Origin of Solar System
The Origin of the Solar System One of the most intriguing questions in astronomy today is how our solar system formed. Not only does the answer add insight to other similarly forming systems, but also helps to satisfy our curiosity about the origin of our species. Although it is highly unlikely that astronomers will, ever know with absolute scientific certainty how our system originated, they can construct similar theoretical models with the hopes of gaining a better understanding. A basic understanding of the current physical aspects of our solar system are helpful when trying to analyzing its origin.
Our solar system is made of the Sun, nine major planets, at least sixty planetary satellites, thousands of asteroids, and comets that all span an immense distance. Each planet has its own individual characteristics and seven of which have one or more satellites. There are thousands of asteroids, mainly congested in the area between Mars and Jupiter, as well as countless comets that all travel in a spherical orbit around our Sun. The Sun contains approximately 99 percent of the mass in the solar system, but only 2 percent of the system’s angular momentum. It lies in the center of our system while all planets, asteroids, and alike rotate in elliptical orbits around it in the same plane. The smaller inner planets have solid surfaces, lack ring systems, and have far fewer satellites than the outer planets. The atmospheres of most of the inner planets consist of large quantities of oxidized compounds such as carbon dioxide. While on the other hand, the outer planets are far more massive than the inner terrestrial planets and have gigantic atmospheres composed mainly of hydrogen and helium.
Asteroids and comets make up the smallest portion of the entities of the solar system and are composed of the remnants left behind while planets were forming. For over 300 years, there has been a very long history of conjecture on the origin of the solar system. These many theories stem from two general categories. The first category called monistic involves the evolution of the Sun and planets as an isolated system. The second group of theories called dualistic suggested that the solar system formed as a result of the interaction between two individual stars. The dualistic formation theory has been almost entirely dropped and monistic formation has become the general consensus on the basic formation of our solar system.
Most modern theories of the origin of the solar system hypothesize that all bodies in the solar system, including the sun, accreted from the formation and evolution of a single primordial solar nebula. It is believed that our solar system began to form around 4.56 billion years ago from a dense interstellar cloud of gas. Because of the conservation of angular momentum, the cloud of gas formed a rotating flattened disk approximately the size of the planetary system. It was this flattened disk that is referred to as the primitive solar nebula and from which our current solar system evolved.
Ordinarily, the internal pressures of the cloud are sufficient to prevent it from collapsing. However, from time to time local increases in pressure of the interstellar medium cause the additional compression of interstellar clouds. These compressions caused the clouds to reach their threshold of gravitational collapse.
Once the gravitational attraction of matter is greater than any tendency to expand due to internal pressures the cloud begins to collapse inward. Theoretical models suggest that the presolar nebula continued to collapse until the center of the cloud became so dense that heat started to form. This heat increased the thermal pressure of the cloud until the collapse was eventually halted.
The existence of our system of planets is entirely due to the angular momentum of the initial cloud. If there were no angular momentum, then the interstellar cloud would have collapsed from a single star. While at the same time, if the collapse had occurred under a system with too much angular momentum then a binary star would have resulted from our system.
Our system formed under intermediate conditions allowing the planets to evolve. The fact that the Sun contains 99 percent of the solar system’s mass but only 2 percent of its angular momentum raises questions about the distribution of masses during the early formation of the solar system. It is suggested that certain processes transported nebula mass inward to form the Sun, and angular momentum outward to the preplanetary region.
Thus decreasing the total angular momentum of the Sun. Three separate hypothesizes have been suggested to explain the processes for such a transport. The main theories suggest that gravitational torques, viscous stress, and magnetic fields may have acted individually or in some combination to produce our present system.
The first theory including gravitational torques arises from the gravitational forces between segments of asymmetric mass. One example of this case would be between the inner and outer regions of the trailing spiral arms of the nebula. Assuming there is a source of asymmetry, then these torques can result in significant outward transportation of angular momentum.
Viscous stresses are another possible source of the shift in angular momentum of the solar system during its evolution. Viscous stresses are caused by the friction between adjacent fluid parcels trying to move past each other at different speeds. These stresses result in the outward transport of angular momentum and are one more possible explanation for the outward spread of momentum.
The third theory postulates that magnetic fields are the source of this momentum transfer. Magnetic fields may have been produced during the collapse of the initial cloud or even electrically generated between the proto-Sun and the solar nebula. This would eventually end in the same result of an outward spread of angular momentum. Therefore, the evolution of the solar nebula involved both the transportation of mass into the central proto-Sun region and the increased angular momentum in the planetary regions.
This meant that most of the primitive cloud’s mass fell into the proto-Sun’s legion while the remainder formed the planets. It is not only important to study the evolution of the solar nebula, but also the formation of the planets. There is a general consensus that once the solar nebula settled to rest that solid dust particles began to move toward the central plane of the nebula. It was at this stage that the planets began to form.
There are two current theories that resulted in the development of the planets. The first theory suggests that the planets formed in a very basic process where dust particles accumulated into planetesimals which in turn grew to the present planets. The second theory proposes that planetesimals resulted from a gravitational instability in the gaseous portion of the solar nebula. The first theory states process of planet formation began with the settling of dust in into the central plane of the nebular disk. Soon after, the first dust particles began to coagulate into small solid bodies. These bodies then accumulated through a collective gravitational instability in the dust disk.
The thin dust disk became more massive through continual sedimentation and resulted in its breakup into a large number of planetesimals. Through a process of random collisions, these planetesimals continued to grow and accumulate mass. There are two possible extremes that ended this process of accumulation.
The first involved runaway accreting where one object grows extremely large through the collection of all smaller planetesimals within its area. The alternative extreme would involve the uniform growth of a number of masses resulting in many equal mass planetesimals. It is currently believed that the formation of the planets resulted from a combination of these two processes.
The equal mass accumulation is presumed to have dominated during the early stages of planet formation while the runaway accumulation is suggested to have taken over during the latter stages. However, there is one substantial problem with this explanation of the planet’s formation. The accumulation theory fails to take into account the rapid formation of the giant planets. By the slow process of coagulation, it would take much longer than the lifetime of the solar system to form the giant planets of Jupiter and Saturn.
This incorrectness in the first theory led scientists to contrive the second theory. The second theory of planetary evolution involves a gravitational instability of the gaseous portion of the solar nebula. It is suggested that if the solar system were massive enough then the instability would lead to the fragmentation of e gaseous nebula and the formation of giant gaseous protoplanets. This theory allows plenty of time for the formation of the very large planets Jupiter and Saturn. The one flaw of this theory is its contingency on a very massive planetary nebula, one much larger than ours.
Because of this problem, many cosmogonists have begun to doubt that the gaseous disk instability led to planet formation in our solar system. Although many of the details on the theories of our solar system will most likely change in the near future, the fundamental concept of solar system formation appears to remain the same. The Sun and planets began forming approximately 4.56 billion years ago out of a solar nebula produced by the collapse of a rotating interstellar cloud of gas and dust.
Following soon after, the terrestrial and Jovian planets eventually formed from the collision and accumulation of smaller planetesimals. While there is significant evidence supporting the formation of the Sun and planets in this way, it is not likely that scientists will know with complete certainty about the solar system’s origin for some time. It is highly likely that the details in the theory of the solar system will change.
With continued improvements in technology and significant advances in astronomical fields of observation, further understanding of our solar system will undoubtedly come. In recent years, the idea that the Solar System formed from the evolution of a primordial solar nebula, has received significant conformation.
The use of satellites such as the Infrared Astronomical Satellite (IRAS) has detected disks of solid particles around several nearby stars, including Fomalhaut, Beta Pictoris, and Vega. The uses of satellites have provided scientists with most of the information they currently have on the system’s origin. Another source of information lies in our neighboring planets. Investigations of the other planets in the solar system by means of interplanetary spacecraft have provided a wealth of data pertaining to the origin and history of the solar system. Through the observation of solar-type stars in the Galaxy, we can learn critical information about the properties of the interstellar cloud that collapsed to form our own solar nebula. It is likely that future explorations and observations will help to solidify our understanding of the solar system.
