When we mentally imagine a planet, the first thing that comes to mind is to imagine a large spherical ball. A very round planet, with a more or less smooth surface, which spits flames or which reaches extremely cold temperatures.
If these characteristics vary, one remains immovable : its round shape. Why not represent a planet as a cube or cone, for example ? Or in the form of a flat disc, to satisfy earth flat fans ? Quite simply because all planets are spheres. Round like a balloon.
A question of gravity
Although many of us would dream of being able to observe planets shaped like Doritos or Cornettos through a telescope, it is not for tomorrow. Sorry to disappoint you, but all planets have more or less a round shape because of a phenomenon that we know well : gravity.
When a planet forms, the dust particles clump together, collide, until they merge and create a large stone. This large stone grows gradually by conglomerating more and more material, thanks in particular to the presence of gas, which acts as a fixative. When matter reaches a significant quantity, the phenomenon of gravity operates.
A planet's gravity then pulls equally from all sides toward the center of the body. Like the spokes of a bicycle wheel, everything revolves around the central point. The ends of the planet are thus at an equal distance from its heart, transforming the unsightly pebbles into a sphere, a three-dimensional circle.
Imperfect circles
Well, if we are a little nitpicky, we should still point out that the planets are not completely round. Except for Mercury and Venus, which approach perfection by resembling real marbles. For the others, their shape is spherical, of course, but most have irregularities. Starting with Saturn.
The sixth planet in the solar system in order of distance from the Sun looks more like a slightly flattened basketball. With 10.7% more thickness in the middle, Saturn is “the most rounded planet in our solar system,” explains NASA. A strange shape that is also found in Jupiter.
On the other hand, it is nothing compared to other celestial bodies, such as asteroids, comets and certain satellites. They really don't look like anything. With a much smaller rotation, they are subject to collisions and interstellar rotations, which means they take on diverse and varied shapes. Like the moon Phobos, a natural satellite which orbits Mars : it looks like a potato.
No, the Earth is not round
But rest assured, our blue planet is not flat either (sorry Flat Earthers). The Earth is indeed spherical, but is slightly flattened at the poles and bulges at the equator. A characteristic that makes the distance from a pole to the center of the Earth shorter than that from the center of the planet to the equator.
The Earth would in fact be 0.3% thicker in the middle, according to NASA. Much less than Saturn and Jupiter, but not far from Mars, which is 0.6% thicker in the middle.
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