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
A common physics factoid is that Saturn will float in water. This is because Saturn has a density of 687.00 kg/m³ and water has a density of 999.97 kg/m³. As Saturn has a lower density than water, it should float. This works with objects like ice cubes, which also have a lower density than water, so float. It makes sense on the surface, but scaling this up to planetary sizes makes make things behave differently. After spending some time reading about it, I’ve found that despite having a lower density than water, Saturn wouldn’t actually float, and here is why.
First of all, Saturn couldn’t float anywhere on Earth because there isn’t a body of water big enough on Earth for it. If you imagine an iceberg in the ocean, or an ice cube in a glass, a small amount sticks up above the water, and there is a massive amount below. Saturn would behave the same, but Saturn is so big that there isn’t a body of water on Earth big enough to accommodate the bit of Saturn below the water. So, first of all, we would need to find a planet which would hold a body of water deep enough.
Fortunately the guys over a Wired have done this calculation for me (so you can check their maths), but they say you would need a body of water as deep as 6x the Earth (sounds massive, but remember that Saturn is nearly 10 times bigger than the Earth – its huge!).
So if we assume we find a massive planet which can support this much water, we still have another problem. Having a body of water as deep as 6 Earths is going to put the water at the very bottom under an immense amount of pressure -much much more than can be replicated in a lab. What water would do under all this pressure is a mystery. The amount of pressure is massive its hard to comprehend, but I’ll try and give you some sense of scale. The depth of water would be about 7600km, and the deepest part of the ocean on Earth is the Mariana Trench, which is only 11km deep.
Usually under large amounts of pressure liquids would become solid, but water is unique in that its solid state floats on its liquid state, and this is much more pressure than is required to just chance a liquid to a solid. Its unknown what would happen to water under all this pressure, but I have come across a number of theories, some say you would end up with a superheated liquid with a reduced volume and others suggest you would end up with something called electron-degenerate matter, which from what I can tell, will probably result in a star forming – quite dramatic.
Either way, all theories suggest that the conditions created would be extremely unstable, and so unlikely to allow a gaseous giant to float on them. What’s more likely is that the chemicals which make up Saturn (which are largely hydrogen) would also undergo fusion reactions and contribute to the forming star.
Finally, if you manage to overcome the very real problem above, there is one more issue. Like our planet, Saturn has a core which is made up of liquid metal and rock, neither of which would float on water. In fact, most of Saturn is gas, mostly hydrogen, and so if you were to bring it onto a planet which could support a big enough body of water to support Saturn, the gravitational pull would probably disintegrate the gas, and the core would sink to the bottom of water – not float.
It might seem like Saturn would float on water because it has a lower density than water does, however, in practice, it will probably be a very different story.
First of all, the sheer volume of water that is needed would create a very unstable and reactive environment, which potentially could cause a star to form, which would probably destroy Saturn rather than keep it afloat.
Secondly, if you somehow solve the problem of the first point, the core of Saturn would fall to the bottom of the water, and the gas would probably disband – again, not really floating.
It really is a theoretical nightmare, and not something that my Physics A level really equipped me to solve, but it does seem very unlikely that Saturn would float in water.
This Youtube video will give an overview of the information found on the article tab. If you want to know more about the topic, or want to see where the information came from, have a read of the article after you watch the video.
No – the conditions are impossible. There is more to consider than just density.