The condensation theory of the solar system explains why the planets are arranged in a circular, flat orbit around the sun, why they all orbit in the same direction around the sun, and why some planets are made up primarily of rock with relatively thin atmospheres. Terrestrial planets such as Earth are one type of planet while gas giants -- Jovian planets such as Jupiter -- are another type of planet.
The GMC Becomes a Solar Nebula
Giant molecular clouds are huge interstellar clouds. They are made up of about 9 percent helium and 90 percent hydrogen, and the remaining 1 percent is various amounts of every other type of atom in the universe. As the GMC coalesces, an axis forms at its center. As that axis rotates, it eventually forms a cold, rotating clump. Over time, that clump becomes warmer, denser and grows to encompass more of the GMC’s matter. Eventually, the entire GMC is swirling with the axis. The GMC’s spinning motion causes the matter that makes up the cloud to condense closer and closer to that axis. At the same time, the centrifugal force of the spinning motion also flattens the GMC’s matter into a disc shape. The GMC’s cloud-wide rotation and disc-like shape forms the basis for the solar system’s future planetary arrangement, in which all the planets are on the same relatively flat plane, and the direction of their orbit.
The Sun Forms
Once the GMC has formed into a spinning disc, it’s called a solar nebula. The axis of the solar nebula -- the densest and hottest point -- eventually becomes the forming solar system’s sun. As the solar nebula spins around the proto-sun, pieces of solar dust, which is made up of ice as well as heavier elements such as silicates, carbon and iron in the nebula, collide with each other, and those collisions cause them to clump together. When the solar dust coalesces into clumps of at least a few hundred kilometers in diameter, the clumps are called planetesimals. Planetesimals attract each other and those planetsimals collide and clump together to form protoplanets. The protoplanets all orbit around the proto-sun in the same direction as the GMC rotated around its axis.
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The Planets Form
A protoplanet’s gravitational pull attracts helium and hydrogen gas from the portion of the solar nebula that surrounds it. The farther the protoplanet is from the hot center of the solar nebula, the cooler the protoplanet’s surroundings’ temperature and therefore, the more the area’s particles are likely to be in a solid state. The greater the amount of solid materials near the protoplanet, the larger the core that the protoplanet is able to form. The larger a protoplanet’s core, the greater the gravitational pull it is able to exert. The stronger the protoplanet’s gravitational pull is, the more gaseous matter it’s able to trap near it, and therefore the bigger it is able to grow. The planets closest to the sun are relatively small and are terrestrial, and as the distance between the planet and the sun grows, they become larger and more likely to become Jovian planets.
The Sun’s Solar Wind Halts Planet Growth
As the protoplanets form cores and attract gasses, nuclear fusion is ignited at the proto-sun’s core. Because of the nuclear fusion, the new sun sends a strong solar wind through the burgeoning solar system. The solar wind pushes out the gas -- though not the solid matter -- from the solar system. The planets’ formation is halted. The farther a protoplanet is from the sun, the farther apart the particles in the area are, which leads to slower growth. Planets at the edges of the solar system might not be finished with their growth when they’re halted by the solar wind. They may have a relatively thin gaseous atmosphere, or they still only be made up of an icy core. When the solar wind blows through the solar system, the solar nebula is approximately 100,000,000 years old.