The solar system is flat. Not each of the planets, satellites and asteroids that form it, which are spherical or simply deformed, but all of it, as a whole . The why has to do with its origins. More than 4.5 billion years ago in this region of the galaxy, there was a huge cloud of gas and dust , that cloud was composed mainly of hydrogen and helium and small amounts of the rest of the elements. Attracted by its own gravity, after many millions of years, that cloud contracted until it formed the Sun , the planets, its hundreds of moons and the thousands of asteroids and comets that populate this solar system.
That original cloud of gas and dust was spherical (or approximately spherical, at least) and yet it ended up giving rise to a solar system where the vast majority of bodies orbit on the same plane . We might expect that, after the orbits are cleaned up and the solar system is left with its current appearance, each planet will orbit in a different plane. From that giant and chaotic cloud we arrive at a solar system where all the planets revolve around the sun in the same direction and on the same plane. Why?
The cloud of gas and dust that eventually formed our solar system was indeed roughly spherical. Furthermore, within it, each particle moved in one direction and with a certain speed , so we could also say that it was somewhat chaotic. However, even though the movement of each particle was different, if we consider the movement of the entire cloud as a whole , if we consider the movement of all the particles that formed it, there must be a total movement that defines this cloud .
That is, the cloud as a whole rotated in a certain direction and in a certain plane . it moved at a certain speed through the Milky Way, etc. As a consequence, the sum of the movements in the other directions had to cancel out, because if they did not, that movement would contribute to redefining the plane of rotation, changing it. Therefore, after enough time, the collisions of the particles of this cloud ended up reducing the movement in any direction other than the main one, giving rise to a flat solar system in which all its bodies rotate in the same direction.
This same mechanism is what gives the rings of Saturn or galaxies like ours their flat shape . However, what we have explained is the ideal situation . In the real world, the original cloud is not uniform throughout its extension and the inner regions have a higher density than the outer ones, facilitating the collisions between the particles, which end up canceling each other. This, added to the fact that an infinite time has not passed since the cloud began to contract and that the collisions between the particles are not perfect, makes things like Pluto happen in the outer regions, whose orbit around the Sun is tilted with compared to the rest of the planets .
Also the dwarf planet Eris , or the asteroid Pallas , have highly inclined orbits. This also explains why some galaxies and their dark matter halos have a spherical or ellipsoidal shape. The galaxies that are formed from very large clouds of gas and dust undergo the same process that we mentioned for the solar system but on a larger spatial and temporal scale. Over billions of years, they end up disk-shaped. But when a galaxy forms without large amounts of gas and dust , as occurs in the collision of already formed galaxies, the stars , being so far apart from each other, rarely collide or interact in such a way that their motions are not cancelled . maintaining the spherical and chaotic shape.
On the other hand, since dark matter interacts so weakly with other matter and with itself, it doesn’t have these collisions and doesn’t end up as a disk .
We might now ask if this same mechanism could give the earth or another planet a flat shape . Could this be the definitive answer to the prayers of flat earthers? The difference between the Earth and the cloud of gas and dust that formed the solar system lies in the forces that hold them together. A cloud of gas and dust is very easily deformable , it is held together only by gravity, but a sphere formed by solid rock, magma and other components, remains more cohesive due to intermolecular forces and friction. All this makes it much more resistant to these deformations.
Which does not mean that you do not feel them, and that we cannot observe them. It is precisely the rotation of the Earth in a certain direction (analogous to the rotation of that cloud of gas and dust that formed the solar system) that makes it not perfectly spherical and that it is wider at the equator than at the poles . Of course this doesn’t explain why the Earth should be spherical in the first place. Why the laws of physics force an object of this size and composition to be more or less spherical. But we’ll talk about that another time.
