The eight planets in our solar system differ in many ways. They are of different sizes. They are at different distances from the sun. Some are small and rocky, and some are big and gassy. But they are all round. Why don’t they have other shapes?
Planets form when material in space begins to collide and clump together. After a while, you have enough material to have a good amount of gravity. That is the force that holds things together in space. When a planet in formation is large enough, it begins to clear its way around the star it orbits. It uses its gravity to catch chunks of space material.
Thus, the spherical shape of large celestial bodies is due to gravity. Any object around it creates a gravitational field that acts as if all the mass of the body is concentrated in the center and draws matter towards itself. During the long period of formation of a planet, matter flows, subjected to the heat of its internal nuclear reactions, and succumbs to the strong pull of its gravitational center. The spherical distribution, which is symmetrical in all directions, is the only geometric shape that causes all the matter on the planet to be as close to its center as possible.
The gravity of a planet attracts equally from all sides. Gravity pulls from the center to the edges like the spokes on a bicycle wheel. This makes the general shape of a planet a sphere, which is a three-dimensional circle.
According to the most widely accepted model of star and planet formation, also known as the nebular hypothesis, our solar system began as a swirling cloud of dust and gas (that is, a nebula). According to this theory, about 4.57 billion years ago, something happened that caused the cloud to collapse. This could have been the result of passing a star or shock waves from a supernova, but the end result was a gravitational collapse at the center of the cloud.
Due to this collapse, pockets of dust and gas began to accumulate in denser regions. As the denser regions attracted more matter, the conservation of momentum caused them to begin to rotate, while the increase in pressure caused them to heat up. Most of the material ended up in a ball in the center that formed the Sun, while the rest of the matter flattened into a surrounding disk, that is, a protoplanetary disk.
The planets were formed by accretion of this disk, where dust and gas merged to form larger and larger bodies.
According to Resolution 5A, the definition of a planet is: A “planet” is a celestial body that:
(a) is in orbit around the Sun,
(b) it has sufficient mass for its self-gravity to overcome the rigid forces of the body so that it assumes a form of hydrostatic equilibrium (almost round) and
(c) has cleared the neighborhood around its orbit.
A “dwarf planet” is a celestial body that
(a) is in orbit around the Sun,
(b) it has enough mass for its self-gravity to overcome the rigid forces of the body, so that it assumes a form of hydrostatic equilibrium (almost round),
(c) has not cleared the neighborhood around its orbit, and
(d) it is not a satellite.
