Astronomy enthusiasts know well that besides the Sun, which dominates local gravity, and our own planet, there are a variety of classifications for the bodies in the Solar System. There are rocky planets, like Earth itself, as well as Mercury, Mars and Venus. Larger and more distant from our star, there are gaseous planets, like Jupiter, Saturn, Uranus and Neptune. There are also dwarf planets, like Ceresand Pluto, and asteroids, either concentrated in the famous belt between Jupiter and Mars, or in other places. And, of course, the moons that orbit many of the planets. From Pluto beyond, there is also the Kuiper Belt, where rocky bodies are concentrated. And, also at the edge of the system, the Oort cloud. It is from this distant and icy region, moreover, that comets originate.

Several of the objects mentioned have similarities among themselves. Sometimes, the classifications even intersect. As, for example, in the case of asteroids and the recent label of dwarf planet, based on certain technical criteria. Observing models of the movements of bodies, however, we can notice what seems to be a coincidence. The objects, or at least the larger ones, such as planets,orbit in the same plane, that is, as if one orbit were contained within the other, with no inclination between them. This, however, is not just an impression, nor does it occur by chance. The explanation actually takes us back to the beginning of the Solar System.

The primordial cloud

The orbital plan of the planets of the Solar System had already been noticed long ago by scientists. In fact, it was one of the evidences for the formulation of the first theories about the origin of our star system. In Europe of the 17th and 18th centuries, science was called natural philosophy. It was even common for great names of philosophy to venture into scientific investigations and vice-versa.For this reason, it was precisely with the philosopher René Descartes that scientific ideas about the origin of our surroundings first took shape on the continent. For a time, it was thought that the matter that gave rise to the planets had been torn from the Sun in an impact. Later, other thinkers such as Kant and Laplace gave voice to the idea of the primordial nebula, or cloud. Thisidea is the source of our current model of the origins of the Solar System.

Image: ESO/VISION survey

Today, we know the primordial cloud as a large, massive agglomeration of rotating dust and gas. It hovered around the universe about 4.5 million years ago. It is also thought to have been tens of thousands of astronomical units (AU) in diameter. To get a better idea of what that represents, 1 AU is equivalent to the approximate distance between the Earth and the Sun. That is, about 150 millionThe size scale was so large that as more matter continued to add, the cloud collapsed by its own gravity. As it collapsed, the nebula flattened out, like a disk. This was because, by the principle of conservation of angular momentum in physics, if the radius of rotation decreases, the speed of rotation must increase.

Planets in formation

In the center of the cloud, the gas molecules accumulated. With the increase of temperature and pressure, the atoms, mostly of hydrogen, began to collide. The conditions became sufficient for their nuclei to fuse, starting a successive process of nuclear combustion. And thus giving rise to what would be the Sun. The rest of the disk generated by the primordial cloud, still incollapse, it also flattened out and expanded in the process. Eventually a thin, long structure, called a protoplanetary disk, formed.

Image: ALMA

As the Sun grew, in addition to heating up the region, the star collected gas from its surroundings. The central part of the Solar System, therefore, tended to be empty of hydrogen and helium, leaving dust particles. These continued traveling through the protoplanetary disk and eventually colliding with each other. Over millions of years, the solid matter was gathering, forming particles each timeFrom this process, the rocky planets were formed. In the outermost region of the disk, however, the Sun's influence was smaller. There, therefore, the gaseous planets were formed by the accumulation, collapse and gravitational balance of gas accumulations.

Anyway, the fundamental point to answer the question of the orbital plane is the thin protoplanetary disk. It is from its structure that the planets formed. Just as it is from its rotation that the planets started to orbit the Sun. And, just like for the Solar System, similar processes are forming at this very moment other star systems throughout the universe.

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