“Why are the planets round, not in any other shape?” Elon asks (9 years old)

When you think of the shape of a familiar celestial body, Earth, The sun, நிலா, PlanetsIs the sphere of recollection.

But once we see the key members of the Sun family, our curiosity will lead to seeing other stars. There, surprise: public law no longer applies. MarchThere are, for example, two satellites that are not spherical in shape. Their dimensions are in the tens of thousands of kilometers, and they look like large stones. The Holly’s CometHis last visit to Earth before 1986 was photographed on this occasion: it looked like a huge pebble ten kilometers long.

Why are large celestial objects spherical and not small? In their history, is there anything that seriously distinguishes each other?

Asteroids are not spherical
Asteroids are not spherical – NASA, CC BY (via 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. Today we understand it as follows: Following some violent events such as the eruption of a galaxy Supernova (The eruption of a giant star at the end of its life), a cloud of gas and dust begins to slide by itself.

This decline is accompanied by an increase in temperature at all levels: the temperature of the central star, whose temperature reaches several million degrees; That Rock planets Moreover, it shrinks and reaches a temperature of thousands of degrees; And even gaseous planets far from the sun. Our initial question becomes: why a liquid or gaseous celestial body sphere?

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

Oil example

1st Experience: Pour a little olive oil (slowly) in a glass of water; It is well known that oil forms a film on the surface of water.

2nd test: Pour a little oil (slowly) over a glass of rubbing alcohol; We see the oil sink into the bottom of the glass and form an image on the bottom of the glass.

Oil is less dense than water and more concentrated than alcohol. In the first case, it is subject to a force Archimedes drive More than its weight, it floats to the surface. Second, this drive is not enough and the oil is down. What happens if you (slowly) pour water into a glass of alcohol? As water and alcohol mix, the density of the mixture increases slightly, and the Archimedean impulse of the mixture increases, a time when the mixture and the oil have the same density. What form will the oil take at this time?

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

Left: Oil film on the bottom of a glass of alcohol (yellow reflection of the surface is only a reflection of the background) - Right: Oil drops oil in a water-alcohol mixture of the same density
Left: Oil film on the bottom of a glass of alcohol (yellow reflection of the surface is only a reflection of the background) – Right: Oil droplets in a water-alcohol mixture of the same density as the oil – Jacques Treiner (via conversation)

What does this experience tell us? Oil molecules attract each other, and they are subject to the Earth’s gravitational pull. When the water-alcohol mixture has the same density as the oil, everything happens as if gravity were suppressed, as Archimedes compensates for the weight, and under these conditions, the oil takes on a spherical shape. This is a very compact format.

The physics problem has become a problem Geometry : What is the “most abbreviated form”? Form of the smallest surface area for a given volume, or, equivalently, shape of the largest size for a given surface. It can be shown that the sphere meets these two possible definitions.

A fluid, subject only to internal keys, always accepts a spherical structure. This is why Earth-like rocky planets form in liquid form and have a spherical shape. And why like products that are always solid Asteroids And this CometsNot spherical in shape.

What about gaseous celestial objects? On Earth, a gas occupies all the volume it is given, and gravity is not enough to play a role. But when a large amount of gas is involved, it is different, the force of gravity can hold it in a small form. Starting with the sun, or ThursdayIts mass is one thousandth of that of the Sun (and about 300 times that of Earth), or that of Saturn, 100 times that of Earth.

This analysis was written by Jack Trainer, a theoretical physicist at the University of Paris.
The original article was published on the site Conversation.

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