Everything about Planet Formation totally explained
In
cosmogony, the
nebular hypothesis is the most widely accepted model explaining the
formation and evolution of the Solar System. It was first proposed in 1734 by
Emanuel Swedenborg.
Immanuel Kant, who was familiar with Swedenborg's work, developed the theory further in 1755. He argued that gaseous clouds—
nebulae, which slowly rotate, gradually collapse and flatten due to
gravity and eventually form
stars and
planets. A similar model was proposed in 1796 by
Pierre-Simon Laplace. His book
Evolution of the protoplanetary cloud and formation of the Earth and the planets, which was translated to English in 1972, had a long lasting effect on the way scientists think about the formation of the planets. In this book almost all major problems of the planetary formation process were formulated and some of them solved. The Safronov's ideas were further developed in the works of
George Wetherill, who discovered
runaway accretion. Over millions of years giant molecular clouds are prone to
collapse and fragmentation. These fragments then form small, dense cores which in turn collapse into stars.
The initial collapse of a solar-mass protostellar nebula takes around 100,000 years. The core gradually grows in mass until it becomes a young hot
protostar. The collapse is often accompanied by
bipolar outflows—
jets, which emanate along the
rotational axis of the inferred disk. The jets are frequently observed in star-forming regions (see
Herbig-Haro (HH) objects). The luminosity of the Class 0 protostars is high— a protostar of the solar mass may radiate at up to 100 solar luminosities. This birth of a new star occurs at approximately 100,000 years after the collapse has begun. A pair of bipolar jets is usually present as well. The emission lines actually form as the accreted gas hits the "surface" of the star, which happens around its
magnetic poles. As a result the young star becomes a weakly lined T Tauri star, which slowly, over timeframe of hundreds of millions of years, evolves into an ordinary sun-like star. They exist from the beginning of a star's formation, but at the earliest stages are unobservable due to the
opacity of the surrounding envelope. The heating of the disk is primarily caused by the
viscous dissipation of
turbulence in it and by the infall of the gas from the nebula. The result of this process is the growth of both the protostar and of the disk
radius, which can reach 1,000 AU if the initial angular momentum of the nebula is large enough.
The lifespan of the accretion disks is about 10 million years. The signatures of the dust processing and
coagulation are observed in the infrared spectra of the young disks. Further aggregation can lead to the formation of
planetesimals measuring 1 km across or larger, which are the building blocks of
planets. However it's only possible in massive disks—more massive than 0.3 solar masses. In comparison typical disk masses are 0.01–0.03 solar masses. Because the massive disks are rare, this mechanism of the planet formation is thought to be infrequent. whereas the outer part can evaporate under the star's powerful
UV radiation during the T Tauri stage or by nearby stars. This results in coagulation of purely rocky grains and later in the formation of rocky planetesimals. Slowing of the accretion is caused by gravitational perturbations by large bodies on the remaining planetesimals. During this stage embryos expel remaining planetesimals, and collide with each other. The result of this process, which lasts for 10 to 100 million years, is the formation of a limited number of Earth sized bodies. Simulations show that the number of surviving planets is on average from 2 to 5. Hypotheses don't predict a merger stage, due to the low probability of collisions between planetary embryos in the outer part of planetary systems. Some combination of the above-mentioned ideas may explain the formation of the cores of gas giant planets such as
Jupiter and perhaps even
Saturn.
Once the cores are of sufficient mass (5–10 Earth masses), they begin to gather gas from the surrounding disk. In this model ice giants—Uranus and Neptune are failed cores that began gas accretion too late, when almost all gas had already disappeared. The post runaway gas accretion stage is characterized by migration of the newly formed giant planets and continued slow gas accretion. On the one hand, if giant planets form too early they can slow or prevent inner planet accretion. On the other hand, if they form near the end of the oligarchic stage, as is thought to have happened in the Solar System, that'll influence the merges of planetary embryos making them more violent. In addition, the size of the system will shrink, because terrestrial planets will form closer to the central star. In the Solar System the influence of giant planets, particularly that of
Jupiter, is thought to have been limited because they're relatively remote from the terrestrial planets.
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