What is now believed to be the story of how our universe and everything in it including our solar system came into being is referred to as the "Big Bang Theory." One of the things I think is important to point out to students is that scientific theories are not just off the top of your head speculation. That is they may start out as that but for them to have any credibility they must reflect and take into consideration what information is already known. As was pointed out in some of the readings the development of the Big Bang theory had to take into account what was scientifically accepted as characteristics of the universe.
Three of the most important ideas that any theory that explains the formation of the solar system would have to explain would include:
1. The terrestrial planets (Mercury, Venus, Earth , and Mars) which are composed of rocky substances and are relatively small compared to the larger Jovian planets (Saturn, Jupiter, Uranus, and Neptune,) are compose primarily of hydrogen and helium. Pluto belongs to neither category and is icy.
2. All of the planets orbit the sun in the same direction and in the same plane.
3. The terrestrial planets orbit close to the sun and the Jovan planets orbit a greater distance away as if in their own subgroup.(1)
The Big Bang Theory suggests that the universe began some 13.7 million years ago.(2) In his book
The First Three Minutes,
Steven Weinberg describes near the beginning of the universe when the temperature was at 100,000 million degrees Kelvin. The universe was "filled with an undifferentiated soup of matter and radiation."(3) It is incorrect to thing of the "bang" as an explosion akin to a bomb going off. There was no destruction of material but rather the rapid expansion of the universe. The universe began cooling off and as it did the collisions of different particles were able to combine and form new forms of matter. Along with the matter, radiant energy spread out in all directions. All of the matter spread outward evenly and very fast.(4)
As the matter spread collisions took place and as molecules mixed new elements formed. Hydrogen was one of the first formed. Over time hydrogen has been recognized as the building block of the universe. It is the most abundant element found in the solar system and Helium is the second. Together these two gases make up 98% of all the mass of material in the solar system. The predominance of hydrogen and helium does not only occur in our solar system, but the entire universe. In addition to hydrogen and helium, lithium and beryllium which are among the lightest elements also emerged. As the young universe expanded it began to cool in regions with slightly more mass than others gases clumped up due to gravity. The gases became like clumps and the swirling material formed the early galaxies. Within theses early galaxies the first stars were born. The heavier elements were made much later by thermonuclear fusion in stars. The dust particles were only possible through the formation and death of some particularly massive stars.(5)
The hydrogen and helium formed clouds. As the clouds cooled they swirled and shrank becoming early galaxies. In the early galaxies there could have been no planets like the rocky Earth forming because there was only hydrogen and helium but no carbon. It would be some time before carbon was produced.(6)
The galaxies that formed differed in shape. Some were elliptical and some were spiral. They are enormous – trillions of miles wide. Sometimes the galaxies smashed into one another. Material from these collisions spread around and in certain instances combined. This led to the birth of the first stars. These early stars ended up providing the chemicals for the different galaxies. The smaller stars turned hydrogen into helium and then helium into carbon and on up to the development of more complicated elements. The larger stars made heavy metals like iron, gold and uranium. As time past these stars exploded they showered the galaxies with metal and chemicals. These supernova remnants scattered across space.
As new stars formed they incorporated the heavy elements which could only come from earlier stars. We can therefore be pretty sure that our Sun is a descendent of some of these earlier stars, because in addition to hydrogen and helium our earth contains heavy elements that could only have come from previous stars. Since the Sun is 5 billion years old there has been lots of time for stars to form and die.
Our solar system was formed from a cloud of interstellar material made mostly of hydrogen and helium and other chemical elements such as carbon, oxygen, and silicon. There were also some interstellar grains which are believed to be made of silicates, iron compounds, carbon compounds, and water frozen into ice. The transformation of this cloud began when the gravitational attraction between the particles in the densest part of the cloud caused it to collapse inward. It did not fall into itself but because it was rotating it flattened much like what happens when a pizza maker would toss the round shaped piece of dough into the air to help flatten it. This process took a few million years. The disk or solar nebula condensed into the planets while the bulge in the middle became the Sun.(7)
The development of the planets occurred in two stages. First, dust condensed and clumped together to form planetesimals. Later, the planetesimals joined together to form planets and satellites.(8)
All of the planets revolve around the Sun in the same direction which must be indicative of the way the cloud turned while the process of collapse was happening. The terrestrial planets are similar because event though the temperature was hotter near the sun the materials making up the terrestrial planets remained partially of molten rock. The iron-rich minerals sank to the middle while the silicon-rich minerals floated to the top. This iron core gave rise to the magnetic fields in the terrestrial planets.(9)
Like the terrestrial planets, the outer planets began with planetesimals which were accumulating more and more materials. However because the outer solar system had lower temperatures gas atoms were moving very slowly and could be gathered by these out planetesimals along with the rocky materials. The core of rocky material was able to suck up the gases and in that way the outer planets except for Pluto were formed with enormously thick hydrogen envelopes surrounding rocky cores. The main difference between the terrestrial and Jovian planets is that it was cold enough in the outer regions where the ice particles and ice coated dust grains could survive. Since there was a great deal of icier material there to form solid objects the Jovian planets are much larger than the terrestrial planets.(10)
Some of the remaining planetesimals crashed into the new planets making large craters. They also may have caused the tilted rotation axes of some of the planets. Today some of the planetesimals survive as asteroids and comets.(11)