There are two major theories that have been advanced to explain the origins of the solar system. Any of the theory that is accepted need to explain or satisfy three leading questions. 1. The differences in composition between the terrestrial planets and the Jovian planets. 2. The fact that all the planets orbit the sun in the same direction and all their orbits are in nearly the same plane and 3. The terrestrial planets orbit close to the sun while the Jovian planets orbit far from the sun. The two theories that have been advanced by astronomers are the nebular theory, and the Core Accretion hypothesis.
The Core Accretion hypothesis
This theory has been studied for a long time by astronomers. It states that planets are formed by the accretion of planetesimals and gases from the solar nebula
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. Under conditions of low temperatures and low pressure a substance cannot remain as liquid, its condensation temperature determine whether it will end up either as a solid or a gas. The gas cloud from which the solar system formed had initial temperature near 50 Kelvin so these substances could have existed in the solid form.
This condition changed as the central part of the solar nebula heated up to form the proto-sun. As the proto-sun collapsed and heated up, the temperature of the inner solar system rose and that of the outer planet remained cool. This explains why the inner planets are rocky and the outer planets remain icy.
The planets became larger by the accretion onto smaller bodies. As a planet grows, it tends to get even larger because its gravity causes more material to accrete on it. The planets captured smaller bodies, creating more space between each planet.
The Nebular Theory
The nebular theory that has been accepted was the theory proposed by Immanuel Kant in 1755. His proposal was not tested, but with today's data the theory seems to describe the true origin of the solar system.
Observations have shown that regions of diffuse matter between the stars have dark globules, huge clouds of cold gas and dust. These large clouds contract under their own gravity. As they shrink, their matter is compressed, thus becoming hotter. As they contract they rotate faster and faster and their shape become distorted. A distinct equatorial bulge is developed as a result of the contraction. The rotation and contraction continue until the material in the disc acquires orbital velocity about the sun. This concentration of gas continues to revolve around the sun in a circular motion. After the concentration of the disc stops, the heating of the disc ceases. The sun prevents the inner regions from cooling off rapidly, while the outer regions become colder by radiation of heat.
The radiation of the solar system depends on the chemistry of the cooling gases. There are three facts that are consistent with respect to the gases that form the solar system
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1. The gases consist mostly of hydrogen and helium. The existing temperature in the disc causes the hydrogen and helium to remain as gasses.
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2. The rest of the matter in the disc consists of some light atoms, such as oxygen, carbon and nitrogen. These can form solids at very low temperature.
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3. The less common, heavier atoms such as magnesium, aluminum, silicon, calcium, and iron are capable of forming solids at higher temperature.
The formation of the Planets.
In the cold outer part of the disc the temperature remained very low. Low enough for the molecules to form into ice. In the inner part of the disc, the Sun's heat prevents it from becoming cold, but remains hot so that only heavier atoms like iron and molecules of materials can condense. These particles orbit the Sun, and as they collide with each other and pick up other particles. They become large and therefore attract the particles that come near to them. The largest particles attract most of the matter, thus growing large and becoming planets.
In the inner regions where it is too hot for lighter elements to condense, the rocky and metallic planets like Mercury, Venus, Earth and Mars were formed
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. Mercury was formed in the hottest region. It has the highest iron content, and the highest density. The elements that form the rocky planets are relatively scarce and are very heavy these planets are very small and dense. The planets formed in the colder regions are more massive and less dense. Jupiter and Saturn are so massive that their gravity captured large amounts of hydrogen and helium gasses. It should be noted that neither of the hypothesis presented explain planetary systems other than our own. Hence other hypotheses are being examined.
The classification of the Solar System
The solar system consists of other bodies beside the planets and the Sun. There are the satellites of the planets; the asteroids and the comets. In addition the planets can be differentiated by their composition (the terrestrial planets and the Jovian planets), by their location (the inner planets, and the outer planets), and by their sizes (the larger planets, and the smaller).
The Structure and Composition of the Comets
Comets are small objects composed of ices and dust. They spend million of years in the outer regions of the solar system in their frozen state before being perturbed into smaller orbits around the Sun. The particles that form the dust do not evaporate. They are released and orbit the Sun with great eccentricity, when they are near the Sun they vaporize, and form a luminous head and a long tail. The spectrum of a comet is the reflection of the sunlight by the dust particles.
The head of the comet expands as it nears the Sun. The head is surrounded by a huge hydrogen cloud. The hydrogen comes from water molecules and hydrocarbon molecules broken up by the sunlight. The tail of the comets always points away from the Sun. This is caused by the radiation pressure of the solar winds
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. Therefore the tail of the comet precedes it in parts of the orbit. When the comet gets close to the Sun more material is fed into the tail. The tail gets larger as the comet passes the Sun and diminishes as it moves away from the Sun.
The brightness of the comets increases as they get close to the Sun, because more material evaporate and the sunlight excites the gasses and reflect the dust become more intense. As a result the brightest comets are those that get closest to the Sun. Each time the comet passes the Sun they lose material by evaporation. In some cases the effect is so strong that that the comet can split in pieces. After a number of solar passages the comet will evaporate. Most comets are found to have elliptical orbits and approach the sun with great speeds. These planets on elliptical orbits are bound to the Sun and belong to the solar system. As the comets enter the inner region of the solar system can have their orbits affected by the planets. Some of the planets speed up and their orbits are converted into a hyperbolic orbit. These comets make their last trip about the Sun and then depart from the solar system.
The orbits of comets unlike the planets are not restricted to a flat system but can have any inclination. The orbits of comets have semi-major axes of almost 25, 000 AU. This proves that they are at very far distances from the Sun.
