# How asymmetric creates symmetry, or Why all planets rotate in the same plane

Scientists often say that anything can happen in an infinitely large universe. However, observations, calculations and simulations show that in star systems, planets always revolve around a star in the same plane and in the same direction. We find out why this is happening.

Order reigns in the solar system: four inner planets, an asteroid belt and four gas giants revolve around the sun in the same plane. And even if you go beyond these boundaries, it turns out that the Kuiper belt is also in this plane. Considering that the Sun has a spherical shape and stars appear in space, around which the planets revolve in any direction, the fact that everything in our system is arranged in this way seems too much of a coincidence. Moreover, we have observed that in almost every star system, the planets line up in the same way. Let's try to figure out what this is connected with.

To date, scientists have calculated the orbits of the planets with amazing accuracy. They found that celestial bodies revolve around the Sun in the same two-dimensional plane with a difference of no more than 7 °.

Moreover, if you remove from this equation Mercury - the planet closest to the Sun - it becomes noticeable how true everything else is ordered in relation to each other: deviations from the unchanging plane of the solar system are no more than two degrees.

In addition, the planets revolve around the Sun in the same direction in which it rotates around its axis. As you might have guessed, the Sun's axis of rotation is also within 7 ° of deviation compared to the orbits of all the planets in the system.

Nevertheless, it is difficult to imagine that everything turned out this way by itself, and not someone from the outside squeezed all the bodies into one system and made them move in one plane. Intuitively, one might assume that the orbits should be oriented randomly, since gravity works the same in all three (spatial) dimensions. It is also more likely to assume the formation of a swarm of scraps of matter than an ordered set of ideal circles. The fact is that if you move very far from the Sun - farther than planets and asteroids, farther than Halley's comet and the like, even go beyond the Kuiper belt - this is exactly what you will see.

So why did the planets end up in the same disk? Why are they all located in the same plane, and not randomly flying around the star? To understand this, you need to go back to the time when the Sun was just beginning to form from one of the molecular gas clouds from which all stars in the Universe are formed.

When a molecular cloud becomes massive enough, gravitationally bound, and cold enough to collapse and collapse under its own gravity - like the Tube nebula (top left) - it forms dense enough regions in which new star clusters appear (indicated by the circles in the picture, in the upper right corner).

You will immediately notice that this nebula, like any similar to it, does not have an ideal spherical shape, it is rather unusually oblong.Gravity does not tolerate imperfections, and due to the fact that it is an inertial force that quadruples with each decrease in the distance to a massive object by half, it perceives even small differences in the original form and significantly enhances them in a short time.

The result is a star-forming nebula with an asymmetrical shape: stars in it form in regions with the highest density of gas. But if we look inside it and look at individual stars, we will see that they are almost ideal spheres - like the Sun.

However, as the nebula itself became asymmetric, so the individual stars that formed in it were formed from superdense asymmetric clumps. These clumps collapse in one of three dimensions, and since the substance of which we are composed, atoms, atomic nuclei and electrons, is attracted to itself and interacts when colliding with another substance, the result is an oblong disc of matter. Yes, gravity will pull most of it toward the center of the disk where the star will form, but what scientists call a protoplanetary disk will form around it. And thanks to the Hubble Space Telescope, we were able to see these discs directly.

This is the first kind of hint, indicating that the result will be something ordered in one plane. To move on to the next step, we will have to turn to simulations, since we have not existed for long enough and simply have not had time to observe this phenomenon - and it takes about a million years - in a young star system.

After the protoplanetary disk is "flattened" in one dimension, it will continue to shrink with more and more matter entering its center. But, despite the fact that most of the material will be concentrated in it, a large part of the gas and dust will be released into stable rotating orbits in this disk.

Why? There is a physical quantity that must be conserved: angular momentum, which tells us how much the entire system is spinning - gas, dust, star and everything else. The way the angular momentum works and how it is equally distributed among all the particles in the system, in fact, indicates that everything in the disk should move, roughly speaking, in one direction - clockwise or counterclockwise. Over time, this disk will reach stable size and density, and then small gravitational instabilities will begin to turn these instabilities into planets.

Of course, there are small differences between the parts of the disc, as well as small differences in the initial conditions. A star forming in the center is not a single point, but rather an extended object - about a million kilometers in diameter. When you add all these parts together, you will not get an ideal plane, but something very close to it will come out. In fact, we recently found the first planetary system outside the solar, in which we were able to observe the process of the formation of young planets in the same plane.

The young star HL Tauri, located about 450 light years from Earth, is surrounded by a protoplanetary disk. The star itself is estimated to be about a million years old. Obviously, this is a disk in which everything is in the same plane, but there are dark "breaks" in it. Each of these breaks corresponds to a young planet that has attracted all the matter in its vicinity. It is not yet known which of them will eventually merge, which will be ejected from the disk, and which will move inside it and be absorbed by the parent star. Meanwhile, we had the opportunity to observe a turning point in the development of a young stellar system. And although earlier scientists were able to observe young planets, it was not possible to study this stage. All stages of the formation of a star system are amazing and correspond to the same story.

But why are the planets in the same plane? Because they are formed from an asymmetric gas cloud, which first collapses in the shortest direction, then the substance “flattens” and “sticks” to itself, and then contracts towards the center. But instead of falling on him, it begins to revolve around him. As a result, planets are formed from inhomogeneities in this young disk, which continue to rotate in the same plane with a difference of several degrees.

This is one of those cases where observations and simulations based on theoretical calculations are surprisingly consistent with each other. So, wherever you are in the Universe, any planets around any stars will always rotate in the same plane.