What if planets were not round?

Look at pictures of the planets. Every one is round!

Why aren't any shaped like cubes or pyramids? Let's find out.

The eight planets are very different. Some are small and rocky, some are giant balls of gas, some are close to the Sun and some are far away. But NASA points out that they're all round. None is shaped like a cube, a pyramid or a pancake.

So what if planets were not round? What would a cube-shaped planet be like, and why don't we find any? This is a thought experiment. Choose a path to find out how gravity makes balls, why asteroids are lumpy, and how big a world has to be before it turns round.

Roundness is part of what makes a planet. In 2006 the International Astronomical Union defined a planet as a body orbiting the Sun that has enough mass for its self-gravity to overcome rigid-body forces, so it takes a hydrostatic equilibrium, nearly round, shape, and that has cleared its orbital neighborhood.

This thought experiment asks what a non-round planet would imply. Evidence comes from NASA's measurements of planetary shapes and of asteroids and dwarf planets visited by spacecraft. Descriptions of life on a cube-shaped world are ideas from simple reasoning about gravity, not observations or model results.

Mercury and Venus are almost perfect balls.

Saturn and Jupiter spin fast, so they bulge a little around the middle.

NASA says Mercury and Venus are the roundest planets, nearly perfect spheres like marbles. Others are a bit squished. Fast-spinning Saturn is 10.7 percent wider around its middle than from pole to pole, and Jupiter is 6.9 percent wider.

Earth and Mars are much closer to round. Earth is only 0.3 percent wider at the middle, and Mars 0.6 percent. Uranus and Neptune are in between. That extra width at the middle is called the equatorial bulge.

NASA Space Place compares equatorial and polar diameters. Mercury and Venus are nearly perfect spheres. Saturn has the largest equatorial bulge, 10.7 percent, followed by Jupiter at 6.9 percent, Uranus at 2.3 percent and Neptune at 1.7 percent. Earth's bulge is 0.3 percent and Mars's 0.6 percent.

The bulge comes from rotation. Material at the equator moves fastest as a planet spins and tends to fling outward, while gravity holds it in; the balance leaves a slightly flattened sphere. Large, rapidly rotating gas giants flatten the most. NASA describes them as like basketballs squashed by someone sitting on them, while Earth and Mars, smaller and slower, stay within one percent of round.

Imagine a planet shaped like a giant box. The corners would stick out far from the middle.

Gravity pulls toward the middle. So the corners might slowly slump down, and the box might turn into a ball!

Imagine a planet shaped like a giant cube. Its corners would be much farther from the center than the middle of each flat side. Gravity pulls everything toward the center, so walking from the middle of a face toward a corner might feel like climbing a huge hill.

For a big world, that pull might be strong enough to crush the corners down. Over time, a cube planet might slump into a ball. These are ideas from thinking about gravity, not something scientists have seen.

On a cube of planetary mass, the corners would lie about 1.7 times as far from the center as the face centers, since a cube's corner-to-center distance is √3 times its half-width. Gravity pulling toward the center might make the faces behave like enormous slopes running downhill toward their middles.

If the body were massive enough to be a planet, its self-gravity would, by the IAU's criterion, overcome the rock's rigid strength, so the corners might yield and flow until the shape approached a sphere. So a lasting cube-shaped planet might not be possible; the scenario is an idea used to show why mass and roundness go together.

Gravity pulls things together. A planet's gravity pulls from its center, the same in every direction.

That pull squeezes the planet into a round ball.

Planets form when bits of material in space bump and clump together. NASA explains that once a clump has enough material, its gravity gets strong. A planet's gravity pulls equally from all sides toward its center, like the spokes of a bicycle wheel.

That makes the overall shape a sphere, a three-dimensional circle. Smaller objects can be different: NASA says asteroid Eros, about 32 kilometers long, is not massive enough for its own gravity to have pulled it into a ball, so it keeps its odd slipper shape.

Gravity pulls toward a body's center from every direction, which NASA likens to the spokes of a wheel. In the IAU's words, a body massive enough for its self-gravity to overcome rigid-body forces assumes a hydrostatic equilibrium, nearly round, shape. Rotation then adds a slight flattening, as the equatorial bulges NASA lists show.

Small bodies fall short of that: NASA notes that Eros, a 20-mile (32 km) slipper-shaped asteroid, is not massive enough for its own gravity to have shaped it into a planet-like sphere. Where the line falls is not settled. NASA reports that when some scientists suggest defining a planet simply as a body massive enough to be roughly spherical, others object that this does not say what degree of roundness is needed, and that the shapes of distant objects are often hard to measure.

Most asteroids are lumpy. Big Vesta is almost round. Bigger Ceres is round!

Big worlds have enough gravity to pull themselves round.

NASA says most asteroids are irregular, lumpy shapes, though a few are nearly round. Asteroid Bennu, visited by NASA's OSIRIS-REx, is only about half a kilometer wide, formed from rubble and looks like a spinning top.

Vesta, the largest asteroid at about 530 kilometers across, is almost spherical, so it nearly counts as a dwarf planet. Ceres, with a radius of 476 kilometers, is the largest object in the asteroid belt and is a dwarf planet: big enough for gravity to make it round. NASA's Dawn spacecraft visited both.

Shape tracks size and mass. Most asteroids are irregular and often pitted; Bennu, about one-third of a mile wide at its equator, coalesced from leftover rubble 1 to 2 billion years ago and is shaped like a spinning top, a shape whose origin NASA says scientists don't fully understand. Vesta, about 530 km across, accounts for almost 9 percent of the total mass of all asteroids, is differentiated into crust, mantle and core, and is almost spherical, nearly a dwarf planet; a giant impact left the Rheasilvia crater, about 95 percent of its mean diameter.

Ceres, radius 476 km and the belt's largest object, meets the dwarf-planet criterion of a hydrostatic equilibrium shape. NASA's Dawn orbited Vesta in 2011 and 2012 and reached Ceres in 2015.