Life's Little Mysteries

Why are things in space round?

The planets and larger moons to scale with the Sun.
Why are these planets and moons, illustrated here to scale next to the sun, so round? (Image credit: Ron Miller/Stocktrek Images via Getty Images)

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.

Donavyn Coffey
Live Science Contributor

Donavyn Coffey is a Kentucky-based health and environment journalist reporting on healthcare, food systems and anything you can CRISPR. Her work has appeared in Scientific American, Wired UK, Popular Science and Youth Today, among others. Donavyn was a Fulbright Fellow to Denmark where she studied  molecular nutrition and food policy.  She holds a bachelor's degree in biotechnology from the University of Kentucky and master's degrees in food technology from Aarhus University and journalism from New York University.

  • Tranent@69
    I would say the tatti shaped moons are not the finished article at the minute
    So isn't earth for that matter
  • pse001
    admin said:
    Why are most planets and moons (mostly) spherical?

    Why are things in space round? : Read more
    What I wonder about is: Why aren't most galaxies spherical? Are they not bound by the same gravity?
  • SerengetiLion
    admin said:
    Why are most planets and moons (mostly) spherical?

    Why are things in space round? : Read more
    Because they "spin"
  • David A. Johnson
    There are 3 reasons though not equal proportional. Spinning is one has this force makes fluffy by throwing all on earth outward. We get a lot of visitors that come and stay. Over meliniums this is substantial so again Spinning insure a evened blanket. Gravity pull and holds things together by a pulling in or toward core. The core is a bit responsible by being flexible from being
    Molten. But the most responsible thing for all the round spheres would be vacuum. Vacuum pulls out in a anticollapsing or else all would give way and fall by gravity laws pulling everything toward center making a smaller by condensing planet/star/satellite. There are bodies along with hosting sun(s)/stars also pulling outward and those are factors of persuasion that may or not be considered.
  • waxwing
    Relatedly, water blobs in space are spherical. Liquids are far more likely and faster to deform to spheres than solids are.