Through telescopes on Earth and in space, astronomers can glimpse the far-flung reaches of the universe. And no matter how far away or how strange the planet, at least one thing seems to hold true in space: a lot of stuff is spherical.
So what makes these celestial bodies round? In short, it’s gravity.
“It’s pretty amazing that we know of so many things being round in space,” Anjali Tripathi, an astrophysicist at NASA’s Exoplanet Exploration Program, located at the Jet Propulsion Laboratory in California, told Live Science. Gravity’s rounding effect is a result of self-gravity, the gravity that an object — in this instance, a celestial body — exerts on itself. Once a planet, or maybe a moon, accumulates enough mass, its self-gravity will pull it into a sphere-like shape.
The universe’s bodies formed after the Big Bang exploded about 13.8 billion years ago. Tiny dust particles circulating in enormous doughnut-shaped dust clouds began to collide. If the collision was gentle enough, according to NASA, the dust particles fused. Collision after collision created a snowball effect; The more mass a budding planet accumulated, the more its gravity grew and the more matter it attracted.
Related: Why do the planets in the solar system orbit on the same plane?
That “gravity pulls all the matter toward the center of gravity,” said Bruno Merín, an astronomer and head of the European Space Agency‘s ESAC Science Data Centre in Madrid. It’s like the kitchen sink, he said: “All water will flow through the hole in the bottom.” In the case of planets, “every piece of matter is trying to get as close as possible to the center of gravity.”
Planetary bodies will continue to shift matter around until they find an equilibrium, a state in which every point is as close as possible to the center. And the only shape that achieves this kind of equilibrium in space is a sphere, Merín told Live Science.
Mercury and Venus are nearly perfect spheres because they are slower-spinning rock planets. Ice planets also tend to be almost perfectly round, as the “layer of ice distributes very evenly,” Merin said.
But “round” doesn’t mean that every planet is a perfect sphere; the gas giants Jupiter and Saturn bulge at their equator because of how fast they spin. Instead of a perfect sphere, Saturn looks like a basketball someone is sitting on, according to NASA. Even Earth has a tiny bulge of less than 1%, due to centrifugal force, the outward force on a spinning object. So Earth is oblate, or a slightly flattened sphere.
Although the universe teems with spheres, many bodies in space aren’t even remotely spherical. Asteroids and comets can come in any shape, altered by crashes and interstellar spinning. Mars has a potato-shaped moon called Phobos; in fact, only about 20 of the nearly 300 known moons in the solar system are the familiar round shape we expect, the rest are more irregular. The reason for all these nonspherical bodies: Their lower mass means they don’t have enough gravity to even out their shape, Tripathi said.