The nebular theory it explains the formation of our solar system and by extension of the other planetary systems. Its details were made known for the first time in 1796 thanks to Pierre de Laplace (1749-1827). According to the theory, the Sun and the other planets in the solar system originated in a slowly rotating cloud of gas and dust..
The force of gravity pushed the matter toward the center of the cloud, decreasing its extent. Then the speed of the cloud increased, to respect the conservation of angular momentum. And the temperature also increased.
Thanks to the rotation, most of the material condensed into a huge spherical central region, and the rest formed a disk around it..
Several forces came into play: gravity, which tends to accumulate matter towards the center, the friction between the particles and the pressure that is generated inside the cloud, which expands it..
From the central sphere originated the Sun, and from the disk of matter around it, called protoplanetary disk, the planets that we know arose, including Earth.
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From ancient times to the present day, numerous thinkers have speculated about how celestial objects were created.
Democritus of Abdera (460 - 370 BC) had already hinted at the idea that celestial objects originated in a vast region full of atoms.
Somehow the matter could be condensed and give rise to the Earth and the other stars.
Much later, the French mathematician René Descartes (1596-1650), presented a similar idea: there were vortices or eddies of matter, full of rotating particles. From one of them the matter arose that later condensed into the Sun and the planets.
This was stated in his treatise Le Monde, written between 1632 and 1633, whose publication was not immediate due to fear of the Inquisition. This work could only see the light after the death of its author.
Years later, in 1734, the Swedish thinker and mystic Emanuel Swedenborg (1688-1772) wrote a three-volume work called Essay on the Principles of Natural Things.
There he affirmed that the Earth and the other planets originated together with the Sun, and then separated from it. The text contained careful explanatory diagrams of the process.
Swedenborg's ideas served as inspiration to the German theologian Immanuel Kant (1724-1804) for his work The general history of nature and the theory of the heavens, published anonymously in 1755.
In it he tells about the nebula of dust that filled space. At one point the nebula began a rotary motion, which eventually led to the rise of the Sun and the planets..
However, it was Pierre Simon de Laplace (1749-1827) who gave a rigorous framework to the theory, providing it with a mathematical foundation. For this reason the nebular theory is also known as Kant-Laplace hypothesis.
By Laplace's time, Newton's laws of dynamics were firmly established and the observations of the astronomers William Herschel and Charles Messier were counted on. The latter had a catalog of celestial objects that contained numerous nebulae.
According to Laplace, nebulae condensed to give rise to stars. Contrary to what Kant believed, for Laplace the nebula was made of incandescent gas and not dust, and it was already animated by rotational movement.
The nebular theory is based on the law of universal gravitation and the conservation of angular momentum. According to her, the creation of the solar system occurred like this:
- In the beginning there was a large rotating gas cloud, which gradually condensed thanks to the force of gravity.
- As the radius of the cloud decreased, it was necessary to increase its velocity so that the angular momentum was constant. Something similar is what happens when a spinning skater contracts his arms: his speed increases immediately.
- Because of the rotation, the cloud took the shape of a disk, the nucleus of which was called protosol, from which the Sun arose, while a gas ring was formed from the external regions, which eventually detached itself, since as we know the objects that rotate at the edges have a tendency to be thrown.
This force was intensified enough to counteract the gravity pulling inward. Thus, the Solar System stopped contracting in one plane, but continued to contract in another perpendicular, and thus the solar nebula ended up as a disk..
The friction caused the region to become so hot that the gas began to undergo nuclear reactions, thus forming our Sun, although these processes were not known in Laplace's time..
- The nucleus contracted again and increased its speed of rotation, emitting a second gaseous ring and then a third and so on..
The dislodged rings were fragmented because of their uneven density, although later they were joined to form planetesimals (objects such as 1km in radius or little more), depending on planetesimal theory, derived precisely from nebular theory.
This theory ensures that planetesimals later gave birth to planets, satellites and other objects in the solar system, through the processes of accretion.
- Over time, the solar wind displaced the gas remaining from space between the planetary orbits and everything was as we see it today.
Currently it is considered that the theory satisfactorily explains why the planets follow a translational movement in the same direction, with elliptical orbits almost circular and in very close planes, since they originally came from the plane of the disk.
It is also consistent with the types of planets observed in the solar system: the small and rocky ones like Earth, closer to the Sun, and the outer gas giants..
However, the theory has some important limitations:
It does not explain the retrograde spin of planets like Venus and Uranus, as well as the numerous retrograde satellites out there..
Each planet has a different inclination on its axis of rotation.
It also happens that a rotating gaseous mass does not condense in a large object such as the Sun and therefore the size of the solar system should be much smaller.
Finally, the theoretical calculation of the angular momentum necessary for the gaseous rings to form is 200 times greater than that observed and almost everything should belong to the Sun..
However, most of the angular momentum of the solar system is held by the planets, although although the angular momentum of an isolated system is constant, it is possible that there would be a redistribution of it between the Sun and the planets..
To settle the matter, the astronomer Carl Von Weizsacker proposed in 1940 a different origin for the Sun and the planets, stating that the Sun formed first and then the disk of matter around it was created that gave rise to the planets..
This is perfectly possible, given that the first exoplanet was discovered orbiting a pulsar, which is a star remnant that catastrophically disappeared..
A planetary system of such a star would have been destroyed by the event, so everything indicates that this planet was formed much later or was somehow captured..
Other hypotheses propose that a star can get rid of its excess angular momentum by expelling jets of rotating matter that would end up in the protoplanetary disk..
So despite the drawbacks, the nebular theory has been successively modified thanks to new contributions, becoming the planetesimal theory..
It is the hypothesis accepted today by the vast majority of scientists to explain the origin of this and the other planetary systems observed in the universe, since the disks of matter have been detected, in effect, in young stars of recent formation, like the T-Tauri type.
But theories are always subject to continuous modifications as knowledge increases, and this is what happens with the nebular theory.
As more is known about the formation of extrasolar planets, the picture of how our own solar system formed will become clearer and clearer..
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