“Why are the planets round and have no other shape? Eloan (age 9) asks

When one thinks of the shape of a familiar celestial object, Earth, Sun, Moon, planets, this is the circle that comes to mind.

But once we see the main members of the Sun family, our curiosity can lead us to visit other stars. And there, surprisingly: the general law seems to no longer apply. For example, Mars has two non -spherical satellites. Their dimensions are ten kilometers, they look like big rocks. Halley’s comet, whose last visitation near Earth began in 1986, was photographed on this occasion: it also resembled a large rock about ten kilometers long.

Why do the big celestial objects circle while the small ones do not? Is there, in their history, something radically differentiating them from each other?

Asteroids are not spherical – NASA, CC BY (via The Conversation)

Gas and dust …

The answer is yes and depends on how a star system is formed: a star and all the objects around it. Our understanding of it now is as follows: after some violent event in a galaxy, such as the explosion of a supernova (explosion of a large star at the end of its life), a cloud of gas and the dust begins to crumble. of himself.

This collapse is accompanied by an increase in temperature at all levels: that of the central star, whose temperature reaches several million degrees; that of rocky planets also, which, by compacting, reach temperatures of thousands of degrees; and even gas planets farther away from the Sun. Our first question became this: why does a liquid or gaseous celestial body revolve?

As a result, we can observe around us the forms taken up by liquids and gases. Let’s start with a liquid. Here is a series of simple experiments that everyone can do in their kitchen that answers the question.

The example of oil

First experience: pour (slowly) a little olive oil in a glass of water; it is well known that oil forms a film on the surface of water.

2nd experiment: pour (also gently) a little oil into a glass of rubbing alcohol; we can see that the oil sinks to the bottom of the glass and forms a film under the glass.

Oil is less dense than water and more dense than alcohol. In the first case, it passes through a force called buoyancy that is greater than its weight, which causes it to float to the surface. Second, this push is not enough and the oil stays low. What happens now if you (slowly) pour water into a glass of alcohol? As the water and the alcohol mix, the density of the mixture gradually increases, the Archimedean thrust of the mixture also increases and there comes a time when the mixture and the oil have the same density. What form does oil take today?

Look: we got beautiful spherical drops of oil floating in the mixture!

Left: oil film under a glass of alcohol (the yellow of the face is the same reflection in the background) - Right: drops of oil in a water -alcohol mixture of the same density as' oil
Left: oil film under a glass of alcohol (the yellow of the face is the same as the background mirror) – Right: drops of oil in a water -alcohol mixture of the same density of oil – Jacques Trainer (via The Conversation)

What does this experience teach us? The oil molecules attract each other and they are also under the gravity of the Earth. If the water-alcohol mixture has the same density as the oil, everything happens as gravity is restrained, because Archimedes’ pushing changes the weight, and it is known that under these conditions, the oil has a circular shape. This is the most compact form possible.

The problem of physics becomes the problem of geometry: what exactly is meant by the “most compact form”? It is the shape that, for a given volume, has the smallest surface, or, similarly, the shape that, for a given volume, has the largest volume. It can be shown that this is the sphere that meets these two possible meanings.

A fluid, subjected to internal force alone, always adopts a spherical configuration. This is why rocky planets, such as the Earth, formed in the liquid state, have a circular shape. And also why things that are always solid, like asteroids and comets, are not spherical.

What about gaseous celestial objects? On Earth, a gas occupies all the volume it has to offer, gravity is not enough to play a role. But if a large mass of gas is involved, then it is different, gravitation is able to hold it in a compact form. Starting with the Sun, or Jupiter, whose mass is one thousand times that 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, theoretical physicist at the University of Paris Cité.
The original article was published on the site at The Conversation.

Leave a Comment