Hello friend,

When we first learn about space, it’s easiest to imagine the solar system as a neat set of planets moving in perfect circles around the sun.

But in reality, almost every orbit is an ellipse, which is just a stretched-out oval.

And when enough objects orbit together, like in a galaxy or Saturn’s rings, the system almost always settles into a wide, rotating disc instead of an evenly filled sphere.

So why do gravity and motion produce ovals and discs, rather than circles and spheres?

In the early 1600s, Johannes Kepler analyzed decades of careful astronomical observations and discovered that planetary paths are not circular, but elliptical, with the sun located at one focus of each ellipse. This insight later helped produce Isaac Newton’s laws of motion and universal gravitation.

A circle is a particularly unlikely orbital path because it is fragile. For a truly circular orbit, an object requires a very specific balance between inward gravitational pull and sideways velocity, with nothing to disturb it. Not exactly a common situation in a chaotic universe.

In practice, objects form with a range of velocities and angular momenta. Gravity pulls towards something large, but sideways motion keeps it moving in a roughly tangent direction. The result is a closed orbit whose exact shape depends on the object’s total energy and angular momentum.

Circular orbits are therefore not impossible, just unlikely. They require exact conditions that are rarely met in a universe filled with turbulence, collisions, and continual gravitational perturbations. Ellipses are not imperfections, they are just the most likely solution.

Now, zoom out even farther.

Imagine a large cloud of gas and dust spread through space. Gravity causes the cloud to contract toward its center of mass. But unless the cloud has exactly zero net rotation, it possesses some angular momentum.

As the cloud shrinks, conservation of angular momentum forces the rotation rate to increase, just as an ice skater spins faster when pulling in their arms. Random motions within the cloud lead to frequent collisions, which dissipate energy but do not remove angular momentum.

Over millions of years, motions perpendicular to the average plane of rotation are damped away, while motion within the plane is preserved. The cloud therefore collapses into a flattened, rotating disc.

That’s why the Milky Way Galaxy, Saturn’s rings, and planetary systems all form discs. Gravity tries to pull everything together, but the spin keeps things spread out in a plane, instead of a neat sphere. Spheres do (kind of) exist as singular bodies, like stars and planets, but the choreography of many bodies always flattens out.

In the cosmos, neat circles and perfect spheres are rare. The universe, left to its own devices, much prefers ovals and discs.

Tool: Fantasy Visualizer

Prompt: An astronomer views the night sky

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PS—Understanding these ideas is one thing, but Isaac Newton had to discover differential and integral calculus in order to explain it.