Why Are Planets Round?

by James Hunt

When they tried to figure out how to define a planet, one of the criteria that astronomers settled on was whether its body was spherical, instead of irregular. Indeed, the fact that Pluto is spherical is one of the reasons the category of "dwarf planet" was created when it was downgraded from a fully fledged planet—because its shape meant it was still too planet-like to be called anything else.

But what is it that makes planets round? Why are they that shape and not something else? And what do potatoes have to do with it?

The answer to those first two questions is fairly straightforward: planets are round because of gravity.

Planetary bodies are formed when material in space clumps together under its own gravitational pull to form a larger body. When that body's mass reaches a sufficiently high point, its gravity becomes strong enough to overcome the structure of the materials of which it is composed and they begin to deform (greatly helped if the planet goes through a molten phase early in its history). The rock (or ice, or gas, or whatever else) gets dragged toward the gravitational center of the body, and over time this pulls the material into the simplest shape that can satisfy the forces upon it: a sphere (ignoring any odd bulges from rotation).

It might take a while, but without any other major external force involved, it seems to be inevitable. The gravitational center will become the physical center, and large enough planets will always become spheres.

Of course, "large enough" is a relative term. Pluto is much smaller than Earth, but still big enough to have formed into a sphere. Pluto's biggest moon, Charon, and Earth's moon have also managed this feat. Even Ceres, a dwarf planet found in the asteroid belt, is large enough to be spherical. If being spherical were the only requirement for being a planet, we'd have to consider countless other bodies as planets. Indeed, Earth would rightly have to be considered a binary planet system rather than a planet with a moon.

This is the other reason why the designation "dwarf planet" was invented. It applies to bodies which are large enough to be spherical but not to have achieved "gravitational dominance" over their orbital region. The moon is in the thrall of Earth, Charon is in the thrall of Pluto, Pluto has other Kuiper Belt Objects in the vicinity, and Ceres is surrounded by asteroids which it has (thus far) failed to clear.

But dwarf planets still have planet in the name, so you might be wondering where the cut-off point actually is. And that's where potatoes come into it: Although there are various factors that can produce planetary outliers (rotational speed and mass are two major ones), it seems that most bodies become spherical at a radius of around 200 to 300 kilometers. This is known as "the Potato Radius," because it's when bodies lose their "potato" shape and begin deforming into spheres.

Ultimately, the question of why planets are round has a pretty fundamental answer: it's because, according to the physical laws of the universe as we understand them, it's that they simply don't have any other choice.

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