Why do planets all have the same shape?


Sometimes scientists like to describe the Earth as a blue marble because it is round and contains a lot of water. The oceans of the Earth are very special: other places in the solar system have no seas or no trace of liquid water is locked up under miles of ice. But the roundness of our planet is another story. From Mercury to Neptune, our neighbors are shaped like giant balls. Even the worlds beyond our solar system are pretty round.

To understand why, we must go back to the birth of a planet – about 4.5 billion years in the case of the Earth. Planets form in clouds of dust around new stars. When dust particles collide, they clump together, forming tufts larger and larger.

As the planet grows like Earth, its gravity becomes stronger. The gravity of the Earth is why we do not float in space; when we jump in the air, it brings us back to the ground. All objects in the universe, including yourself, tug all the rest because of gravity. But it is only when an object becomes really huge (like the Moon or the Earth) that we can feel this tightness.

Finally, a whole new planet becomes so big that its gravity is powerful enough that its surface really deforms. It's like a cardboard collapsing if you sit on it, says Mark Sykes, CEO and Director of the Planetary Science Institute in Tucson, Arizona. On a new planet, this occurs from all directions at the same time, so that the planet is crushed into a rounded shape.

Many other things in the space are also round because of their gravity, like the Sun and our Moon. An object must be big enough – about 600 km wide – for this to happen. Small things like asteroids and comets, which have a lower gravity, can have rather strange shapes. Saturn's moon, Prometheus, looks like a potato. A comet-shaped rubber duck also floats around our solar system.

But even the Earth is not a perfect sphere. As the planet rotates, the earth and water sink into space, much as one must stick to a ride to stay in place faster than it turns. Earth's gravity is powerful enough to keep everything in place while it's spinning, but it just overflows at the equator.

The moon does not help, further deforming the shape of the Earth. The tides on this blue marble are the result of the gravity of the moon pulling on our oceans. But even the solid earth spills a little more in the middle.

Every object in the space experiences its own pushes and prints. Mars is running as fast as the Earth, but with less gravity, it overflows even more, says Mark Panning, scientist in planetary sciences at NASA's Jet Propulsion Laboratory in Pasadena, California. The dwarf planet Haumea, which lies beyond Neptune, rotates so fast that it has more the shape of a football than a basketball.

Keep in mind however that in the grand scheme of a giant planet, it is really tiny imperfections. If you look at the Earth or even Mars of the eye, they will look nicely round.