Why are planets all the same shape? Gravity makes the Earth round.

Pedro_P

Our home planet, as seen by astronauts in the 1970s.
NASA
▼ Sometimes scientists like to describe Earth as a blue marble because it’s round and has so much water. Earth’s oceans are pretty special—other places in the solar system have no seas, or any trace of liquid water is locked up  under miles of ice. But our planet’s roundness is another story. From Mercury to distant Neptune, our neighbors are shaped like giant balls. Even worlds beyond our solar system are pretty round.
To understand why, we have to go all the way back to the birth of a planet—about 4.5 billion years in Earth’s case. Planets form in clouds of dust around new stars. As specks of dust collide, they stick together, forming bigger and bigger clumps.
As a planet like Earth grows, its gravity becomes stronger. Earth’s gravity is why we don’t float off into space; when we jump into the air, it pulls us back to the ground. Every object in the universe—including you—tugs at everything else because of gravity. But only when an object becomes really huge (like the Moon or Earth) can we feel this tugging.
Eventually, a brand-new planet gets so big that its gravity is powerful enough to make its surface actually crumple. It’s like how a cardboard box will collapse if you sit on it, explains Mark Sykes, CEO and director of the Planetary Science Institute in Tucson, Arizona. On a new planet, this happens from all directions at once, so the planet is crushed into a round shape. (▪ ▪ ▪)

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