Nebular Hypothesis:
According to this theory, the Sun and all the planets of our Solar System began as a giant cloud of molecular gas and dust. Then, about 4.57 billion years ago, something happened that caused the cloud to collapse. This could have been the result of a passing star, or shock waves from a supernova, but the end result was a gravitational collapse at the center of the cloud.
From this collapse, pockets of dust and gas began to collect into denser regions. As the denser regions pulled in more and more matter, conservation of momentum caused it to begin rotating, while increasing pressure caused it to heat up. Most of the material ended up in a ball at the center while the rest of the matter flattened out into disk that circled around it. While the ball at the center formed the Sun, the rest of the material would form into the protoplanetary disc.
The planets formed by accretion from this disc, in which dust and gas gravitated together and coalesced to form ever larger bodies. Due to their higher boiling points, only metals and silicates could exist in solid form closer to the Sun, and these would eventually form the terrestrial planets of Mercury, Venus, Earth, and Mars. Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large.
Planetesimal Theory:
The planetesimal theory, put forth by Viktor Safronov in 1941, explains planet formation in the early solar system from accretion of small bodies, growing in size as gravity attracted more and more objects. As the small bodies orbit, their gravity is very weak and they must rely on non-gravitational forces to stay together, such as radiation pressure and the emission of thermal photons.
Growth of the small planetesimal caused the strength of its gravity to increase. Growth to approximately one kilometer in size allowed the gravity of planetesimals to attract objects to them, increasing their size to seeds for planetary formation.
Collisions were common during the formation of the solar system and planets. Planetesimals continually collided and destroyed each other, but some planetesimals were able to withstand the impacts and grow, eventually forming into planets.
In contrast, the giant planets (Jupiter, Saturn, Uranus, and Neptune) formed beyond the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid (i.e. the Frost Line). The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium. Leftover debris that never became planets congregated in regions such as the Asteroid Belt, Kuiper Belt, and Oort Cloud.
Tidal Theory:
involving the approach near the Sun of another star. This set up tidal forces, and the instability of the Sun resulted in part of its mass being torn off to form the planets. The theory was proposed by Sir James H. Jeans (1877–1946) and Sir Harold Jeffreys.
