Introduction
There are a number of theories that have been raised to explain the gradual formation of the solar system. This paper however focuses on the solar nebular theory which was formulated by Immanuel Kant and proposes that the formation of the sun and other identified planets occurred at the same period due to contraction whirling of gas and dust. According to this theory, the huge concentration of gas was composed mainly of hydrogen and the accompanying dust had heavy particles that led to formation of rocky planets like Mars and Earth. This dust is believed to have resulted from the supernova remains together with primordial hydrogen and helium gases before they contracted to form the Solar system (Laurune, 2010).
The other current theories that explain the formation of the solar system are;
Protoplanets Theory
This theory has some similarities to Nebula theory but it proposes that the condensed mass that formed the Solar system was as a result of numerous number of small particles of ice and dust (Laurune, 2010). These tiny grains of dust and ice joined up together to form bigger clouds called proto-planets within the same period that the sun was forming. This theory however, does not provides details of how the ecliptic angle but only provides what composed Jupiter by making an assumption that the sun moved across a different space from where its formation took place (Woolfson, 2000).
Tidal Theory
This theory is a modification of the encounter theory that was first propagated by Georges Buffon. According to this theory, a moving star passed very near the sun causing a withdrawal of a stream of solar gases that eventually condensed to form the different planets (Laurune, 2010). The theory is supported by many as it is seen that the stream of gases drawn from the sun would have been expected to bulge at the centre and tapper at the end a fact that can be seen in the current distribution of the solar system.
The theory also explains why the stream of gases drawn from the sun is on a single plane yet it does not represent the solar equator but forms a divergence of an eclipse drawn from the plane of solar equator (Anderson, 2007). However, the theory is limited in explaining how the hot matter drawn from the sun could not have dispersed away and be blown off by the wind as it would be expected based on different scientific laws. In addition, the possibility of a star getting so near the sun is not regarded as very practical scenario as most of the stars in the milky way revolve on define orbital paths around the centre of galaxy (Woolfson, 2000).
Capture Theory
This theory is a kind of inverse of the tidal theory because while the tidal theory postulates that a star extracted a stream of matter from the sun, this theory explains that it is the sun that actually extracted matter from a fast moving protostar that was undergoing through a process of formation (Laurune, 2010). This theory has been significant in accounting for the reason of planets having more angular momentum than the sun by considering the captured materials as well as accounting for the ecliptic plane. The Theory however is criticized due to the fact that the dust from the protostar would have been expected to have a greater diameter than that of a star. Chances of the sun getting close to a protostar that orbits around the galaxy are also very minimal (Barnes, 2010).
Speculative Theory
This is a recent idea that explains the formation of the solar system as having taken place within the disordered parts of the milk way galaxy and eventually got from it. The theory is significant in that it accounts for what composes Jupiter by considering acquired more supernova materials after the sun was formed (Barnes, 2010). The problem with the theory is the fact that the sun does not move as fast as the other surrounding stars and therefore the chances of it being pushed from a more active place and eventually slowing it down is minimal.
The Angona Theory
According to this theory, cold ice together with enormous dust condensed into planets that were then driven out to their specific orbits by the heat of the sun depending on their weight. Like other current theories, it postulates that a part of dust from a moving object resulted to formation of planets and that the remains of supernova accounts for the presence of elements that are more heavy than iron in some planets (Laurune, 2010).
The Solar system
The solar system comprises of the sun (an average star) and the following nine planets: mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. It also includes the satellites of the planets and other interplanetary medium. The sun comprises of 99.85% of the solar system matter while the planets take a fraction of 0.135%. In addition, the satellites contain 0.015% (Zellik, 2002). Figure 1 bellow shows an image of the sun and its planets as seen by space explorers.
Crary and Boulder (2010) are of the opinion that any authentic theory that explains the formation of the solar system should amicably address four factors that are clearly evident from the solar system. These are; the fact that massive solar bodies appear to have an orderly motion system- according to them, satellites and planets have perfect circular orbits, occurring in similar planes and occurring in the same direction. Additionally, such a theory would be able to explain the fact that planets can broadly be classified into two major categories namely the hydrogen rich and large Jovian planets that are located far from the sun and the rocky and small planets located near the sun.
In their view, the Jovian planets consist of many moons and rings and are made up of ice and rock particles. Such a theory should also explain the fact that there are myriads of comets and asteroids spread all over the solar system (Crary & Boulder, 2010). Additionally, there should be an explanation on a number of variations such as very large moons and planets with significantly varied tilts. The nebula theory, described below is one of the most comprehensive in its explanations that to a great extent covers the four dimensions mentioned above.
There are three major processes that that take responsibility for the diversity of matter in the solar system and they are directly responsible for the internal structures of planets. They include:
- Condensation of low pressure and temperature from the solar matter or the interstellar medium
- Condensation of high pressure and temperature from solar matter that is related to planetary formation
- Stripping of primordial volatile components from the inner part of the solar system
Collapse of the solar nebula
The solar nebular is a disc-shaped cloud of gas and dust left over from the sun’s formation. Most theories that endeavour to explain the origin and development of the solar system depend on the information about the composition of the material from which the sun and the planets were formed. 99% of the solar nebula is preserved in the outer layers of the sun hence the components of these layers are alleged to look like those of the solar system. The nebula is believed to be the conversion state between the residues of the stellar ancestors and planetary matter which include the sun and planets that advanced from it.
The solar system did not exist 4.6 billion years ago and there was only a single cloud of cold molecular gas and dust. The disintegration of the solar nebula was an effect of the super nova explosion hence generating areas of larger mass where gravity took over. During this process, materials drew inwardly into dense pockets where stars formed afterwards. It’s in these regions that the solar system started forming in the solar nebula.
In reference to the condensation theory, the high temperature that was emitted by the narrowing of the nebular cloud dissolved ice crystals, dissociated hydrogen molecules to atoms and left very tiny grains of nebular materials which continued to revolve around the undeveloped sun. Because of the physical and dynamic conditions that accompanied the collapse of the solar nebula, very high temperatures were reached.
Planet formation and evolution using the nebula theory
The different planets are all said to have been formed from the solar nebula and the currently acceptable method through which planet formation occurred is accretion. This is the process through which planets started developing as dust grains in a trajectory around the central protostar. Through more collisions, the planetesimals increased in size with time. High temperatures in the inner solar system caused high volatile elements such as methane and water to condense and hence the planetesimals that could form there consisted only of hardened elements.These included metals like iron and rocky silicates; as a result, the rocky bodies transformed into terrestrial planets including Mercury, Venus, Earth and Mars.
Since these compounds are so scarce in the universe with a composition of 0.6% of the nebula, the terrestrial planets couldn’t grow any bigger. The terrestrial planets were largely blanketed by dust and gas during this process. This gas was moderately sustained by pressure hence did not revolve round the sun as fast as the planets. This speed of movement was directed by temperature deviation in the disk but the general drift was for the central planets to shift inwardly as the disks degenerated leaving the planets in their present orbits.
Systematically, the nebula theory can be summarized stepwise as follows; first, the solar nebula (interstellar gas cloud) is disturbed and consequently collapses. As mentioned above, the disturbance is assumed to have originated from a shock wave source, such as from a nearby supernova. The collapse in the initial step involved substantial motion and therefore, a significant amount of heating took place. After the heating, the collapsed cloud then compressed at the centre, a period in which all dust generated from the collapse should have vaporized. After the compression, the centre that condensed formed a protostar while the vaporized dust orbited around it. Most of the vaporized dust that surrounds the centre star flows inwards to add to the outer parts of the condensed star (Landgraf, et al, 2002).
A centripetal force pulls some gas towards the center. A resultant centrifugal force prevents some gas from reaching the central parts and gas is left rotating around the central body. The gas held by the centrifugal force forms a cyclic, ‘accretion disk’ round the central star. The rotating gas eventually cools off to an extent that particles of metal, rocks and ice in it condenses to form tiny particles.
These tine particles later collide and interact with each other leading to formation of bigger particles that later form boulder in the form of small steroids (Irvine, 2007). As more collision and interaction between the particles increases, most of them merge leading to formation of larger particles which increases their growth and nontrivial gravity. This implies that they develop their own gravity and their run away ability grows, hence they start moving away from each other.
According to the nebula theory, each of these merged boulders moving away carry with it a substantial column of gas. A very strong wind generated by the nebula star later blows all the wind surrounding these proto-planets with two significant results. First, if a proto-planet is large enough, it is able to pull back some gases towards itself resulting to the formation of a massive gas giant (Lissauer & Stevenson, 2006). If it’s small, it’s unable to pull any gases and is left as a bare rocky proto-planet.
The proto-planets would eventually with time collide with each other, leading to formation of much larger bodies, both of the bare rocky bodies and the massive gas bodies. The bodies that remain separate, which are large enough to generate their own gravity are stabilized by attraction forces and they form stable orbits, similar to the one that characterizes the solar system as it is known today.
Conclusion
Though there are numerous theories explaining the processes that led to the formation of the solar system, there is still a limited understanding on the its exact origin, formation and evolution and the ideas of how it came into being are still not yet proven. Hence there is no single theory that fully explains the whole process (Woolfson, 2000). As science started to discard a Judeo-Christian view of creation 200 years ago, a tendency inclined to sole naturalistic explanations arose. As scientists continue with their search for answers in the contention that surrounds the contemporary theories, a lot can be credited on the work that has been completed. With the rise of more sophisticated equipments and unearthing of new evidences, the hopes are ripe that a more consistent theory shall be birthed.
Reference List
Anderson D. L. 2007. New Theory of the Earth, London: Cambridge University Press.
Barnes R. 2010. Formation & Evolution of Exoplanets, New York: Wiley Publishers.
Crary, F & Boulder, C. 2010. The Origin of the Solar System. Web.
Irvine, M. 2007. The Chemical Composition of the pre-solar nebula. Web.
Landgraf, M., Liou, J., Zook, H. & Grun, E. 2002. Origins of Solar system dust beyond Jupiter. The astronomical Journal, Vol. 123(5):2857-2861.
Laurune B. 2010. Monmatia revisited. Web.
Lissauer, J. & Stevenson, D. 2006. Formation of Giant Planets, California Institute of Technology: NASA Ames Research Centre.
Woolfson, M. 2000. The Origin & evolution of the Solar system, Astronomy & Geophysics, Vol. 41:1-12.
Woolfson, M. M. 2000. The origin & Evolution of the solar system, New York: Institute of Physics Publishers.
Zellik, M. 2002. Astronomy: The Evolving Universe (9th ed.), Cambridge: Cambridge University Press. pp. 240.