Introduction
The difficulty in defining the theories and processes leading to the formation and evolution of the solar system and our planet lies in restrictions imposed on experimentation. Limited opportunities are explored under the subject of cosmogony, the field which focused on the question: ‘Which processes contributed to the formation of Solar System, as well as how it evolved since its formation?’ Despite the divergence in the ideas and theories on the creation of the Universe, all the scientists agree that the Solar System emerged about 4.500 million years ago (Woolfson 2007). Since that time, the Solar System has undergone substantial changes, both the reversible and the irreversible ones. The introduction of numerous theories and frameworks has provided grounds for discussion in such scientific disciplines as physics, geology, astronomy, and planetary science.
The reversible and irreversible processes are fundamental concepts used in studying the origins and processes of the Solar System formation. While considering a reversible change, the system could factually “run backwards so that the past states of the system can be deduced from its present state” (Woolfson 2007, p. 6). For instance, the scientists are aware of the mechanics governing the Earth motion and, therefore, it is possible to imagine the reversal of this motion. In contrast, the irreversible processes imply the impossibility of predicting the events which caused a specific change during the evolution of the universe. Thus, the past state cannot be used to analyze the present state. In order to find out the origins of specific irreversible processes, various models have been initiated to find out whether some of these remind of the current state of the Solar systems. Though the given approach to defining the sequence of processes is quite risky, it is among the most efficient methods in this case.
Processes Contributing to the Evolution of the Solar System
While studying the theories of the Solar System formation, two major theories can be highlighted – evolutionary and catastrophic (Backman and Seeds 2012). The first one involves gradual processes whereas the second one is connected to specific events that gave rise to creation processes. Despite the fact that many scientists are inclined to use catastrophic theories, evidence demonstrates that all natural processes are more gradual and evolutionary. Therefore, adhering to this theory often excludes the possibility of the unlikely and unpredictable events.
With regard to the above-presented opposition, the scientists have introduced the solar nebula theory according to which “…planets form in the rotating disks of gas and dust around young starts” (Backman and Seed 2012, p. 406). The evidence of the proposed theory is strong because of the fact that the planetary system was created in the disk-shaped from. In addition, the Solar nebula theory suggests that the Earth, along with other planets of the solar system, evolved several billions age because of the sun condensing from gas and dust. In case planet formation is typical of stars evolution, other evolutionary processes should also have planets rotating around them.
A wide range of models and theories has been implemented to discover the processes contributing to the formation and evolution of the Solar System. The analysis of the chronology of stars creation can be carried out through the observation of supernova starts formation, as well as their classification. According to Rose (2004), “very massive starts were more common and played a more decisive role in the early universe than at the current epoch” (p. 56). In particular, 30 Mo stars are expected to generate supernova, which explains the abundance of HE01075240, a metal-poor star that has 80 % mass of the Sun (Rose 2004). In contrast, pair production supernova that has a mass of more than 130 Mo has eruption substance much more enriched with iron (Rose 2004). With regard to the above calculations, the researcher also explains the formation of Saturn and Uranus: “The rapid decrease in H and He abundance between Saturn and Uranus may be a consequence of photoevaporation of the outer solar system before Uranus and Neptune could form” (Rose 2004, p. 57).
Preconditions of Planet Formation
In 1940, Carl von Weizsacker, the German Physicist, discovered that the rotation of gas within a disk around the Sun had different speed of rotation (The inner rotating circles were moving faster as compared to the outer ones) (Koupelis 2010). The figure below illustrates the way the Sun attracts gas from the outside and shapes planets (Figure 1). The given research has put forward a new theory of the planet formation. These gas rotations were also identified with a set of eddies around the sun, with the smaller ones inside the rotation and larger ones outside it.
As soon as gas disk cools down, it starts condensing liquids and solids, including such substances as silicon and iron, which are further transformed into solid dust grains. Each of the formed grains has elliptical orbit rotating around the center, which later condenses on the surface. The collision of particles as result of surface has two outcomes, gentle collision that leads to formation of larger particles and collision of particles that are forced into orbits (Koupelis 2010). The shaped chunks of matter have higher mass and, as a result, these particles acquire a greater gravitational force. As the particles continue collapsing, the gravitation leads to shrinking the celestial object, which leads to acceleration of the collision and friction processes (See Figure 2). The gravitational energy leads to the heating effect which is much higher at the core of the particles collision.
In order to understand the processes which trigger the planet formation, specific attention should be given to the analysis of primordial disks, the main precursors of planet origins. In this context, Mamajek (2009) refers to the primordial stage of exoplanet formation to define whether it corresponds to the stage of the planet formation within the Solar System. According to the researcher, “the existence of primordial disks around young stars was originally inferred through the spectroscopic evidence for accretion among classical T Tauri stars…and evidence of circumstellar dust structures” (Mamajek 2009, p. 4). The observations have triggered the discussion concerning the origin of such Solar system planets as Saturn and Jupiter.
In particular, the scholar also asserts, “Saturn itself had accreted the majority of its mass…from the Sun’s protoplanetry disk within ~ 3.4.-5.4 Myr” (Mamajek 2009, p. 4). Thus, the nature of giant planet formation should be at least plausible and amount to ~106-7 yr lifetime of protoplanetary disks, in case the physical property meets the observed characteristics of primordial disks (Mamajek 2009). Analysis of planet formation and its evolution, therefore, is possible through exploring the processes of formation of the planets outside the Solar System. However, there is no evidence whether these processes and stages are applicable to the formation of our Solar System.
The possibility of exploring the planet evolution by searching for the planet outside the Solar System was one of the most efficient methods to define the origin and stages that solar planets undergo. Ollivier et al. (2008) have defined that first exoplanets have been discovered at the end of the twentieth century by means of pulsar timing. This discovery has fostered the search for protoplanetary disks that could uncover the unknown processes of the planet formation within the Solar system. With regard to this theory of formation, “the Solar System formed from a disk, resulting from the collapse of a rapidly rotating nebula of dust and gas” (Ollivier et al. 2008, p. 12). As soon as the planets formed, the protoplanetary disks were dispersed during the stage of intense activity, which was previously called as T-Tauri stars phase. Such types of stars exist around MS starts that are almost identical to the Sun.
Nowadays, the accepted model of formation of the Solar system suggests that it appeared almost 4.5 billions ago out of the radioactive atoms which were applied to terrestrial rocks, meteorites, and lunar samples. In addition, because all the planets are located within the Solar System, they have one common feature. They may be split into two major categories – the terrestrial planets, namely Mercury, Venus, Earth, and Mars, and giant planets – Jupiter, Saturn, Uranus, and Neptune – which have low density (Ollivier et al. 2008). Their atmospheres are not dense and do not have a solid surface.
Evolution of Earth as a Planet
According to scientists’ calculations, about 100 million years passed before the proto-Earth model has been formed. During the first stage of the Earth’s formation, which refers to the 0.8 billion years, the Earth has a diameter of 10 km (Martin 2011). The phase stretches to near 3.9 billion years ago and is called the Hadean Eon because the Earth developed under extremely high temperatures (Martin 2011). The next stage of Earth’ formation refers to the Precambian era, which ends 540 million years ago. This period is marked by the emergence of first fossils. The sources of high temperatures are diverse. During the first stage, the Earth was heated because of radioactive decay driving seafloor and spreading plate tectonics (Martin 2011).
There is also evidence that most of the heat was produced by radioactive isotopes which had been vanished. Further, gravitation played an enormous role in accretion of particles and dust (Martin 2011). In the course of the planet formation, the particles were closer to the center and their energy released as a result of friction. Meteors bombarding is another factor that contributed to the heating processes (Martin 2011). Finally, the core accretion of the Earth was another underpinning for releasing higher temperatures because iron and nickel, heavier elements, were transformed to the centre of the Earth’s core.
The next stage of the Earth evolution refers to the period of mantle differentiation. Due to the intense heat, magma ocean stretched from the Earth’s surface almost to 400 km (Martin 2011). As soon as the magma reached the surface, it lost its heat and, as a result, it was crystallized quickly, which led to the crust formation. However, the magma eruptions continued forming volcanoes spewing gases and lava into atmosphere (Martin 2011). The eruptions were one of the sources of the atmosphere formation as the Earth had enough gravitational force of the magnetic field to accumulate gases. At this phase, the primitive atmosphere has been formed out of such gases as methane and ammonia. Hydrogen and oxygen were also presented due to the evaporation processes. Existence of reducing atmosphere caused a number of problems during the early period of the Earth’s evolution. To begin with, the magnetic field could not have been created as long as the Earth’s core formed (Martin 2011).
Consequently, there was a possibility of the sun blowing off the primitive atmosphere because it had much greater gravitational force. Second, the fact of the presence of abundant ammonia in the atmosphere could have been impossible as well because it could have been subjected to photolysis. Third, the rich carbon-rich organic compounds had been rare during that evolutionary period and, therefore, it was impossible to suggest that methane had been in excess amounts. Due to the above-presented arguments, the majority of researchers are inclined to believe that the atmosphere origin was secondary in nature. It implies that the atmosphere was formed after the emergence of the magnetic field of the Earth. Such a suggestion is corroborated by the fact that oxygen should not have participated in the reaction with iron, which was placed in the centre of the core. Instead, it entered into reaction with carbon and hydrogen.
The thermal history of the Earth evolution is its main driving force. In fact, the heating and cooling processes either directly or indirectly influence many stages of planetary evolution. More importantly, “in a silicate-metal planet like Earth, thermal history determines when and if a core will form” (Condie 2012, p. 2). In particular, it controls the core formation, as well as identifies the presence of the magnetic field, which interacts with cosmic rays and the solar. The interaction in its turn affects the life processes on the planet. In addition, the thermal history of the planet has a strong impact on crustal, magmatic, and tectonic history. For example, “only planets that recycle lithosphere into the mantle by subduction, as Earth does, appear capable of generating continental crust, and thus having collision orogens” (Condie 2012, p. 2). Such an assumption explains the emergence of hills, mountains, and cavities on the Earth’s surface.
Conclusions
The discussion has touched upon various theories and hypotheses of the Solar System formation, as well as evolution of the Earth as a planet. Most of the researchers mentioned in the discussion agree with the nebular theory of the Solar System origin. According to this theoretical framework, the Sun, along with other planets, emerged as a result of accretion of dust and gas. The circles of the matter were more intense in its inner part, whereas external processes were less intense. The theory also explains why the first four planets are of terrestrial nature, whereas the next four planets – the giants of the solar system – are composed of gas. It has also been defined that the planets formed out of primordial disks. Such a discovery is based on the exploration of extrasolar planets with similar structure and composition.
References
Backman, D & Seeds, MA 2012, The Solar System, Cengage Learning, New York.
Condie, KC 2011, Earth as an Evolving Planetary System. Earth as an Evolving Planetary System, Academic Press, US.
Koupelis, T 2010, In the Quest of the Universe, Jones & Bartlett Learning, US.
Mamajek, EE 2009, ‘Initial Conditions of Planet Formation: Lifetimes of Primordial Disks’, AIP Conference Proceedings, 1158, 1, pp. 3-10.
Martin, 2011, Earth’s Evolving Systems: The Hystory of Planet Earth, Jones & Bartlett Publishers, US.
Ollivier, M, Encrenaz, T, Roques, F, Selsis, F, and Casoli, F 2008, Planetary Systems: Detection, Formation, and Habitability of Extrasolar Planets. Springer, US.
Rose, WK 2004, ‘Early Solar Systems and the Formation of Massive Stars’, AIP Conference Proceedings, 713, 1, pp. 55-58.
Woolfson, MM 2007, The Formation of the Solar System: Theories Old and New, Imperial College Press, US.