“Why are the planets round and never have a different shape? Eloan (9 years old) asks

“Why are the planets round and never have a different shape?  Eloan (9 years old) asks

As soon as one remembers the shape of a famous celestial object, Earth, sun, Moon, planetsis the realm that comes to mind.

But when we see the main members of the solar family, our curiosity can lead us to visit other stars. And here’s a surprise: the general law no longer seems to apply. Marchfor example, it has two non-spherical satellites. Their dimensions are in the tens of kilometers, they look like big stones. The Halley’s Cometwhose last visit near Earth dates back to 1986, was photographed on this occasion: it is also like a large pebble about ten kilometers long.

Why are large celestial objects spherical while small ones are not? Is there anything in their history that radically sets them apart?

Asteroids do not have a spherical shape
Asteroids are not spherical – NASA, CC BY (via The Conversation)

Gas and dust …

The answer is yes, and it depends on how the star system is formed: the star and all the objects that revolve around it. Today we understand this: after some violent event in the galaxy, such as the explosion and supernova (explosion of a big star at the end of her life) a cloud of gas and dust begins to collapse on herself.

This collapse is accompanied by an increase in temperature at all levels: the central star, whose temperature reaches several million degrees; of which stone planet also by compaction reaching temperatures of thousands of degrees; and even gaseous planets farther from the sun. Our initial question turns into this: why is a liquid or gaseous celestial body spherical?

As a result, we can observe the forms that liquids and gases take on. Let’s start with the fluid. Here are a series of simple experiments that everyone can do in their kitchen to answer the question.

An example of oil

1st experience: pour (gently) a little olive oil into a glass of water; It is well known that oil forms a film on the surface of water.

Experiment 2: pour (also gently) a little oil into a glass of alcohol; we see that the oil drops to the bottom of the glass and forms a film at the bottom of the glass.

The oil is less dense than water and denser than alcohol. In the first case, the force of the so-called Archimedes’ move greater than its weight, causing it to float to the surface. In the second, this thrust is not enough and the oil remains at the bottom. What happens when you (gently) pour water into a glass of alcohol? As water and alcohol mix, the density of the mixture gradually increases, so does the Archimedes’ draft, and the time comes when the mixture and oil have the same density. What form is the oil taking at the moment?

Look: we get beautiful spherical drops of oil that float in the mixture!

Left: oil film at the bottom of the glass with alcohol (yellow surface is only a reflection of the background) - Right: oil droplets in a mixture of water and alcohol of the same density as' oil
Left: oil film at the bottom of the glass with alcohol (yellow surface is only a reflection of the background) – Right: oil droplets in a mixture of water and alcohol of the same density as the oil – Jacques Treiner (via The Conversation)

What does this experience teach us? Oil molecules are attracted to each other and also subject to the Earth’s gravity. When a mixture of water and alcohol has the same density as oil, everything happens as if gravity is suppressed, because Archimedes’ pull compensates for the weight, and it is found that under these conditions, the oil takes on a spherical shape. This is the most compact form possible.

The problem of physics has become a problem geometry : what exactly does “most compact form” mean? It is the shape that has the smallest surface for a given volume, or equivalently the shape that has the largest volume for a given surface. It can be shown that it is a sphere that meets these two possible definitions.

A fluid that is only affected by internal forces always has a spherical configuration. This is why rocky planets such as Earth, which formed in a liquid state, have a spherical shape. And also why items that have always been solid, like asteroids and cometsthey are not spherical.

And what about gaseous celestial objects? On Earth, gas occupies all the volume that is offered to it, gravity does not play a role here. But when it comes to a large mass of gas, it’s different, gravity can keep it in a compact form. Starting with the Sun, or Jupiterwhose mass is one thousandth the mass of the Sun (and about 300 times that of the Earth), or Saturn, whose mass is about 100 times that of the Earth.

This analysis was written by Jacques Treiner, a theoretical physicist at the University of Paris Cité.
The original article was published on the web Conversation.


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