The Solar System’s Nebular Model

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Table of Contents

Introduction
Pre-solar nebular
Planetesimals
Formation of the moons
Features explained by nebular model
Conclusion
References

Introduction

Nebular model, an explanation about the origin of the solar system was first proposed by Laplace in 1796. He suggested that the matter from which the solar system formed was at one time a nebula or a slowly rotating cloud of hot gas and dust. The dust and gas cooled and the cloud began to shrink. As the cloud became smaller, it began to spin more rapidly, and somehow became flattened. The rotation could have resulted from a combination of centrifugal forces while gravitational force caused the fragments of gaseous matter to be left behind. The rings fused into planets and moons while the larger part of the cloud formed the sun.

The nebular model is the most widely accepted hypothesis in cosmology, but has several flaws. First concerns about the speed of the rotating sun. The model predicts the speed of the rotating sun to be 50 times fast that its actual speed. Secondly, there are doubts that the rings hypothesized to form the planets would ever condense. However, this model seems to explain most of the phenomena observed in our solar system. These have been considered to be the evidence of the theory for a very long time. This has been supported by scans of the universe which indicate the process to be taking place elsewhere. In response to this, the paper is aimed at explaining the nebular model of the solar system in details and the features of the solar system that the model can explain. Despite the many theories about the formation of the solar system, the nebular model seems to be the most inclusive and which is associated with observable evidence.

Pre-solar nebular

The nebular model maintains that our solar system began to form when a fragment of a giant cloud of dust and gas began to collapse due to the gravitational forces exceeding the forces related to gas pressure that expanded it (Montmerle, et al, 2006. p.47). This collapse was triggered by a range of perturbations such as density waves in rotating galaxies and a supernova blast wave. Montmerle and others (2006) assert that the cloud had the size of about 20 pc while the collapsing fragment was about 1pascecs across (p.47). The fragments continued to collapse resulting to the formation of dense cores, about 0.01 to 0.1 pc in size11. The pre-solar nebular (one of the collapsing fragments) was to form our solar system. The mass of this fragment which was to form the sun was just larger that the mass of the sun. This part contained elements like hydrogen, helium as well as lithium while the other included heavier elements formed earlier.

*Fig.1: Nebular collapsed by gravity and starts to spin rapidly.*

Stable daughter nuclei of transitory isotopes like iron-60 which form only in exploding transitory stars have been revealed from studies of earliest meteorites. Therefore, a supernova must have occurred in a region near the sun. it is likely that the hypothesized formation of the sun was initiated by the supernova shock waves. The nebular became denser and caused the fragmentation. And since only enormous, transitory stars produce supernova, this formation must have taken place in the region where massive stars are formed, probably similar to Orion Nebular (Hester, Desch, Healy & Leshin, 2004. p.1117). Revelations from Kuiper belt and the strange materials it contains suggest that the formation of the sun occurred within a cluster of stars. The width of the cluster was between 6.5 and 19.5 light years and a mass of about 3,000 suns (Simon & Zwart, 2009. p.13).

In the Nebular model, the collapsing cloud begins to spin faster and faster because of the angular momentum being conserved. The condensation of the matter in the cloud was characterized by the bombardments of the molecules with escalating frequency, and their kinetic energy changing into heat energy. The core of the nebular, where the mass was concentrated, acquired much heat than the surrounding regions. For many years, the competing forces associated with gas pressure, gravity, rotation, and magnetic fields caused the contracting cloud to flatten into a spinning pancake shape (protoplanetary disc) and formed a hot, dense protostar or a star prior to hydrogen fusion at the core (Greaves, 2005. p.68).

*Fig.2: The spinning cloud begins to flatten into a protoplanetary disc.*

During this stage in the formation of solar system, the sun is suggested to have been developed into a T Tauri star. In the presence of T Tauri star, it means that there are protoplanetary plates having smaller masses than the star itself. These discs may extend to several hundred light years and are somewhat cool, with the highest temperature being a thousand kelvins only (Küker, Henning & Rüdiger, 2003. p.397).

The Hubble Space Telescope observed potoplanetary discs that are 1,000 AU wide in regions where stars are formed like the Orion Nebular. With time, the temperature and pressure of the center of the sun went to an extent that the hydrogen gas contained started to fuse and created a source of energy within the disc that counteracted the gravitational contraction leading to a hydrostatic balance. At this juncture, the sun entered into the principal phase of its evolution, often referred to as the main sequence. Stars in this phase obtain energy from hydrogen fusion in their centers. Since the formation halted, the sun has existed as a main sequence.

Planetesimals

According to nebular model, the planets in our solar system formed from the same nebula as the sun, the sola nebula. The cloud fragments left from the formation of the sun were responsible for the planets formation (Boss, & Durisen, 2005. p.137). The common and accepted method by which this formation took place is referred to as accretion. In this method, the planets started out as dust particles orbiting around the inner protostar. These particles collected into bigger objects through fusion and later collided to form planetesimals. Through further collisions, these bodies ultimately increased in size at a rate of several centimeters annually for several million years that followed.

The solar nebular formed from very hot gases and dust and the heat could not allow volatile molecules to condense. Therefore, only silicates and metals (heavier elements) were the only constituents in the formation of terrestrial planets. These rocky planets became the so called terrestrial and include: Mercury the closest to sun; Venus the second nearest, Earth the third nearest and finery Mars the farthest of the rocky planets. Compounds with high melting points are very scarce in the universe and so the rocky planets grew to relatively small sizes. The terrestrial planetesimals grew to a small fraction of the Earth masses and stopped accumulating matter approximately 100,000 years after the sun was formed. Through collision and fusions of the terrestrial embryos that followed, the rocky planets are believed to have grown into their present sizes (Lin, 2008. p.58).

During the formation of the rocky planets, these planets remained engrossed in a gas and dust cloud disc. And because the gas had its own pressure plus the gravitational pressure, it orbited slower than the forming planets. The difference in pressure resulted in a drag which changed the angular momentum causing the planets to eventually move to new positions. The temperature differences in the disc controlled the rate by which the planets moved, yet the overall trend was for the planets nearest to the core to move inward as the nebular dissipated, leaving them in their present orbits.

Jovian planets essentially formed further away from the sun. This is past the snow line which is the area between Jupiter and Mars. In this region, the temperatures were lower and volatile elements could condense. These materials constituted the larger part of the Jovian planets. These compounds are more abundant in the universe than silicates and metals that formed the rocky planets. As the planet increased in size, they were able to consume the lighter gases that were most abundant in the solar nebular. Formation of the planets past the snow line collected to several times the earth masses in a period of about three million years. At present the Jovian planets comprise almost 99 percent of the total mass rotating around the sun.

Furthermore, it is believed that the existence of Jupiter near the snow line is not an accident. As the falling ice approached the snow boundary, it encountered a change in temperature and evaporated causing the surrounding area to accumulate a lot of water. This resulted in a reduction of the pressure and dust particles could spiral faster and thus stopped moving towards the sun. Effectively, the snow line formed a barrier that made the matter to accumulate fast at a short distance from the sun. The excess matter combined into a large body of several Earth masses that grew swiftly by acquiring hydrogen from the adjacent disc to the largest planet in the solar system. The lower masses for Saturn resulted from its later formation when most of the gas to consume had been swallowed by Jupiter.

All the T Tauri stars including the sun are characterized by strong stellar wind. Other stable stars may have weaker winds. Neptune and Uranus must have formed after Saturn and Jupiter, when the stellar winds had cleared much of the matter within the disc. Therefore, the planets acquired very small amounts of hydrogen and helium, probably one Earth mass each. In effect, the two planets are usually called the “failed cores”. Though, the formation assumptions of these planets bring a problem relating to the time taken to form them.

The actual distance of the planet from the sun suggests that the planets formation or the accretion could have taken a much longer period of time. This means that the formation of the planets took place a closer distance from the sun…probably near or between Saturn and Jupiter…and they later migrated to their present positions (Levison et al., 2007. p.258). Planet migration was on both directions during their formation, either to the warmer region or the cold regions. The growth of the planetisimals could have halted after many years when the strong stellar winds forced the material out of the solar nebular into interstellar region.

Formation of the moons

Moons have been known to revolve around many planets as well as other bodies in the Solar system. Three mechanisms could have been responsible for the formation of the moons and include: from a solar disc through co-formation, from bombarding fragments and confinement of the passing objects.

Saturn and Jupiter have several large moons including Europa, Io, Titan, and Ganymede, which might have formed from discs surrounding each planet in a similar way the giant planets originated from the discs surrounding the sun (Takato, et al., 2004. p.2224). This formation is indicated by the nearness of the moons to the planets and their relatively larger sizes. The indicated attributes cannot be attained through capture neither can the moons form from bombardment fragments due to their gaseous nature. Moons further away from the giant planets tend to have smaller sizes and have peculiar orbits with random inclinations. Only captured bodies could be having such characteristics. Unfortunately, such moons have been reported to be revolving in the opposite direction. Evidence on the capturing of passing objects is based on Triton which is a moon of Neptune which has many irregularities. This moon could probably have been captured with Kuiper belt.

The formation of the moons within the terrestrial planets could probably have been a consequence of collision or capture. Phobos as well as Deimos is believed to be captured asteroids outside the snow line. Stevenson (1987) suggested that the Earth’s moon might have formed from a particular, slanting collision (p.271). The object that caused the impact had a mass similar to that of Mars, while the collision possibly took place as the end of giants bombardment period approached. The bombardment released some of the fragments into the orbit, which then united to form the moon (Canup & Asphaug, 2001. p.710). Perhaps, the collision was the last in the sequence of fusions that formed the primary. The earth-Sun Lagrangian points are also believed to be some of the areas where the moons could have been formed. Charon, a moon in Pluto is believed to have formed from a large collision.

Features explained by nebular model

The nebular model explains many features of the sola system including:

Density difference between Jovial and terrestrial planets
Terrestrial planets have fewer and smaller moons than Jovian planets
Most of the planets carry a disc shape
All the planets revolve in one direction

The terrestrial planets have approximately similar densities as none accreted much of the low-density material, which still existed in vapour form in the region near the core of the solar system. Moving towards Mars and Asteroids, volatile condensates such as water are more available leading to lower densities. However, the bodies could grow big enough to allow for the gravitational swallowing of gases. Saturn and Jupiter grew large enough to accrete gasses which lowered their densities. The increasing density of the outermost planets is due to the low-density methane condensates which makes the planets to have higher levels of heavy materials which increase the density.

The nebular model explains the formation of moons to have occurred as a result of co-formation, impact fragments, and capture of passing objects. Only co-formation was possible in the region beyond frost line and resulted in moon almost the size of terrestrial planets. The circum-planetary disc as a result of solar nebular fragmentations resulted in the formation of many fragments that fused to form the planets and moons. However, the moons in the terrestrial planets formed as a result of bombardments and capture of the passing objects.

The collisions were not very frequent and could have occurred outside the terrestrial region sending the fragments into the interstellar region. In addition, the passing objects could only be captured within the Kuiper belt region in order to form a moon. Unfortunately, many objects passed outside this region and thus only a few could be captured.

Due to the competing forces from gas pressure, gravitational force, and angular momentum, the constricting nebular starts to flatten resulting into a spinning flattened object with a swelling at the core. The decreasing angular momentum support close to the poles indicates that the material will easily fall close to the top, and not at the equator. Therefore, there results a swell which finally leads to the disk shape. The disc should not necessarily be flat, but is typically thicker on the outside than inside.

The entire solar system including the sun and the planets were formed from the solar nebular in which an angular motion was involved. In turn, the resulting fragments also spiraled in the same direction but with varying velocities due to their accumulating materials. As the disc materials reduced and the stellar winds brew them into the interstellar region, the revolving bodies could not change the aspects that defined their flow and rotated on a specific orbit at a specified speed. However, the nebular model does not explain satisfactorily about the moons that rotate in the opposite direction, yet it claims to have been formed together with the planets. The only explanation could be that the moons formed from either collision outside the solar system or capture of a passing object to assume their present direction. There is also a possibility that the moons collided with other bodies and thus changing their direction of rotation.

Conclusion

The nebular model of the solar system is a comprehensive theory that explains the origin of the solar system basing on the existence of a cloud of gases and dust. The cloud contracted to form a disc-shaped nebular. The nebular then contracted to form small planetisimals. The planetisimals fused to form the planets. The moons could have formed from bombardments or capture of passing objects, but most of the larger moons for the jovial planets formed just like the planets. The nebular theory can explain why the planets revolve in the same direction or why most planets are disc shaped. It can also explain the difference in densities between jovial planets and terrestrial planets and why the rocky planets have fewer moons that the giant planets.

References

Boss, A. P. & Durisen, R. H. 2005. Chondrule-forming shock fronts in the solar nebula: a possible unified scenario for planet and chondrite formation. The Astrophysical Journal, 621(2), pp.137–140. Web.

Canup, R. M. & Asphaug, E. 2001. Origin of the Moon in a giant impact near the end of the Earth’s formation. Nature, 412 (6848), pp.708–12. Web.

Greaves, J. S. 2005. Disks around stars and the growth of planetary systems. Science, 307 (5706), pp.68. Web.

Hester, J. J., Desch, S. J., Healy, K. R. & Leshin, L. A. 2004. The cradle of the Solar System. Science, 304 (5674), pp.1116-1117. Web.

Küker, M., Henning, T. & Rüdiger, G. 2003. Magnetic star-disk coupling in classical t Tauri systems. Astrophysical Journal, 589 (1), pp.397. Web.

Levison, H. F. et al 2007. Origin of the structure of the Kuiper belt during a dynamical instability in the orbits of Uranus and Neptune. Icarus, 196 (1), pp.258. Web.

Lin, D. N. C. 2008. The genesis of planets. Scientific American, 298 (5), pp.50–59. Web.

Montmerle, T. et al. 2006. Solar system formation and early evolution: the first 100 million years. Earth, Moon, and Planets, 98 (1-4), pp.39 –95. Web.

Simon F. & Zwart, P. 2009. The Lost siblings of the Sun. Astrophysical Journal, 696 (13/16), p.13. Web.

Stevenson, D. J. 1987. Origin of the moon–The collision hypothesis. Annual Review of Earth and Planetary Sciences, 15 (1), pp.271. Web.

Takato, N. et al. 2004. Detection of a deep 3- m absorption feature in the spectrum of Amalthea (JV). Science, 306 (5705), pp.2224–7. Web.

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