From the planets in our solar system, to pictures of far off and alien planets, planets are always seem very round, and smooth. Planets always look like a ball. Whether they are big or small, near a star or far away, gaseous or solid – planets are always round. You don’t find planets which are shaped like a potato (although asteroids are). In fact, if you were to shrink Earth to the size of a billiard/ pool ball, Earth would be smoother (but not quite as spherical). That is just how smooth planets are! So why are planets always round and smooth, and why aren’t asteroids?
Planets begin their life as space dust. This space dust will slowly clump together over many many many years, because it all has a very small gravitational pull on each other. As the clump forms, its gravitational pull becomes greater and greater, and so attracts more dust. This forms a rough rock clump, similar to the picture on the right, which will continue to attract more and more space dust/ rocks.
This rock clump’s gravitational force pulls from all sides equally to its centre. No single side is able to pull in more dust than the other, so the space dust builds up equally. I know, the space rock in the image isn’t smooth, yet. As the pre-planet rock gets bigger and bigger, its gravitational force gets bigger too, which will force dust and debris to find the lowest point closest to the centre (similar to you putting a rock on a steep hill, and it rolling down), which fills in any grooves or ditches.
One way to imagine this is if you have a rough and sharp stone, and each day you dip it in thick paint. The more you dip it in the paint, the more its rough and irregular features smooth out. Over time, the sharp paint covered stone would begin to look smooth and round. It is a very similar thing that happens here.
Sometimes, these clumps of space dust/ early planets collide at great speed. This can cause the rocks and dust to become molten. The liquid rock will form a very smooth surface, and the gravity will naturally cause it to go in a sphere. Remember, liquids always try and find their lowest level, and as the centre of gravity is in their middle, they will form evenly around it. As the rock cools, it will maintain a very circular shape. This cooling of the molten rocks is thought to be a cause of the mountains and ditches on Earth, because when liquids cool, it tends to wrinkle/ crack.
Planets may be smoother than a billiard ball, but they are not as round as one. If planets didn’t rotate on an axis, they would be pretty round. However, they do rotate, and quite fast! When they rotate the bits of the planet around the equator have to more extremely fast (and the poles move relatively slowly). This is because the poles have a much smaller distance to travel than the equator does.
To put this into perspective, imagine that someone has to walk around the North Pole in a day, and another person has to walk around the equator in a day. The person at the equator would have to go over 1,000 miles an hour, whereas the person on the pole can take their time. They only have to do a 360 degree turn, and have 24 hours to do so!
The spinning force of planets a the equator is very strong, but the gravity of the planet is pulling the planet in, and this causes the planet to bulge around the equator. This impacts Saturn the most of all the planets in our solar system, and Saturn’s equator is 10.7% wider than its height form pole to pole. In comparison, Earth’s equator is only 0.3% wider.
Asteroids are neither round or smooth simply because they are too small. This means their gravitational strength is too weak to force a circular shape. As their mass increases, they will slowly become more round, like a planet.
Planets are always round because of the gravitational forces that form them. Gravity pulls objects down to a central point, and as the gravity pull is equal, so is the distribution of space dust on the planet. Additionally, planets can go through molten stages in their formation, where their rock melts. Liquids will always find a level ground around the strongest gravitational pull, and so form a sphere around their centre.
The rotation of planets does cause their equator to bulge out though, making some planets, like Saturn, more like an oblong that a sphere.
Image courtesy of This is Yu