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Why are planets round? – Daniel B., La Crosse, Wisconsin
The ancient Greeks proved over 2,000 years ago that the Earth is round and used simple observations of the Sun to determine how big it is.
But how do people know this today? When you drop something, gravity causes it to fall directly toward the center of the Earth, at least until it hits the ground. Gravity is a force that is caused by almost everything that has mass. Mass is a measure of how much material is in something. It can be in the form of rocks, water, metal, people – anything. Everything has mass, and therefore everything causes gravity. Gravity always pulls towards the center of mass.
Earth and all the planets are round because when the planets formed, they were made of molten material – essentially very hot liquid. Since gravity always points toward a center of mass, it squeezes out the stuff that the Earth has built up equally in all directions and forms a ball. When the earth cooled and solidified, it was a round ball. If Earth did not rotate, it would be a perfectly round planet. Scientists call something that is perfectly round in all directions.
The gas cloud that the Earth is made of was moving slowly in one direction around an axis. Above and below this axis are the Earth’s North and South Poles.
Now, hold your right hand. Point your thumb directly at your right hand, and rotate your fingers in the direction of rotation. Your thumb is pointing towards the North Pole. The equator is defined as the plane halfway between the North and South poles.
If you’ve ever played on a merry-go-round, you know that spinning merry-go-round throws you off. The faster it spins, the harder it is to stay on it. This tendency to flow is called centrifugal force and it pushes the mass outwards at the equator. This makes the planet bulge at the equator.
The faster the spin, the more unruly it becomes. Then, when it cools and hardens, it retains that shape. If a molten planet began to spin faster, it would be less round and have a larger bulge.
Saturn is very flat – spherical – because it spins very fast. Because of gravity, all planets are round, and because they rotate at different rates, some have a thicker equator than their poles. So the size of the planet and the speed and direction of its rotation depend on the initial state of the material from which it is formed.
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