Since time immemorial, humanity has been trying to answer the question of how the universe came into being. However, they began to seriously deal with this issue only with the beginning of the scientific revolution, when theories began to dominate in the world, the proof of which was carried out empirically. It was from that moment - the interval between the 16th and 18th centuries - that astronomers and physicists began to derive evidence-based explanations of how the life of our Sun, planets and the entire Universe began.
When it comes to the solar system, the nebular hypothesis of the origin of worlds is the most popular and widely accepted view. According to this model, the Sun, planets and all other objects in the solar system were formed many billions of years ago from dense clouds of molecular hydrogen. Originally proposed as an explanation for the origin of the solar system, it remains the most widely accepted.
Nebular hypothesis
According to this model, the Sun and all the planets of our solar system began their history with a giant molecular cloud of gas and dust. Then, about 4.47 billion years ago, something happened that caused the cloud to collapse. Perhaps the cause was a passing star or supernova blasts, no one knows for sure, but the end result was a gravitational collapse at the center of the cloud.
From that moment on, denser clumps began to form from clouds of gas and dust. Having reached a certain density, the bunches began to rotate according to the law of conservation of momentum, and the increasing pressure warmed them up. Most of the matter gathered in the central clot, while the remaining matter formed a ring around this clot. The clot in the center eventually turned into the Sun, and the rest of the matter formed a protoplanetary disk.
The planets were formed from the matter of this disk. Particles of dust and gas attracted to each other gathered in larger bodies. Near the Sun, only those clumps with the highest concentration of metals and silicates were able to form into denser objects. This is how Mercury, Venus, Earth and Mars appeared. Because metallic elements were weakly present in the primary solar nebula, the planets were unable to grow very much.
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In turn, such giant planets as Jupiter, Saturn, Uranus and Neptune were formed somewhere in the point between the orbits of Mars and Jupiter - somewhere beyond the freezing temperatures, where the material freezes so much that it allows volatile compounds to maintain a solid form in the form of ice. The variety of this ice turned out to be much wider than the variety of metals and silicates from which the planets of the inner part of the solar system were formed. This allowed them to grow so huge that they eventually had entire atmospheres of hydrogen and helium. The remaining material, which was never used to form planets, concentrated in other regions, eventually forming the asteroid belt, Kuiper belt and the Oort cloud.
The early solar system as seen by the artist. The collision of particles in the accretion disk led to the formation of planet earths and ultimately planets
Over the next 50 million years, the pressure and density of hydrogen at the center of the protostar became high enough to trigger a thermonuclear reaction. Temperature, reaction rate, pressure and density continued to increase until hydrostatic equilibrium was reached. From that moment on, the Sun turned into a main sequence star. The solar winds created the heliosphere, sweeping away the remaining gas and dust from the protoplanetary disk into interstellar space and marking the end of the planetary formation process.
History of the nebular hypothesis
For the first time, the idea that the solar system was formed from a nebula was proposed in 1734 by the Swedish scientist and theologian Emmanuel Swedenborg. Immanuel Kant, familiar with Swedenborg's work, took up the further development of the theory and published the results in his work "General Natural History and Theory of the Sky" in 1755. In it, he stated that gas clouds (nebulae) rotate slowly, gradually collapse and, under the influence of gravity, shrink, forming stars and planets.
A similar but less detailed model of formation was proposed by Pierre-Simon Laplace and described in Exposition of the System of the World, which was published in 1796. Laplace theorized that the Sun originally had an atmosphere expanded to the entire solar system, and at some point this "protostellar cloud" began to cool and shrink. With the increase in the speed of rotation of the cloud, it threw out excess matter, from which the planets were subsequently formed.
The Sh 2-106 nebula. Compact star-forming region in the constellation Cygnus
Laplace's nebular model gained widespread acceptance during the 19th century, although it did contain some obvious inconsistencies. The main issue was the angular distribution of momentum between the Sun and the planets, which the nebular theory did not explain. In addition, Scottish scientist James Clerk Maxwell (1831–1879) argued that the difference in rotational speed between the outer and inner parts of the protoplanetary disk would prevent matter from accumulating. In addition, the theory was also rejected by the astronomer Sir David Brewster (1781-1868), who once said:
“Those who believe that the nebular theory is correct, and are confident that our Earth received its solid form and atmosphere from a ring thrown from the solar atmosphere, which was subsequently enclosed in a solid terraqual sphere, most likely believe that the Moon was formed in the same way. [If viewed from this point of view], then the Moon must also have water and its own atmosphere."
By the end of the 20th century, the Laplace model had lost credibility in the face of scientists and forced the latter to start searching for new theories. This began, however, not earlier than the very end of the 60s, when the most modern and most widely recognized version of the nebular hypothesis appeared - the solar nebular disk model. The merit belongs to the Soviet astronomer Viktor Safronov and his book "The Evolution of the Preplanetary Cloud and the Formation of the Earth and Planets" (1969). This book describes almost all the basic questions and mysteries of the planetary formation process, and most importantly, the answers to these questions and mysteries are clearly formulated.
For example, the preplanetary cloud model successfully explains the appearance of accretion disks around young stellar objects. Multiple simulations have also shown that accretion of matter in these disks leads to the formation of several Earth-sized bodies. Thanks to Safronov's book, the question of the origin of the terrestrial planets (or terrestrial planets, if you like) can be considered solved.
Despite the fact that initially the preplanetary cloud model was applied only to the solar system, many theorists believe that it can be used as a universal system of measures for the entire universe. Therefore, even now it is often used to explain the formation process of many exoplanets that we have found.
Disadvantages of the theory
Although the nebular model is widely accepted, it still contains a number of questions that even modern astronomers cannot solve. For example, there is a question related to tilt. According to the nebular theory, all the planets around the stars should have the same axis tilt with respect to the plane of the ecliptic. But we know that the planets of the inner and outer circles have completely different axis tilts.
While the planets of the inner circle have tilt angles of 0 degrees, the axes of the others (Earth and Mars, for example) have tilt angles of about 23.4 and 25 degrees, respectively. The planets of the outer circle, in turn, also have different axis tilts. Jupiter's axis tilt, for example, is 3.13 degrees, while Saturn and Neptune's are 26.73 and 28.32 degrees, respectively. And Uranus generally has an extreme axis tilt of 97.77 degrees, which actually forces one of its poles to constantly face the Sun.
List of potentially habitable exoplanets according to Planetary Habitability Laboratory
In addition, studying planets outside the solar system has allowed scientists to note inconsistencies that cast doubt on the nebular hypothesis. Some of these inconsistencies are related to the "hot Jupiters" class of planets, whose orbits are close to their stars, and the period of several days. Astronomers have adjusted some points of the hypothesis to solve these questions, but this did not solve all the problems.
Most likely, the unresolved questions are closest to understanding the nature of formation, and therefore it is so difficult to answer them. It's just that when we think that we have found the most convincing and logical explanation, there are always moments that we are not able to explain. Nevertheless, we have come a long way until we arrive at our current models of star formation and planetary formation. The more we learn about neighboring stellar systems and the more we explore space, the more mature and sophisticated our models become.
NIKOLAY KHIZHNYAK