The stars are round because their own gravity pulls them towards their center of mass, naturally shaping them into a spherical form while minimizing their potential energy.
Gravity acts like a kind of cosmic glue: it attracts all matter toward the center of a celestial body. When a planet or star forms, all the particles try to move toward the central point. The result: the celestial object takes on a rounded shape, as it is simply the simplest form where all the matter is evenly distributed around the core. The more massive a celestial body is, the stronger its gravity, and the more pronounced its tendency to become perfectly round. Beyond a certain size, irregular shapes are a thing of the past: it is impossible to have a giant asteroid shaped like a potato, for example, because gravity would quickly force it to round out. That’s why small moons or asteroids remain irregular, while planets like Earth or Jupiter are beautiful, almost perfect spheres.
When a celestial body forms, it grows by accumulating rocks, gases, or other space materials. After a while, the accumulation of matter creates an internal pressure so strong that it pushes the materials outward, like a balloon being inflated. At the same time, the gravity of all this material pulls everything toward the center. When the internal pressure becomes equal to the gravity, it creates an equilibrium called hydrostatic equilibrium. At this stage, the celestial body organizes itself into a spherical shape, simply because it is the most stable shape that best distributes the forces. If you imagine a giant mountain trying to push outward on the surface of a planet, hydrostatic equilibrium quickly calms that down and quickly reshapes things, naturally rounding the surface. That’s why massive planets all look like nice round space balls: it’s simply a matter of balance.
When a celestial body rotates on its axis, the centrifugal force generated by its rotation pushes its matter outward, especially around the equator. This push slightly deforms the shape of the body, causing it to become slightly flattened at the poles and bulging at the equator. This is known as flattening or ellipsoid: our Earth, for example, is not perfectly round due to its rapid rotation; it resembles a slightly squashed orange at the poles. The faster the rotation, the more pronounced the effect, which explains why some gaseous planets like Jupiter, which rotate quickly on themselves, appear much more flattened. In contrast, celestial bodies that rotate slowly (or not at all) remain much closer to a perfect sphere.
Celestial bodies are subjected to two main forces: their gravity, which pulls all matter toward the center, and their internal pressure, which pushes outward. Gravity seeks to crush the star or planet inward, while internal pressure, generated by intense heat and nuclear reactions (for stars) or simply by density and internal heat (for gas and rocky planets), counteracts this compression. It's a kind of delicate balance: gravity wants to compact everything, but as soon as it compacts too much, the pressure increases and pushes back again. The result is a rounded shape where everything stabilizes nicely at roughly the same point in the center. It is precisely this tightrope act between gravity and internal pressure that structures the perfect (or nearly perfect) sphere of celestial bodies.
The mountains on Earth do not exceed a certain height because gravity would act to bring them back down. Mars has the tallest mountains in the solar system due to its low gravity, including Olympus Mons, which is 21 km high!
The phenomenon of hydrostatic equilibrium, which forces the matter of a celestial body to take on a spherical shape, only occurs above a certain mass; this is why some small celestial bodies are often irregular in shape.
The asteroid Hygiea was long considered irregular until high-resolution observations recently showed that it is spherical enough to be potentially classified as a dwarf planet.
If you were to compress the Earth down to the size of a marble, it would become a small but dense black hole. Despite its infinitely small size, this mini black hole would still retain the same mass!
Only celestial bodies with sufficient mass can adopt a spherical shape over time. Objects that are too small generally never achieve the necessary hydrostatic equilibrium. Their gravity is too weak to force them into a round shape. Thus, even over time, a small asteroid will typically remain irregular.
No, not all planets have a perfect sphericity. Most planets rotate on their axes, which generates a centrifugal force. This force tends to slightly bulge their equator, making their shape slightly flattened at the poles, a phenomenon known as oblateness.
Planetary rings often result from rocky fragments and particles trapped in orbit by the planet's gravity. Gas giants like Saturn, Jupiter, and Uranus have remarkable ring systems. These rings depend on formation processes, collisions, as well as the mass and gravity of the planet, which explains why they are not observed around all planets.
Yes, the tidal effect, caused by the gravity of one celestial object on another, can influence the shape or deformations of celestial bodies. For nearby bodies, such as the Earth and the Moon, gravity exerts differential forces, creating tides and even slightly deforming their initial shape, thus creating a tidal bulge.
Asteroids, unlike the large planets, do not have sufficient mass for their gravity to cause them to adopt a spherical shape. Their low gravity does not allow for the hydrostatic equilibrium needed for sphericity. Therefore, they retain irregular shapes due to collisions and their chaotic formation.
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