Why does gravity prefer to build flat disks instead of spheres?
In short: If you throw a handful of sand into the air, it forms a messy cloud. But if you spin that same sand rapidly, it flattens out. Our solar system followed this exact rule 4.6 billion years ago, transforming from a shapeless blob into a structured disk.
Roughly 4.6 billion years ago, a gaseous and dusty nebula gave rise to our solar system. While gravity pulled these materials together, the initial state was a roughly spherical, shapeless blob. To understand why it didn't remain a sphere, we have to look at the physics of angular momentum. Any isolated system has a total amount of spin; in our universe, this total remains constant.
Because the cloud had an initial rotation, there was a mathematical plane around which the entire mass spun. In a three-dimensional space, particles can move 'up and down' relative to that plane. However, as these particles moved, they frequently collided with one another. These collisions acted as a mechanism for energy loss: while the total spin of the system was preserved, the random vertical motion was canceled out by these crashes.
Over time, this process stripped the cloud of its 'loft,' forcing it to settle into a roughly two-dimensional disk shape—the same reason we see flat spiral galaxies and Saturn's rings. This preference for flatness is a consequence of living in three dimensions. In a four-dimensional universe, the math allows for two complementary planes of rotation, meaning particles wouldn't lose their vertical energy through collisions, allowing clouds to remain spherical.
This phenomenon isn't unique to our neighborhood; many exoplanet systems and black hole accretion disks exhibit this same flatness. It is a fundamental structural outcome of gravity, motion, and the geometry of our space.
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Sources
- Summary by Intellect · source: Why is the Solar System Flat?
- Also: Is the solar system flat? | Britannica
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