The Nebular Hypothesis

     This theory is the most accepted model in Cosmogony to explain the formation of the Solar System. Basically, this theory suggests that our Solar System was formed from Nebulous material.

(https://en.wikipedia.org/wiki/Immanuel_Kant)

• Developed By: Immanuel Kant
• Published: Allgemeine Naturgeschichte und Theorie des Himmels ("Universal Natural History and Theory of the Heavens")
• Year: 1755
• Modern Variant: Solar Nebular Disk Model (SNDM)

     Originally, this theory was created specifically for our Solar System, but now scientists believe that this theory also applies to our Universe. The actual details of Solar System formation are very complicated and can't be explained simply without cutting off many important factors. Which is why I will quote the actual process from Wikipedia and insert a video explaining the theory below.

(http://www.wolaver.org/Space/seahorse.htm)

According to the nebular hypothesis, stars form in massive and dense clouds of molecular hydrogen—giant molecular clouds (GMC). These clouds are gravitationally unstable, and matter coalesces within them to smaller denser clumps, which then rotate, collapse, and form stars. Star formation is a complex process, which always produces a gaseousprotoplanetary disk around the young star. This may give birth to planets in certain circumstances, which are not well known. Thus the formation of planetary systems is thought to be a natural result of star formation. A Sun-like star usually takes approximately 1 million years to form, with the protoplanetary disk evolving into a planetary system over the next 10–100 million years.^[1]

The protoplanetary disk is an accretion disk that feeds the central star. Initially very hot, the disk later cools in what is known as the T tauri star stage; here, formation of small dust grains made of rocks and ice is possible. The grains eventually may coagulate into kilometer-sized planetesimals. If the disk is massive enough, the runaway accretions begin, resulting in the rapid—100,000 to 300,000 years—formation of Moon- to Mars-sized planetary embryos. Near the star, the planetary embryos go through a stage of violent mergers, producing a few terrestrial planets. The last stage takes approximately 100 million to a billion years.^[1]

The formation of giant planets is a more complicated process. It is thought to occur beyond the so-called frost line, where planetary embryos mainly are made of various types of ice. As a result, they are several times more massive than in the inner part of the protoplanetary disk. What follows after the embryo formation is not completely clear. Some embryos appear to continue to grow and eventually reach 5–10 Earth masses—the threshold value, which is necessary to begin accretion of the hydrogen–helium gas from the disk. The accumulation of gas by the core is initially a slow process, which continues for several million years, but after the forming protoplanet reaches about 30 Earth masses (M_⊕) it accelerates and proceeds in a runaway manner. Jupiter- and Saturn-like planets are thought to accumulate the bulk of their mass during only 10,000 years. The accretion stops when the gas is exhausted. The formed planets can migrate over long distances during or after their formation. Ice giants such as Uranus andNeptune are thought to be failed cores, which formed too late when the disk had almost disappeared.

     To explain the broad idea found in the above quote, planets are formed thanks to the birth of new stars. While stars are formed by 'dense clouds of molecular hydrogen -- giant molecular clouds' or more commonly known as GMC. The matter within the GMC collapse due to gravity and form a star.

(http://www.everythingselectric.com/planet-formation-theory-debunked/)