Since time immemorial, mankind has been tryinganswer the question of how the universe appeared. However, they began to seriously study this issue only with the beginning of the scientific revolution, when theories began to dominate in the world, the evidence of which was carried out empirically. It was from this 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.
If we are talking about the solar system, then the mostA popular and widely accepted view is the nebular hypothesis of the origin of worlds. According to this model, the sun, planets and all other objects of the solar system formed many billions of years ago from dense clouds of molecular hydrogen. Originally proposed as an explanation of the origin of the solar system, it remains the most widely accepted.
According to this model, the sun and all the planetsOur solar system began its history with a giant molecular cloud of gas and dust. Then, about 4.47 billion years ago, something happened, which led to the collapse of the cloud. Perhaps the reason was a passing star or supernova explosive waves, no one knows for sure, but the end result was a gravitational collapse in the center of the cloud.
From this moment, clouds of gas and dust begandenser clots form. Having reached a certain density, the clots, according to the law of conservation of momentum, began to rotate, and their increasing pressure warmed up. Most of the matter gathered in the central bunch, while the remaining matter formed a ring around this bunch. The clot in the center eventually turned into the Sun, and the rest of the matter formed a protoplanetary disk.
The planets are 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 in which the highest concentration of metals and silicates were able to form into denser objects were formed. Thus appeared Mercury, Venus, Earth and Mars. Since metallic elements were weakly present in the primary solar nebula, the planets could not grow very much.
In turn, such giant planets asJupiter, Saturn, Uranus and Neptune have already formed somewhere at a point between the orbits of Mars and Jupiter - somewhere beyond the freezing temperatures, where the material freezes so much that it allows the 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 in the end they had whole atmospheres of hydrogen and helium. The remaining material, which was never used to form planets, concentrated in other regions, eventually forming the asteroid belt, the Kuiper belt, and the Oort cloud.
The early solar system in the view of the artist. Collision of particles in the accretion disk led to the formation of planet Earths, and ultimately the planets
Over the next 50 million years, pressure andThe hydrogen density at the center of the protostar has become high enough to start 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 star of the main sequence. Solar winds created the heliosphere, sweeping away the gas and dust remaining from the protoplanetary disk into interstellar space and marking the completion of the planetary formation process.
History of the Nebular Hypothesis
First idea that the solar systemformed from the nebula, was proposed in 1734 by the Swedish scientist and theologian Emanuel 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) slowly rotate, gradually collapse and contract under the influence of gravity, forming stars and planets.
A similar but less detailed modelformations was proposed by Pierre-Simon Laplace and described in the work "Presentation of the world system", which was published in 1796. Laplace theorized that the Sun originally had an atmosphere extended to the entire solar system, and at some point this "protostellar cloud" began to cool and decrease. With an increase in the speed of rotation of the cloud, it threw out excess matter, from which planets subsequently formed.
Nebula Sh 2-106. A compact star-forming region in the constellation Cygnus
The nebular Laplace model received widerecognition during the 19th century, although it contained some obvious inconsistencies. The main question 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 rotation speed between the outer and inner parts of the protoplanetary disk would not allow matter to accumulate. In addition, the theory was also not accepted by the astronomer Sir David Brewster (1781–1868), who once said:
"Those who believe that the nebular theory is true, andWe 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 terracal sphere, most likely, they believe that the Moon formed in the same way. [If viewed from this point of view], then the Moon also must have water and its own atmosphere. "
By the end of the 20th century, the Laplace model losttrust in the face of scientists and made the latter begin to search for new theories. True, this did not begin until 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 Pre-Planetary Cloud and the Formation of the Earth and Planets" (1969). This book describes almost all the basic questions and puzzles of the planetary formation process, and most importantly - the answers to these questions and puzzles are clearly formulated.
For example, a pre-planet cloud model successfullyexplains the appearance of accretion disks around young stellar objects. Multiple simulations have also shown that the accretion of matter in these disks leads to the formation of several bodies the size of the Earth. Thanks to Safronov’s book, the question of the origin of the terrestrial planets (or earth-like, if you like) can be considered resolved.
Despite the fact that the original modelthe pre-planet cloud was used only in relation 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 process of formation of many exoplanets that we have found.
Despite the fact that the nebular model haswidely recognized, it still contains a number of issues that even modern astronomers cannot solve. For example, there is a tilt related question. According to the nebular theory, all planets that are around stars should have the same axis tilt with respect to the ecliptic plane. But we know that the planets of the inner and outer circles have completely different axes.
While the planets of the inner circle possessthe axis tilt angle of 0 degrees, the axes of others (Earth and Mars, for example) have a tilt angle of about 23.4 and 25 degrees, respectively. The planets of the outer circle, in turn, also have different axes. The inclination of the axis of Jupiter, for example, is 3.13 degrees, while those of Saturn and Neptune are 26.73 and 28.32 degrees, respectively. And Uranus generally has an extreme tilt of the axis of 97.77 degrees, which actually makes one of its poles constantly face the Sun.
List of potentially habitable exoplanets according to the Planetary Habitability Laboratory
In addition, the study of planets outside the solar systemallowed scientists to point out inconsistencies that cast doubt on the nebular hypothesis. Some of these inconsistencies are related to the class of planets “hot Jupiters”, whose orbits are close to their stars, and for a period of several days. Astronomers corrected some points of the hypothesis to solve these issues, but this did not solve all the problems.
Most likely, unresolved issues havethe closest meaning to understanding the nature of formation, and therefore it is so difficult to answer. Just 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 come to 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 perfect our models become.