Voyager: Why is the Earth Round?

Q. If massive boulders came together to form the earth, wouldn’t the planet appear from space as a big bunch of chunks? Why does it appear so smooth?

– Submitted by Frank D., 13, San Diego, Calif.

When the Solar System began to take shape some four and a half billion years ago, a disk of swirling gas and dust rapidly separated into increasingly larger “clumps,”ultimately forming planetary bodies, including Earth. This is similar to a ‘snow ball’ effect, with gravity as the driving force behind planetary growth. The increasing gravitational attraction of more massive objects led to them becoming larger at the expense of smaller objects.

The increasing size of planets led to enormous amounts of heating and melting. Heating and melting occurred through “gravitational accretion,” which is when gravity causes material in a disc to spiral inward towards a central body. The fast decay of certain types of common elements such as aluminum, and the pooling of metal to form the cores of planets also contributed to heating and melting. Combined with gravity, which compresses bodies into shapes that evenly distribute gravitational force among their mass, the result is planets that are approximately spherical in shape.

The size and time that the body formed is also important. Objects in space smaller than about 100 miles in diameter, such as most asteroids, lacked the mass to melt, or formed too late for radioactive decay to heat them, and so the weak gravitational force acting upon them could not compress them into spheres. That’s why some asteroids have weird and wonderful shapes.

– James Day, assistant professor, Geosciences Research Division

Related Image Gallery: Voyager: Why is the Earth Round?