The Solar System
During the first nanoseconds of time after the big bang the universe was so hot and dense that it was only filled with energy. However as the universe continued to expand it began to cool and as a result energy began condensing into matter and the first atoms of Hydrogen began to form.
As the universe continued to cool more and more hydrogen would form and eventually cooler temperatures would allow chemical reactions to take place and molecules of Hydrogen (H2) begin to form. Since all matter has mass and mass produces gravity each atom and molecule of hydrogen would also have gravity; and as the universe continued to expand and cool massive clouds of hydrogen would begin to accumulate into patches called nebulae. Within each nebula denser regions (regions that contained significantly more H2) would exert more gravity on the surrounding nebula and therefore attract more H2. As more and more gas (H2) was attracted gravity would increase and the dense region would be compacted into smaller, hotter, swirling, spherical region of gas. As the dense region continued to grow gravity would cause it to contract more and more and, just like an ice skater, as the nebula contracted the faster it would spin.
As this region of gas spins faster and faster it begins to flatten out much like dough being spun to make a pizza and takes on a more disk-like appearance. Eventually the gravity produced by the enormous amount of mass at the center of the swirling disk causes the center to collapse into a dense ball. Increased pressure, as a result of increasing gravity, forces temperatures to rise dramatically causing the dense ball to glow, forming a protostar.
Temperatures within the protostar continue to rise as the protostar becomes denser and denser. When the temperature reaches 10,000,000°, conditions are just right to allow hydrogen nuclei to slam together in a process known as nuclear fusion. During this process atoms of hydrogen fuse together to form atoms of Helium and in the process give off enormous amounts of energy. This reaction sets off a chain reaction "igniting" the protostar and turning it into a star.
This process would have created the first stars when the universe was only 300,000,000 years old. As these stars continued to fuse atoms more and more elements would have been created. Eventually as a star's fuel is used up, the star dies in a violent explosion known as a super nova releasing newly created elements back into space where they can accumulate to start this process over and over again.
Based on the fact that our solar system contains many heavy elements (Fe, Ni, Si, O, and C), elements that are produced by a series of fusion reactions, our Sun can be either a third, forth, or fifth generation star. This means that our Sun and all of the objects in our solar system, including ourselves, have been formed from the products of all previous supernova.
| Animation showing planetary formation. NASA/JPL-Caltech/R. Hurt |
According to the Solar Nebula Theory, our solar system formed from material that was swirling around a massive nebula. Just as the gasses in the early universe were able to accumulate due to their gravitational attraction, as heavy elements began to accumulate in nebulae they to would also be attracted to one another. This is exactly what happened roughly 9 billion years after the formation of the universe in a nebula that would eventually become our solar system.
The planets that form our solar system were produced through a process known as accretion, building things bigger through the addition of small pieces. Unlike the accretion of gasses that would allow our sun to get larger and larger in the center of the nebula, planetary accretion occurs within the flattened nebula disk that swirls around the protostar, called a protoplanetary disk.
The protoplanetary disk was most likely contained al 92 elements. Some, such as Helium (He), existed as isolated atoms while others would exist in compounds such as, H2O, NH3, CH4, and SiO2. As the protostar in the center of the nebula warmed the cloud of material around the star would begin to differentiate. Lighter more volatile elements and compounds would have been pushed further and further from the protostar. Other compound that melt at much higher temperatures would be left behind forming soot-sized particles of dust closer to the protostar.
As the protosun grew larger and larger gravitational attraction between the swirling dust grains in the protoplanetary disk caused clumping dust grains to clump together, and just with the formation of stars, the more mass that clumped together created more gravity which, in turn, attracted more mass. Through this process, animated in the movie below, dust grains grew larger and larger: first sand grains formed, then as sand grains clumped together larger balls formed. Eventually as this process continued planetesimals formed (objects larger than 1 kilometer in diameter) and even they would interact to form larger protoplanets and once most, if not all, debris within its orbit was incorporated into the protoplanet it became a planet.
Computer modeling of this process suggests that planetary formation as described by the Solar Nebula Theory may have taken anywhere between 10 and 200 million years depending on the density of material within the planets orbit. Radiometric dating (a process we will cover in later modules) of meteorites our solar system, including the Earth, was formed 4.5 billion year ago. Due to the differentiation that occurred within the protoplanetary disk the planets that did formed mirror the composition of the disk within their orbit. This means that planets that formed nearer to our sun are composed of materials with a relatively high melting point and have a rocky composition, such as Earth, whereas planets that formed further from the sun contain more volatile and gaseous compositions such as Jupiter and Saturn.