Historic and Current Theories on the Origins of the Solar System

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Submitted by: Angela Britto

Submitted to: Mr. Clarke

Course: SCI 199Y1 – L0111

Date of Submission: Monday, February 13, 2006

 

 

 

 

 

 

 

 

 


The origins of the Earth, Sun, satellites, planets, and celestial bodies which form the Solar System have been a subject for much scientific exploration in a field called cosmogony. Several hypotheses have been presented over the past few centuries; however, an absolute answer to this question has not been determined. The first important theories that came close to doing so were formulated in the 17th and 18th centuries. Since then, many others have been proposed and been built upon, reformulated, and discarded several times as each has not been able to fully explain different aspects of the System. These historical and modern theories will be explored here, which attempt to account for the formation and presence of the Sun at the centre of the Solar System, the orbits of the planets around it, the formation of the terrestrial and gaseous planets, the origin of satellites, and the reasons for the distribution of mass and momentum between the planets and the Sun, also known as the problem of angular momentum.

            First, a description of the Solar System’s features is necessary. It consists of a star called the Sun at its center, with (conventionally) 9 planets orbiting it in slightly elliptical orbits which lie more or less within the same plane called the ecliptic. The distance of each planet’s orbit from the Sun closely follows a simple arithmetic sequence called the Titius-Bode’s law. Most planets are orbited by satellites, numbering 158 in total. The inner Solar System is composed of the terrestrial planets Mercury, Venus, Earth and Mars, as well as the Main Asteroid Belt. The outer Solar System contains the gaseous planets Jupiter and Saturn, as well as Uranus and Neptune. Beyond this lie the trans-Neptunian objects which include the planet Pluto, the Kuiper belt objects, and the Oort cloud which is a source of comets. Meteoroids and interplanetary dust are also found in the Solar System. One important aspect is that the Sun contains approximately 99% of the System’s mass but it only has 1% of its angular momentum, which is the direction and extent to which an object rotates around a reference point – this is known as the problem of angular momentum.[1] Considering these characteristics, one is now in a position to evaluate the major theories which have arisen over the past 400 years, which attempt to account for the origins of such a System.  

            The foremost theory which attempts to account for the Sun at the centre of the Solar System and the orbits of the planets and satellites around it is that of philosopher Rene Descartes. In 1644, he hypothesized that vortex motion prevailed in the fluid primordial matter that filled the universe, and that the matter collected in the center of the largest central vortex (eddy) and this is where the Sun formed.[2] In the smaller vortices surrounding the Sun, the primordial matter collected there formed the planets, which then orbit the Sun. This may be compared to dust that is thrown into an eddying river.[3] Dust will collect and rotate in the centre of every circular eddy, and the largest accumulation in the biggest eddy represents the formation of the Sun. In this largest eddy, smaller ones move around the centre. The accumulations in the smaller eddies represent the formation of the planets. Free-floating particles in these smaller eddies represent the satellites which orbit the planet-eddies. This explained the near circular orbits of the planets, the direction of the orbits, and the Sun’s position in the System. What it did not account for was the formation of the terrestrial and gaseous planets, and the distribution of mass and momentum in the System, and the theory was soon discarded. However, this was the first theory to provide a rational explanation for the origins and characteristics of the Solar System, and the concepts of primordial matter and the central role of the Sun in the formation of the System pervaded the theories that were to follow his.

            The next theory to follow this latter concept was proposed in 1745 by naturalist Georges Louis Leclerc Comte de Buffon. He believed that about 1/650th of the Sun’s mass had been displaced by collisions with comets, and from this were formed the planets.[4] This can be compared to a rotating round disc of wood (Sun) into which a sharp tool is piercing off the edge (comet).[5] The shavings (solar matter) will rotate in the same direction. Smaller shavings (satellites) would move about the larger shavings because they would be drawn to it by fine fibres (gravitational force). The matter with the smallest density would have the greatest velocity and would be thrown furthest away from the Sun,[6] but current data shows that while the planets closest to the Sun are the densest, Neptune is not the least dense planet. Buffon did not have any mathematical data to support him, and his theory failed to explain the formation of terrestrial and gaseous planets, as well as the distribution of mass and momentum in the System. Nonetheless, he did attempt to demonstrate the connection between the Sun and the development of the planets, an element that will reoccur through many of the subsequent theories of cosmogony.

            The most renowned historical cosmogonic theory was presented by Immanuel Kant in 1755, which helped to form the basis of many current theories. He suggested that the Sun and planets had formed from condensations in a cloud of dispersed gas and dust particles with slight density variations, called the solar nebula.[7] Due to gravitational forces, the nebula slowly flattened out and formed a disk, and from this the Sun condensed first, while the disk rotated around it.[8] The particles in the disk fell towards regions of slightly larger density, and particles would collide with each other and be deflected, resulting in movements in closed circular orbits around the centre (Sun).[9] Occasionally, chemical forces would bond them to each other, and form small bodies, and if they fell towards the centre further, this would cause the Sun to rotate about its axis in the same direction.[10] Thus, protoplanets, the precursors to today’s planets formed in this way from nebular concentrations. The larger the planet, the larger the nebular concentration around it would be, and the greater the number of satellites it would have.[11] His limited mathematical background meant his idea did not explain why planets move around the Sun in the same plane, and did not explain the revolution of the satellites.[12] He failed to explain the distributions of mass and momentum between the Sun and planets. He also didn’t see that the particles and gas in the nebula would have been too rarefied (opposite of dense) to tend towards condensation; the attraction between particles in the nebula would not have been stronger than the gravitational forces from the Sun pulling them apart. The flaws in Kant’s theory were minor when compared to the explanations provided for the presence and formation of the Sun at the centre of the Solar System, the orbits of the planets around it, and the origin of satellites. This theory is often combined with that of Laplace to form the definitive nebular hypothesis.

Pierre Simon, Marquis de Laplace postulated his theory about 40 years later, which is combined today in the Kant-Laplace nebular hypothesis. In his “Systeme du Monde,” he assumes a spinning, glowing mass of gas, similar to Kant’s solar nebula. The nebula contracted on cooling, which causes the rotation of the cloud to increase, and the cloud to flatten into a spinning disc. This happens until the centrifugal force balances gravitation along its rim. If the nebula contracts further, a ring of matter will detach itself from this, and not contract any more.[13] This process of division continued until a series of concentric rings were formed, and they cooled down into solid or liquid form.[14] The rings are unstable and will break up into “knots” moving in the same orbit, until they coalesce with each other and form a protoplanet in each ring. This later becomes a fully formed planet with a satellite system.[15] A planet’s satellites would also form from this ring. The central portion of the nebula left over from the rings condenses to form the Sun. This would produce the nearly circular orbits of the planets lying in the same plane orbiting the Sun, and would explain why they rotate in the same direction as the Sun, and why the planets’ distances from the Sun follow a simple arithmetic sequence. Though this theory was generally accepted for the next century or so, there were several problems with it. It did not explain the presence of asteroids with eccentric orbits and moons with retrograde orbits. Secondly, the Sun contains about 99% of the mass of the Solar System, while the planets carry more than 99% of the system’s angular momentum, and for the Solar System to conform to this theory, either the Sun should be rotating more rapidly or the planets should be revolving around it more slowly.[16] It also did not explain the formation of the terrestrial and gaseous planets. Finally, that the rings would coalesce into masses large enough to constitute the planets is unlikely – it would probably result in a collection of circulating small meteorites. Laplace and Kant’s theories involved planets forming from dispersed matter in a solar cloud and thus their theories are often combined to form the Kant-Laplace nebular hypothesis.

            The nebular hypothesis is often evaluated against collision or “catastrophe” theories on the origins of the universe. As seen earlier, Buffon was the first to propose such a theory, and Svante Arrhenius later modified this in the early 20th century. Alternatively, he theorized that the Solar System had been formed from a collision between the Sun and another star.[17] Matter was pulled out from the stars as a result, and it began to move around the Sun. A gaseous disk formed from the matter, and solid nuclei penetrated it from the outside. The planets condensed onto these nuclei. The second star leaves the vicinity so that there is the possibility that the matter expelled can be directed into orbits around the first star.[18] However, this theory failed to explain the origin of the satellites, the formation of terrestrial and gaseous planets, and the distribution of mass and momentum in the System. Nevertheless, this theory gave reasons for the Sun’s position in the System, and the nature of the planetary orbits, and thus was the first of the modern explanations that would be reformulated several times during the 20th century.

            A modification of the catastrophe theory is the tidal theory first proposed by contemporaries of Arrhenius, Thomas Chrowder Chamberlin and Forest Ray Moulton. This theory was formulated primarily to explain the problem of distribution of angular momentum in the System, where the Sun is moving far more slowly than the planets (and it shouldn’t be). The Chamberlin-Moulton theory as it is often called, involves the close meeting of the Sun and a foreign star. Just as the gravitational forces of the moon raises tides on the Earth, the foreign star drew out two arms of filament from the Sun.[19] The two arms would have started revolving around the Sun, and would cool down quickly, condensing into solid particles called “planetesimals.”[20] The planetesimals were drawn together by gravitational forces to form the planets. The problem of distribution of angular momentum is explained by the fact that the foreign star would accelerate the drawn out matter to create the large angular momentums of the later-formed planets, and give the planets a sideward motion, while a planetesimal dropping down onto the Sun would slow down its angular momentum.[21] The matter that did not condense into planetesimals served as resisting materials that would reduce the eccentricities of the orbits of the planets into what is present today.[22] This theory would not explain the orbits of the planets following almost an exact arithmetic progression (Titius-Bode’s rule), the origin of satellites, and the formation of terrestrial and gaseous planets. It did however propose a reasonable explanation for the presence of the Sun at the centre of the System, the orbit of planets around it, and seemed to solve the problem of distribution of angular momentum. The Moulton-Chamberlin component of tidal theory was to be echoed by several others to follow.

            The next notable component of the catastrophe/tidal theory was that of James Hopwood Jeans and Harold Jeffreys put forward in 1916-17. This hypothesis also involves a close encounter between the Sun and a foreign star. The foreign star exerts strong gravitational or tidal forces on the Sun and as a result, a long cigar-shaped strand of filament is drawn away from one side of the Sun.[23] This filament follows the foreign star due to a strong gravitational attraction, and quickly disintegrates into little “knots.” These knots condense into protoplanets and move around the Sun and some may even fall back on the Sun after the departure of the foreign star. This fall causes the Sun to form a rotation in the same direction as the revolution of the planets.[24] The satellites are formed when the protoplanets had material pulled out of it by the Sun’s gravitational tidal forces.[25] The resistance of matter that didn’t form the protoplanets would round out their eccentric orbits. Therefore, the theory explains the central role of the Sun in the System, the nature of the planetary orbits, and the origin of satellites. The problem with this theory lies in the fact that the filament would almost never condense enough to form planets because the matter would have been very hot and would have expanded. Secondly, it turned out to be unable to explain the problem of angular momentum, because a dynamical calculation showed that the material drawn out from the Sun would not have been enough to account for the different-sized planets and their orbits’ distances from the Sun.[26] It also did not account for the formation of terrestrial and gaseous planets. It is for this reason that it was eventually discarded and the way was clear for a new understanding of the origins of the universe and Solar System.

            In 1935, Georges Lemaitre explained these origins with his theory of the primeval atom, more commonly known by the disparaging term given to it by his critics: the Big Bang theory. He believed that the entire universe existed in the form of a highly unstable atomic nucleus which existed for a second before it repeatedly broke up into electrons, protons, and alpha particles.[27] This was accompanied by a rapid increase in the radius of space which the pieces filled. This was the formation of gaseous clouds with very large velocities which would eventually condense into nebulae and stars.[28] The first rapid expansion was followed by a period of deceleration where changes in density in the gas cloud allowed the forces of attraction between the particles to be stronger than the forces of repulsion, and so these areas of the nebula began to contract.[29] When two clouds with great velocities collide, they flatten each other out and remain in contact, increasing the density of the cloud and condensation starts here.[30] This results in a star with several planets rotating around it in the same direction – the Solar System. The universe would begin to expand again after this. The theory of the primeval atom was a change from the previous theories that had expanded, more or less, on either the nebular hypothesis or the catastrophe idea. It provided a an original idea on the way everything began, but did not do much to explain the origin of satellites, terrestrial and gaseous planets, and the distribution of mass and angular momentum in the System. It would, however, spark further attempts to reconcile the origins of the Solar System with this theory.

            George Gamow’s attempt to do so in 1948 is worth examining. He went on to say that after the first five minutes of expansion, particles had begun to coalesce into more complex nuclei like that of helium and deuterium. After 250 million years of this, the gas began to break up into protogalaxies (giant gaseous clouds) which contained matter that would eventually condense into stars and planets.[31] However, no explanation was given for the satellites, terrestrial and gaseous planets, problem of angular momentum, and the properties of the planet’s orbits. This theory soon fell out of circulation and gave rise to a re-emergence of the nebular hypothesis in the mid-20th century.

            Gerard Kuiper explained how the condensation of planets could have occurred from the gas disk that formed from the spinning solar nebula. He postulated that the nebula would have been highly turbulent and unstable because it lay close enough to the Sun so as to be subject to solar tidal forces.[32] Changes in density would have led to condensations which allowed protoplanets to have been formed from them.[33] As the protoplanets continue to contract, the condensable mass collects at the center, forming a planet while some collect to form the planet’s satellites.[34] Solar wind (continuous stream of charged particles from the Sun) would have blown away hydrogen and helium away from the protoplanets near the central part of the nebula, so that the terrestrial planets of high density such as Earth and Mars formed from the remaining dust particles. The gaseous planets of high density such as Jupiter and Saturn formed from the large concentrations of hydrogen and helium further away from the centre of the nebula.[35] Not only did this explain the presence of the sun at the centre of the System, but the formation of gaseous and terrestrial planets. It did not demonstrate the origins of satellites, the orbits of planets, or solve the problem of angular momentum. This theory marked the re-emergence of the nebular hypothesis in all of its various forms.

            One example of this is the dust cloud hypothesis formulated by Fred Lawrence Whipple in 1947. His nebula was a large “smoke” cloud that contracted and produced the Sun. The angular momentum of the cloud is very low, which explains the low angular momentum of the Sun. This cloud captures smaller ones, with a greater angular momentum that is equal to the momentum of the planets today.[36] The captured clouds begin to spiral inward by accretion and the encompassing cloud begins to collapse with increasing speed. The collapse ends when the partially condensed clouds reach the present orbits of the planets.[37] Orbits would be nearly circular because accretion would reduce the eccentricity of the orbits. The orbits would be in the same plane and move in the same direction because the group of small clouds was originally compared to the larger cloud and the motions would be the same.[38] Due to accretion, the protoplanets were heated up to high temperatures and more volatile compounds were lost from the planets moving at the greatest velocity – the planets closest to the Sun.[39] Therefore, the terrestrial planets closest to the Sun formed from dust and heavier, more stable particles, and the gaseous planets would form from the more volatile compounds further away. Whipple’s theory explained well all of the main characteristics of the Solar System: the formation and presence of the Sun at the centre of the Solar System, the orbits of the planets around it, the formation of the terrestrial and gaseous planets, and the reasons for the distribution of mass and momentum between the planets and the Sun. Major shortcomings of this theory is that it failed to describe the origins of the satellites and to use mathematical data to support his conclusions. The uncertainty of this allowed other theories on the formation of the Solar System to be put forward.

            One group of such theories involved the creation of the Solar System as a result of star systems. Raymond Lyttleton (1911-1995) believed that the early Sun was part of a double star. The other star was hit by a third star and was thrown out of the System, while the Sun became enveloped by a cloud which eventually condensed into the planets.[40] He later modified this theory to form a triple star system consisting of a close double star and the Sun. The double star combines due to accretion but will break up due to rotational instability, producing a gaseous filament from which the planets and satellites will condense.[41] Fred Hoyle (1915-2001) believed in a double star, where the Sun was one star, and the other exploded in a supernova. The star’s components of hydrogen and helium were dispelled into space. A small part of the matter dispelled, with a high molecular weight, was captured by the Sun. The Sun was now surrounded by a gas or dust disk out of which the planetary system formed.[42] Though inventive, the multiple-star systems and catastrophe-theories have been generally dismissed today for the inability to account for the problem of angular momentum.

            Today, the Kant-Laplace theory is generally accepted and the Solar System is agreed to have been born about 4.6 billion years ago. Images of young stars have revealed a gas, concentrated in a disk, rotating around the star, similar to the disk put forward by this theory. The modern Laplacian theory states that the central condensing mass has solid dust grains which create a drag. After the centre has slowed down, it heats up and the dust is evaporated. The slowly rotating core becomes the Sun and the planets form from the faster moving cloud around it. [43] This solves the problem of angular momentum distribution in the System. This theory coexists with the modern nebular theory which states that the planets were formed form a dense disk which in turn had formed from material in the gas and dust cloud which had collapsed to give the Sun.[44] The nebula became turbulent due to tidal action of the Sun, and broke into whirlpools of gas called protoplanets. The protoplanets condensed to form the planets. The disk is dense enough to allow condensation of the planets and thin enough for matter that does not form planets to be blown away by solar wind. A third theory that persists today is that terrestrial planets were formed at temperatures high enough to evaporate lighter substances like hydrogen and helium, but low enough to allow for the condensation of heavier substances like iron and silica to form planetesimals.[45] The planetesimals accumulated to form protoplanets, the temperature increased and the metals formed a molten core. At the distance of the gaseous planets, methane, water, and ammonia were frozen, preventing the heavier substances from condensing into small solids and led to their different composition and low density.[46] However, these modern theories are by no means the final explanations for the origins of the Solar System. They simply explain the formation of our own System. The discovery of extrasolar planetary systems like 51 Pegasi in 1995 has revealed large planets orbiting the star at distances less than that of Mercury, and has highly elliptical orbits.[47]

            History has shown that no answer to the question of the Solar System’s origins can be agreed upon. Each proposed theory cannot fully explain the characteristics of the System; characteristics which include the presence of the Sun at the centre of the Solar System, the orbits of the planets around it, the formation of the terrestrial and gaseous planets, the origin of satellites, and the distribution of angular momentum. The theories seem to build upon each other and incorporate other related ideas, and even modern ideas are a synthesis of what has come before. Perhaps there will only be a definite answer to this fundamental question when further extra solar planetary systems which share common characteristics with ours are discovered. Until then, the process of refining and improving our current theories of the Solar System must continue.


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

References



[1] Wikipedia. Solar System. Available from: URL http://en.wikipedia.org/wiki/Solar_System                                                 

[2] Benest, Daniel. Solar System: Formation. In: Encyclopedia of Astronomy and Astrophysics,      November 2000. Available from: URL http://eaa.iop.org/full/eaa-pdf/eaa/2197.html

[3] Berlage H.P. The Origin of the Solar System. Oxford: Pergamon Press, 1968. p. 10-11

[4] Arrhenius, Svante. The Life of the Universe, Vol. II. London: Harper and Brothers, 1909. p. 130

[5] Arrhenius, Svante. The Life of the Universe, Vol. II. London: Harper and Brothers, 1909. p. 131-132

[6] Arrhenius, Svante. The Life of the Universe, Vol. II. London: Harper and Brothers, 1909. p.

 133

[7] Benest, Daniel. Solar System: Formation. In: Encyclopedia of Astronomy and Astrophysics,      November 2000. Available from: URL http://eaa.iop.org/full/eaa-pdf/eaa/2197.html

[8] Berlage H.P. The Origin of the Solar System. Oxford: Pergamon Press, 1968, p. 11-12

[9] Arrhenius, Svante. The Life of the Universe, Vol. II. London: Harper and Brothers, 1909. p. 139

[10] Arrhenius, Svante. The Life of the Universe, Vol. II. London: Harper and Brothers, 1909. p. 139

[11] Jastrow R., Cameron A.G.W. Origin of the Solar System. New York: Academic Press, 1963. p. 7

[12] Solar System. In: Encyclopedia Britannica, 2006. Available from: URL http://search.eb.com/eb/article-242060

[13] Berlage H.P. The Origin of the Solar System. Oxford: Pergamon Press, 1968, p. 14-16

[14] Arrhenius, Svante. The Life of the Universe, Vol. II. London: Harper and Brothers, 1909. p. 146-147                                                                              

[15] Berlage H.P. The Origin of the Solar System. Oxford: Pergamon Press, 1968, p. 16-17

[16] Solar System. In: Encyclopedia Britannica, 2006. Available from: URL http://search.eb.com/eb/article-242060

[17] Berlage H.P. The Origin of the Solar System. Oxford: Pergamon Press, 1968. p. 22

[18] Berlage H.P. The Origin of the Solar System. Oxford: Pergamon Press, 1968. p. 23

[19] Brush, Stephen. Origin of the Solar System. In: Encyclopedia of Astronomy and Astrophysics, November 2000. Available from: URL http://eaa.iop.org/full/eaa-pdf/eaa/2755.html

[20] Berlage H.P. The Origin of the Solar System. Oxford: Pergamon Press, 1968. p. 22-23

[21] Jastrow R., Cameron A.G.W. Origin of the Solar System. New York: Academic Press, 1963. p. 11

[22] Jastrow R., Cameron A.G.W. Origin of the Solar System. New York: Academic Press, 1963. p. 12

[23] Berlage H.P. The Origin of the Solar System. Oxford: Pergamon Press, 1968. p. 24

[24] Berlage H.P. The Origin of the Solar System. Oxford: Pergamon Press, 1968. p. 25

[25] Berlage H.P. The Origin of the Solar System. Oxford: Pergamon Press, 1968. p. 28

[26] Brush, Stephen. Origin of the Solar System. In: Encyclopedia of Astronomy and Astrophysics, November 2000. Available from: URL http://eaa.iop.org/full/eaa-pdf/eaa/2755.html

[27] Munitz, Milton. Theories of the Universe: From Babylonian Myth to Modern Science. Glencoe: The Free Press, 1957. p. 343-44

[28] Munitz, Milton. Theories of the Universe: From Babylonian Myth to Modern Science. Glencoe: The Free Press, 1957. p. 343-44

[29] Munitz, Milton. Theories of the Universe: From Babylonian Myth to Modern Science. Glencoe: The Free Press, 1957. p. 348

[30] Munitz, Milton. Theories of the Universe: From Babylonian Myth to Modern Science. Glencoe: The Free Press, 1957.  p. 350

[31] Munitz, Milton. Theories of the Universe: From Babylonian Myth to Modern Science. Glencoe: The Free Press, 1957. p. 399  

[32] Jastrow R., Cameron A.G.W. Origin of the Solar System. New York: Academic Press, 1963. p. 28

[33] Berlage H.P. The Origin of the Solar System. Oxford: Pergamon Press, 1968. p. 47

[34] Jastrow R., Cameron A.G.W. Origin of the Solar System. New York: Academic Press, 1963. p. 29

[35] Berlage H.P. The Origin of the Solar System. Oxford: Pergamon Press, 1968. p. 52

[36] Jastrow R., Cameron A.G.W. Origin of the Solar System. New York: Academic Press, 1963. p. 27

[37] Jastrow R., Cameron A.G.W. Origin of the Solar System. New York: Academic Press, 1963. p. 27

[38] Jastrow R., Cameron A.G.W. Origin of the Solar System. New York: Academic Press, 1963. p. 27

[39] Jastrow R., Cameron A.G.W. Origin of the Solar System. New York: Academic Press, 1963. p. 27

[40] Berlage H.P. The Origin of the Solar System. Oxford: Pergamon Press, 1968. p. 29         

[41] Jastrow R., Cameron A.G.W. Origin of the Solar System. New York: Academic Press, 1963. p. 14

[42] Berlage H.P. The Origin of the Solar System. Oxford: Pergamon Press, 1968. p. 30

[43] National Maritime Museum. The Origin of the Solar System. Available from: URL http://www.nmm.ac.uk/server/show/conWebDoc.269/viewPage/6

[44] National Maritime Museum. The Origin of the Solar System. Available from: URL http://www.nmm.ac.uk/server/show/conWebDoc.269/viewPage/6

[45] Solar System. In: Columbia Encyclopedia, Sixth Edition. 2000-2005 on Infoplease. Available from: URL http://www.infoplease.com/ce6/sci/A0861168.html

[46] Solar System. In: Columbia Encyclopedia, Sixth Edition. 2000-2005 on Infoplease. Available from: URL http://www.infoplease.com/ce6/sci/A0861168.html

[47] Solar System. In: Columbia Encyclopedia, Sixth Edition. 2000-2005 on Infoplease. Available from: URL http://www.infoplease.com/ce6/sci/A0861168.htm

 

 

 

 

 

 

 

 

 

Bibliography

 

Arrhenius, Svante. The Life of the Universe, Vol. II. London: Harper and Brothers, 1909.

 

Benest, Daniel. Solar System: Formation. In: Encyclopedia of Astronomy and Astrophysics,      November 2000. Accessed on February 1, 2006. Available from: URL http://eaa.iop.org/full/eaa-pdf/eaa/2197.html

 

Berlage H.P. The Origin of the Solar System. Oxford: Pergamon Press, 1968.

 

Brush, Stephen. Origin of the Solar System. In: Encyclopedia of Astronomy and Astrophysics, November 2000. Accessed on February 1, 2006. Available from: URL http://eaa.iop.org/full/eaa-pdf/eaa/2755.html

 

Jastrow R., Cameron A.G.W. Origin of the Solar System. New York: Academic Press, 1963.

 

Munitz, Milton. Theories of the Universe: From Babylonian Myth to Modern Science. Glencoe: The Free Press, 1957.

 

The Origin of the Solar System. In: National Maritime Museum. Accessed on February 2, 2006. Available from: URL http://www.nmm.ac.uk/server/show/conWebDoc.269/viewPage/6

 

Solar System. In: Columbia Encyclopedia, Sixth Edition. 2000-2005 on Infoplease. Accessed on February 2, 2006. Available from: URL http://www.infoplease.com/ce6/sci/A0861168.htm

 

Solar System. In: Encyclopedia Britannica, 2006. Accessed on February 1, 2006. Available from: URL http://search.eb.com/eb/article-242060

 

Solar System. Wikipedia. Accessed on February 2, 2006. Available from: URL http://en.wikipedia.org/wiki/Solar_System