The origin of the universe has been one of the biggest mysteries modern science has been unable to conclusively explain. The solar system is believed to have been formed many years ago, probably 4.5 billion years. Although its formation and evolution are not well understood, the facts are quite well known. There are several models that have been used to explain the origin of the solar system. They include the sun-cantered model, capture model and nebular model among others. Amongst them, the Nebular Model is the most accepted model. The nebular model traces the origin of the universe from the collapse of a gaseous cloud called nebula due to the effect of gravity. More specifically, it clearly elaborates the origin of the basic features of our solar system. From ancient times, the nebular model has mainly concentrated on the intrasolar system, but now it is employed to cover even the extrasolar system. The nebular model explains the evolution of the universe as a slow, gradual and natural process through which the sun and planets amongst other objects condensed from the interstellar gigantic clouds, nebulae. In other words, the nebular models elaborate that the solar system started as the gaseous clouds-nebulae in space which contracted and condensed to form the sun, planets, moons, asteroids and many other objects of the solar system (Jones 2007).
This model was originally proposed by Emanuel Swedenborg in 1734. Later in the 18th century, Immanuel Kant and Pierre-Simon Laplace further developed Swedenborg’s idea. They suggested that the gas and dust clouds, nebulae, under the influence of gravity would slowly rotate, flatten and gradually collide, thereafter condensed and join to form what became the solar system, planets and stars. While the solar nebula is rotating and flattening it gets warmer at the centre. The particles of the nebulae collide and clump together forming planetesimals across the swirling disk. As a result of the great gravitational forces, the larger planetesimals collect the gases and dusts while the disk is spinning. Both the large and small planetesimals collide with each other forming the planets. The remaining gases and dust are removed from the nebula hence leaving a hot gaseous centre, the sun, surrounded by the planets. The forces related to gravity and solar wind contributes to remove the gas and dust while nebula is spinning leaving a central bulged centre. This reduces the angular momentum making it easier for materials to fall on the top rather than the central bulge that eventually had the least angular momentum. According to the nebular model, the central bulge disk (the sun) formed should be rotating rapidly or rather have more angular momentum in the solar nebula (McFadden et al. 2007).
The sun was formed alongside many other stars in the swirling solar nebula. As some astronauts argued, radiation from the nearby stars would have evaporated the disk around the sun. Moreover, the gravitational pressure by these stars could have pulled the gas and dust in the disk away. However, this was not true as the following factors clear the nebula model. These are sun’s radiation, solar wind, and the gravity as a result of planets and planetesimals encounter. Relative to sun’s radiation, although its radiation pressure pulls the materials including gas and dust away from the solar nebula, planetesimals and planets are not affected due to their large size. The strong solar wind from the sun further assists to push away the gas and dust from the solar nebula (McFadden et al. 2007).
As far as the basic features of our solar system, any formation model must clearly explain the existence of these solar system features. As mentioned above, the nebular model is more preferred to illustrate the formation of the universe using the basic features of the universe. The nebula model explains the disc shape taken by the solar system. This shape originates from the movement of solid materials within the solar nebula. Additionally, the motion pattern of these materials forming the sun, planets, moon and other objects rotate and revolve in a similar direction. This is attributed to the fact that they are formed from a similar rotating gaseous cloud, nebula. Furthermore, the planets orbits fall in the same plane since the solar nebula collapsed in the swirling disk where the planets were formed prevention further collapsing. The temperature and density of the disk are also instrumental. An increment in both the temperature and the density towards the centre causes an increase in the internal pressure yielding the gravitational force. This creates a balance between the internal pressure and the gravitational force thus preventing further collapse (Seeds 2011).
Next, the nebula model elaborates the existence of the two different types of planets, terrestrial and Jovian planets. Planets form from the tiny solid particles sticking together to form planetesimals through gravitational process known as accretion. The process of accretion involves gradual growth of the solar system through collision and sticking of small materials of the solar nebula. Accretion process takes place in different phases. First and foremost, the gravity swept all materials in the solar nebula including both the heavier and lighter materials. As these materials interacted, their collision rate became faster where they contracted, condensed and started to clump together. By the end of this stage, millions of planetesimals surrounded by atmospheres of hydrogen and helium gas formed the solar system. In the next phase, the gravitational force amongst the planetesimals triggered them to collide and merge again to form large planetesimals. Because large planetesimals have higher gravitational forces, all the previous planestesimal materials were forced to combined forming few larger protoplanets accumulated with more matter, which eventually evolved into planets we know today (Seeds 2011).
However, the nature of the solid material from which planets form is dependent on the temperature. In addition, the temperature variations in the disk’s inner and outer regions determine the condensates available to form the planets. The inner region of the nebula is hotter, and only heavier materials can condense leading to the formation of terrestrial planets, Mercury, Venus, Earth and Mars. The planetesimals drop into the disk’s plane where they interact, merge and agglomeration takes place forming protoplanets which further become the planets. Once the planetesimals have grown in size, their collisions become destructive hence inhibiting further growth. On the contrast, the large planetesimals are not affected and continue to growing steadily and clump with other planetesimals of similar composition to form protoplanets through accretion. This formation process is further necessitated by the fact that all planetesimals spin in a similar direction and thus they collide in a gentler manner that is not destructive. These heavier materials are very little in the inner solar nebula and thus the planets formed can not grow larger. Moreover, these planets exert minimal pull on helium and hydrogen gases hence no ice can be formed. In the inside part, only materials with high melting point such as metals and silicates can condense into solids (Seeds 2011).
On the contrary, the Jovian planets formation is favoured by low temperatures and, therefore, takes place on the cooler outer solar nebula. This low temperature favours formation of lighter elements and ice. More ice is formed as opposed to the silicates and it coalesces around the planetesimals causing them to grow large rapidly forming the four largest protoplanets. As earlier mentioned large planetesimals have high gravitational force to attract other small planetesimals, as well as gases and dust around them. Therefore, these four protoplanets due to their largeness attract large amounts of hydrogen and helium gas from the solar nebula forming the four jovian planets-Jupiter, Saturn, Uranus and Neptune. Satun and Jupiter are enormously large, and this is contributed by their growth ability through drawing in the cool helium and hydrogen gas form the solar nebula by gravity forming a thick atmosphere. Terrestrial planets are limited in size and thus lack this ability to capture helium and hydrogen gas. In cases where the terrestrial planets had the potential to capture these gases this would not be possible. This is attributed to the fact that they are nearer to the sun and so the sun’s heat would melt up these gases causing them to escape. This hindered the terrestrial planet developing their atmospheres. The smaller, inner protoplanets that developed to terrestrial planets never reached this point and their masses remained relatively low (Seeds 2011).
However, terrestrial planets later came to have their own atmospheres. These atmospheres were acquired as a result of the bombardment of the planetesimals outside our own solar system. In addition, outgassing was another source instrumental for the formation of atmosphere in terrestrial planets. For instance, oxygen found on planet Earth developed from the breakdown of carbon dioxide by plants. Far from our own solar system, beyond Neptune onwards, only a few of the planetesimals survive. This is because of the presence of a colder atmosphere. Nevertheless, due the low disk density these icy and dusty planetesimals did not grow bigger. Additionally, the planetesimals could not accrete the gases around them thus, they remained small forming comets. Comets rotate in all directions and they have highly inclined eccentric orbits. These comets are hydrogen and icy snowballs. Pluto, the farthest solar system’s planet, is classified in the family of comets. Pluto falls between the errestrial and jovian planets. It is referred as aterrestrial planet due to its small size whereas as a jovian planet due to low density (Seeds 2011).
Besides the physical composition and size between the terrestrial planets and jovian planets, the nebular model describes the difference in the number of the solar system objects such as the moon present in both planets. Most planets and other solar bodies have moon(s) surrounding them.. In regards to the jovian planets, there are large amounts of gases present. These gases are captured and are transformed through a similar process that formed the planets of the solar system to form similar but smaller objects surrounding these planets. As they undergo condensation and accretion in the jovian nebulae a miniature solar system is created surrounding each of the jovian planets. This explains why jovian planets, mainly Jupiter, have over a dozen moons. Some of the jovian planets, Saturn, have rings surrounding them. This ring forms from the stray planetesimals that are destroyed by gravitational forces when they get closer to these planets (Seeds & Backman 2010).
Likewise, this collision of planetesimals also took place with the terrestrial planets. Nevertheless due to the small and compact size of the planetesimals the gravitational impact on these planets contributed to the formation of few solar system objects like the moon around these planets. For example, the Earth was hit by planetesimals equal to the size of planet Mars and it ejected debris that condensed and coalesced forming the only moon. Between the terrestrial and jovian planets, there exist thousands of asteroids. These asteroids are considered to be debris that did not consolidate into planets during the solar system formation. This is attributed to the gravitational force by jovian planets, especially Jupiter. Therefore, there exists a large asteroids belt between the terrestrial planet Mars and jovian planet, Jupiter but these asteroids take the composition of terrestrial planets. These heavenly bodies further collide and collapse producing small fragments, meteorites that in some occasions fall on the terrestrial planet, Earth. These meteorites provide significant information on the age of the solar system (Davis 2005).
Another feature of the solar system the nebular model defines is the dated age of the solar system. For a solar system model to be correct, the stated date of the formation of the planets should correspond roughly with that of the sun. As earlier mentioned, the solar system is dated 4.5 billion years. Nevertheless, we can only ascertain this by using radioactive decay. The oldest rocks to have been encountered are the meteorites dated about 4.6 billion years (Davis 2005). These rocks were formed during the solar nebula condensation process from which the planets were formed 100 million years later. Therefore, the age of the planets is estimated to be about 4.5 billion years. Relative to angular momentum, the sun in the solar system has little or no angular momentum. Since the sun and the planets form from a contracting nebula, it is expected that the sun should be spinning rapidly in the counter clockwise direction. Thus, the sun is expected to have more angular momentum than the others objects. Formation of the sun, as explained above, would cause the same effect as it would have occurred when the gases contracted to form the sun.
This would have triggered the sun to spin at a very high speed. But the sun spins very slowly, whereas “the planets move very rapidly around the sun, although the sun has over 99% of the mass of the solar system, it has only 2% of the angular momentum” (Davis 2005, 637). The stars that rotate slowly have the high likelihood of being surrounded with gas and dust in their disks, as opposed to the fast ones. Therefore, the strong magnetic field in the stars extends further into their disks. This triggers angular momentum transfer hence speeding disk rotation and, on the other hand, slowing rotation of the stars. This caters for the deficit in the sun’s angular momentum (Seeds & Backman 2010). Finally, gravitational collision of the planets with the planetesimals may have a wide range of effects such as the formation of craters. Furthermore, this collision also contributed to the ejection of other planetesimals outside our solar system. Most of these planetesimals were left over after formation of the major planets of our solar system. Eventually, these planetesimals collided with the planets or were ejected into the Oort Cloud to form other solar system of the universe (Ollivier et al. 2008).
The Nebular model provides the natural explanation of the solar system using the basic features of the solar system. Most notably, the nebular model elaborates that the solar system evolved from the collapse of a gaseous cloud, the nebula due the gravitational pressure. The nebula collapsed by spinning faster as it contracted due to the conserved energy. Further the collapsed materials of the nebula condense to form the sun, planets, moon, as well as other debris of the solar system. In regards to the known basic features of the solar system, the nebular model illustrates the existence of these features. All planets lie in the same plane with the sun positioned at the middle. In addition, all the planets revolve and rotate in a similar direction with the axis of rotation at a right angle with the orbital plane. The nebular model predicted that as the nebula spiralled inwards, all the planets and comets should rotate on their orbits in a similar direction (prograde) with the exception of Venus. Venus rotates in an opposing direction, called retrograde. Furthermore, comets were also found to have a retrograde orbit (Davis 2005). Despite accounting for all features of the solar system, there is still not core foundation of how the nebular model accounts for these details. Besides these criticisms, the nebular model remains the most preferred model for the formation of the solar system.
List of References
Davis, A M 2005, Meteorites, comets, and planets, Elsevier, Amsterdam.
Jones, B W 2007, Discovering the solar system, England, John Wiley Chichester, West Sussex.
McFadden, L A, Weissman, P R, & Johnson, T V. 2007, Encyclopedia of the solar system, Academic, Amsterdam.
Ollivier, M, Casoli, F, Encrenaz, T, Roques, F, & Selsis, F. 2008, Planetary Systems: Detection, Formation and Habitability of Extrasolar Planets, Springer, Berlin Berlin.
Seeds, M A 2011, The solar system, Brooks/Cole, Cengage Learning, Boston, MA.
Seeds, M A & Backman, D E 2010, Astronomy: the solar system and beyond, Brooks/Cole, Cengage Learning, Belmont, CA.