Our solar system is filled with spherical planets and moons. Why are they round, and not some other shape? A question about round globes and the force of gravity “Why isn’t the Earth a square? And why is it a sphere?”
The answer lies in the same force that holds you to Earth’s surface — the force of gravity. Gravity pulls every part of our planet in toward the center of Earth.
Scientists believe that Earth formed from a cloud of interstellar gases, dust, and ice surrounding our sun when it was young. Eventually these particles bumped into each other and clumped together. When Earth became massive enough, its own self-gravity took over. Since gravity pulls from the center — equally in all directions — over time gravity smoothed out any variations in the shape of the Earth, and our planet became almost perfectly round.
The force of gravity pulls from within all the large moons and planets in our solar system. That’s why they all have the shape of spheres. But, remember the strength of Earth’s gravity comes in part from its mass, or its bulk. Less mass means a weaker tug toward the center. That’s why small moons and asteroids are often lopsided or oddly shaped. Their gravity isn’t powerful enough to overcome the inherent sturdiness of the rock.
Note 1: Some more comments from Dr. David Black, on the critical size a body needs to be.
Dr David Black: “There is a critical mass and that corresponds to a critical size for typical rocky things, roughly 100 km, below which the strength of the solid material is able to resist the efforts of gravity to crush it and form a spherical object. That is why the space rocks (small asteroids) have irregular shapes. There is insufficient gravity (they have small masses) to overcome the crystalline strength of the material. Once you get larger than this critical size (there is no single size, it depends on the material, but the variations are not too large), objects tend to become spherical because gravity wins.”
Note 2: Not convinced a cube-Earth’s edges would be farther from the center? Try this: Draw a square. Inside the square, draw a circle that touches the middle of each of the sides. Put a dot in the center of the square. (This is also the center of the circle.) The line you draw from the center of the circle to the square’s corners will be longer than any line you draw from the center to the circle’s edge.
If the earth’s surface were water, there’d be nothing to hold up the edge — the edge would quickly flow away, so that every point on the earth’s surface was the same distance from the center. Rock doesn’t flow as easily as water, but given enough time, the rock, too, will rearrange and flow into a spherical shape.
Note 3: Jupiter’s large moons, like Callisto and Ganymede, aren’t perfectly circular, says Dr. Schubert. (Although you couldn’t tell if you looked at them through a telescope. They look spherical!) Scientists have been able to measure small distortions in these moon’s spherical shape that result from being so close to Jupiter! Even though the moon’s own gravity has the biggest role in producing its spherical shape, the nearby tug of Jupiter has a small hand in the final form.
Here, the rock has won in the battle between the strength of the rock and the pull of gravity. The small moon, or asteroid, doesn’t have enough mass to create a gravity field strong enough to dominate the rock’s strength. But where does gravity come from? The pull of gravity increases as mass increases. Earth doesn’t really notice the tiny tug we exert on it because it is so much bigger than us.