Why are asteroids & comets weird shapes?

A rubber duck. A spinning top. A pair of pancakes. These are just a few of the shapes astronomers have observed across the solar system.

While planets and some moons are almost perfectly spherical, the smaller bits of the solar system, such as asteroids and comets, come in all different shapes. But why is that?

It all comes down to mass and gravity, said Alessondra Springmann, a researcher who studies asteroids at the Lunar and Planetary Laboratory at the University of Arizona.

As soon as a structure gets big enough, gravity pulls everything equally toward the body's center of mass. That gravitational force creates a spherical shape.

Then there are the asteroids, comets and other small bodies of the solar system, such as some Kuiper Belt objects (KBOs), which orbit the sun beyond Neptune.

These objects are composed of leftover material from the formation of the Solar System, when the growing planets shed most of the Solar System's parent material.

Then there's Comet 67P/Churyumov-Gerasimenko, which is famously shaped like a rubber duck.

Comets come in weird shapes not just because of their size but also because they're made mostly of ice (both water ice and other types of ice), Springmann said. 

When comets get close to the sun, that ice sublimates — it becomes gas, skipping the liquid phase — and jets into space, creating a temporary atmosphere around the comet called a coma.

"Those jets can form all kinds of structures," on the surface of the comet, Springman said.

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