There are several theories that explain the formation of the solar system. However, it has been a tricky question explaining how and why the solar system came into existence. Therefore, many people have tried to come up with myths and theories to explain the origin of the solar system. Each of this theories attempts to explain the reason why the spheres of all the planets are prograde. This means if viewed from above the North Pole of the Sun, they all revolve in the anti-clockwise direction. All the planets have planes inclined by less than six degrees with respect to each other. Pluto is an exception. Terrestrial planets are solid, rocky and tiny while Jovian planets are made of gaseous particles and are gigantic. As a result of major advances in technology, it is possible to estimate the time our solar system came into existence. It is believed that the solar system was formed approximately 4.5 billion years ago. Scientists, astronomers and researchers are still investigating how the solar system came into place. Different scholars have come up with different theories, for example, The Accretion theory, the protoplanet theory, Capture theory, modern Laplacian theory and the Nebular theory.
The Nebular theory was first discovered in the 18th century (Seeds & Backman, 2013). However, there are several variations of this theory and the latest version is the modern nebular theory. The Nebular theory is a widely accepted model that explains the formation of the solar system. It explains how the solar system was formed from the cloud of gases and dust particles called nebula.
About four billion years ago, the dust particles and interstellar gas were drawn together by gravitation. The contraction was initiated by over-density of the clouds. As over-density grew, the contraction became faster. This made gases and cloud particles move in a random direction and at a greater speed causing the clouds rotation (Seeds & Backman, 2013). The rotation speed increased as the contraction continued. The gravitational pull was more efficient along the axis making the rotating ball of cloud collapse into a thin disk. The most of its mass concentrated in the center.
As the contraction of the cloud continued, the gravitational potential energy transformed into kinetic energy of particular dust and gas particles. The particles started moving randomly thus colliding with each other. These collisions between particles changed into heat energy making the nebula the hottest in the center, where most of the mass was concentrated, forming the protosun. This cloud of gas later became the sun (Seeds & Backman, 2013).
With time, the temperature in the center rose to more than 10 million Kelvin and atoms collided so violently causing nuclear reactions. This led to the formation of the sun that contains 99.8% of the mass. As temperature and density towards the center increased, the outward pressure increased until the sun attained hydrostatic equilibrium (a balance between the internal pressure and gravitational force). This stopped the contraction., A thin disk around the sun gave birth to moons, comets, asteroids and planets (Seeds & Backman, 2013). Disks of dust and gas were found surrounding newly formed stars. The protoplanetary disks span and were similar to the solar nebula. The disks contained 0.2% of the solar nebula mass and contained particles moving in a circular motion. 75% of their mass was in the form of hydrogen, and the rest was helium. As the gravitational energy was released, the material got a thousand degrees closer to the center and vaporized. The material was majorly gaseous as helium and hydrogen turn into gases at low temperatures. Gravity could not pull the material down as the disk spread over a large area.
The solar system is mainly composed of rocks, metals, ice and light gases (Seeds & Backman, 2013). The big temperature difference between the cool outside regions and the hot inward regions of the disk determined the amounts of condensates available for the formation of planets in each region. The inner nebula was deficient in ice and gases but significantly rich in heavy solid grains. Meteorites are a good evidence for this theory.
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The initial solid particles were very tiny in size. They moved around the sun in almost circular orbits close to each other just like the gases they condensed form. The intense collisions allowed the flakes to remain and stick together forming larger particles. As a result, additional solid particles were attracted; the process is known as accretion (Seeds & Backman, 2012). The process resulted in small planets called planetesimals; they play an important role in the formation of the planet. In the beginning, the planetesimals were packed at close distance to each other; they then slowly coalesced to form very large objects that in turn formed clumps that extended a few kilometers away in less million years compared to the age of the solar system.
After the planetesimal had grown to immense sizes, collisions were destructive, therefore, causing difficulties in any growth. The biggest of all planetesimals survived the fragmentation process and slowly grew to protoplanets by accretion of the same composition (Seeds & Backman, 2012). After the formation of protoplanets, the materials started to differentiate according to their density; this was caused by heat from radioactive decay of short-lived elements.
In the deeper warmer innermost solar system, planetesimals were formed from metals and rock; it took many billions of years that had passed in the cores of immense stars. The elements combined occupied 0.6% of the entire solar nebula only. This was observed in the formation of terrestrial planets (Mercury, Mars, the Earth and Venus). The planets could not grow bigger and also exert the great pull of helium and hydrogen gas. Even if the planets had these gases, they would heat up and disappear due to their proximity to the sun. These terrestrial planets contained 2 percent of the total number of heavy elements in the solar system nebula.
Jovian planets (Neptune, Saturn, Uranus and Jupiter) were formed in the outer nebula. Here planetesimals were formed from a combination of ice, rock and metal flakes though the major composition element was ice (Seeds & Backman, 2013). Due to this composition, the planetesimals could not grow to huge sizes hence they became the core of the Jovian planets. It is important to note that the core was vast (almost 15 times bigger than the Earth mass). Due to their size, they were in a position to capture hydrogen and helium gases easily from their surroundings to form a thick atmosphere. They ended up holding large amounts of hydrogen and helium gases which formed dense solid foundations that were large and gaseous in nature.
Far from the Sun, that is beyond Neptune, there are the coldest regions of Nebula. In these areas, only icy planetesimals survived. The disks were so low in density that the planetesimals could only grow for a few kilometers. Due to their inability to accrete to the surrounding gases, they remained tiny dull snowballs (Seeds & Backman, 2013). It is notable that Pluto does not fit any planet category (neither jovial nor terrestrial). Just like Jovian planets, it has low density and falls very far away from the Sun but it is small just like terrestrial planets. Some astronomers believe it falls under the category of comets.
The Asteroid Belt is located between the center of the Earth and Jupiter. It is made up of infinite rocky planetesimals that cover a wide area. They are thought to be debris of the solar system formation that could not possibly form a planet as a result of Jupiter’s gravity. Asteroids collide to produce small fragments that once in a while fall on the Earth. The rocks are known as meteorites; they provide useful information concerning the primordial solar nebula. Majority of the fragments are tiny, they are the size of a grain of sand and usually burn up and shine on the Earth like meteors.
The Jovian planets were in a position to trap large amounts of gases. In a similar process that formed the nebula, similar disks that were small in size were formed around these planets. Later condensation and accretion followed within Jovian nebulae that created the miniature solar system all around Jovian planets. Jupiter has quite a number of moons. The planets and their moons came out independently around the same time from similar rock and dust particles.
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The nuclear burning started in the Sun; it later came to a luminous object that cleared nebula. The pressure from the light being emitted and the solar wind forced the material out of the solar system (Seeds & Backman, 2012). Planets were instrumental in cleaning up absorbing and ejecting some planetesimals. Some planets collided with these planetesimals resulting in major craters. It is possible that the Uranus tilt might have resulted from a big Impact. Also, the Earth’s tilt might have been caused by a large impact that resulted in the ejection of debris that coalesced and led to the formation of the moon. Planetesimals came up as a result of gravitational encounters with the planets and were ejected to other remote areas of the solar system.
After the debris had been cleared out of the solar system, planet building came to a halt. All scars that came up as a result of many impacts can be seen on the moon. Today, the scars have been erased by erosion and different geological processes that occur on the Earth. It can be seen that the formation of the solar system is largely a complex issue that is open for debate. The Nebula system beats all other theories and has won the minds of many scientists and geologists as it clearly guides through the stages of the solar system formation.