The life cycle of a planet usually begins when the gravity of the star attracts clouds of rocks and dust until it forms into a blob that attracts more matter towards its center. 
Since gravity works in all directions equally, this is the reason why planets are round-shaped instead of any other form.

The Existence of Round Planets

Although the roundness of the planets is not perfect, every planet belongs in its own spherical category, like the Earth is an "oblate spheroid."
Due to the differences in the planet's sizes, bigger planets such as Saturn may gather extra mass around its middle area and get thicker.

Imperfectly Perfect-Sphered Planets

The idea of a cube-shaped moon is not entirely impossible. However, for it to happen, it will take a much longer time before we are able to see one. 
Asteroids and moons are generally smaller than planets therefore it goes to show that they both have lesser gravity thus they are less round than planets.

The Shape of Moons and Asteroid

The QI Elves explain the part that gravity plays in making planets round, and why our Earth is carrying a little extra weight around the middle.

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Personal Development

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Science & Nature

The QI Elves explain the part that gravity plays in making planets round, and why our Earth is carrying a little extra weight around the middle.

Published: <time datetime="2021-01-06T11:54:40+00:00" dir="ltr">06th January, 2021 at 11:54 </time>

Our Universe has at least one sextillion <a dir="ltr" href="http://sciencefocus.com/tag/planets">planets </a>. A sextillion is a number so huge, it’s virtually impossible to imagine, but to put it into some kind of context, the Earth weighs about a sextillion times more than a polar bear. It’s a big number.

But every single one of the sextillion planets in the Universe is round. There isn’t a single cuboid, dodecahedral or Toblerone-shaped one among them.

A planet begins as a cloud of rocks and dust particles rotating around a star. The particles attract each other due to gravity, until they form a single blob. As that blob attracts more matter, it gets bigger and bigger, until it has collected everything in its path.

Since gravity works in all directions equally, the blob becomes round. Imagine a crowd of a thousand people all trying to get as close as possible to a circular bar in a pub. If the queue was six deep on one side of the bar and two deep on the other, then new arrivals would join the shorter queue and eventually the queues would even out, leaving a circular throng of thirsty punters. That’s how gravity works in this case.

The roundness is not perfect. <a dir="ltr" href="https://www.sciencefocus.com/tag/the-earth/">The Earth </a>, for example, is an ‘oblate spheroid’: it’s nearly spherical but has a bulge at the middle. This is because of the planet’s spin.

Every time the Earth rotates once on its axis, the middle travels further than the top. This means that land at the equator is moving faster than land at the poles, and the faster something spins, the more it is thrown outwards.

Just like the rest of us, once it reaches a certain age, a new planet always gets a bulge around its middle. The Earth’s spare tyre is not very big: a hole through the planet would be only about 43 kilometres longer if it started at the equator than if it was pole to pole.

The effect is much bigger on other planets that are not quite so solid. <a dir="ltr" href="https://www.sciencefocus.com/tag/saturn/">Saturn </a>, which is largely made of gas, has a diameter that is 11,000 kilometres longer between two points on its equator than between its poles. This extra thickness can yield dramatic results too: if a planet gathers enough extra mass around its middle, the whole lot can be flung into space, eventually forming into a moon. But if you’re hoping for a cube-shaped moon, you’ll be waiting for quite a while.

Asteroids and moons are smaller than planets, so they have less gravity, and this means they’re less round. Comet 67P, pictured above, is a trillionth of the weight of Earth (it weighs about the same as a billion polar bears) and is shaped like a very odd rubber duck.

Why are all planets round?

The Existence of Round Planets

Imperfectly Perfect-Sphered Planets

The Shape of Moons and Asteroid

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