Theories about Earth Origins | Prof. Zaigham Kamal, Study notes of Astronomy

Download Theories about Earth Origins | Prof. Zaigham Kamal and more Study notes Astronomy in PDF only on Docsity! Theories about Earth Origins Prof. Zaigham Kamal Theories about Earth Origins The origin of the Earth is a complex and fascinating topic that has been explored by scientists and researchers over the centuries. Numerous theories have been proposed to explain how our planet came into existence. Below are some of the prominent theories about the origin of the Earth, along with their full details: 1. Nebular Hypothesis: The Nebular Hypothesis, first proposed by Emanuel Swedenborg in the 18th century and later developed by Immanuel Kant and Pierre-Simon Laplace, suggests that the solar system, including the Earth, formed from a rotating, flattened cloud of gas and dust called the "solar nebula." According to this theory, about 4.6 billion years ago, a nearby supernova explosion or shockwave caused the nebula to start collapsing under the influence of gravity. The Nebular Hypothesis, also known as the Laplacian Nebular Hypothesis, is a scientific theory that proposes an explanation for the formation of the solar system, including the Earth. It suggests that our solar system originated from a massive, rotating, and flattened cloud of gas and dust known as the "solar nebula." The hypothesis was first formulated in the 18th century and later refined by Immanuel Kant and Pierre-Simon Laplace. As the cloud collapsed, it began to spin faster, and most of the material gathered at the center to form the young Sun. The remaining material in the disk around the Sun eventually clumped together through accretion and collision processes, forming planetesimals and protoplanets. Over time, one of these protoplanets grew large enough to become the Earth. Key Concepts of the Nebular Hypothesis: Formation of the Solar Nebula: The Nebular Hypothesis posits that about 4.6 billion years ago, a nearby supernova explosion or a shockwave from a passing star triggered the collapse of a large region of space containing gas and dust. The gravitational force between particles caused the cloud to start collapsing in on itself, resulting in a spinning disk-like structure. Conservation of Angular Momentum: As the solar nebula contracted, it began to spin faster due to the conservation of angular momentum. This concept is analogous to an ice skater spinning faster when they pull their arms closer to their body. The increased rotation led to the flattening of the nebula into a disk shape, with the center becoming denser and hotter. Formation of the Protosun: In the center of the spinning disk, the concentration of gas and dust increased significantly. Over time, this accumulation led to the formation of a dense and hot region known as the "protosun" or "solar protostar." This process initiated the birth of the Sun. Terrestrial Planet Formation: Eventually, some protoplanets grew massive enough to become the terrestrial planets of the solar system, including Earth, Mercury, Venus, and Mars. These planets continued to accrete and interact with the surrounding material until their orbits and growth stabilized. Terrestrial Planet Characteristics: The Planetesimal Hypothesis accounts for certain characteristics observed in terrestrial planets. These planets have relatively high densities, as they were composed of dense materials like silicates and metals that condensed close to the young Sun. They lack the massive atmospheres found in gas giants like Jupiter and Saturn, as they formed in the inner regions of the solar system, where the solar wind prevented the accumulation of large amounts of gas. The Planetesimal Hypothesis is a key component of modern planetary science and has helped explain the formation of rocky planets like Earth within the context of the larger solar system. It highlights the importance of solid particles in the early solar system's evolution and how these particles clumped together to create the diverse celestial bodies we observe today. However, it's essential to remember that scientific understanding is continually evolving, and ongoing research and observations continue to refine our knowledge of planetary formation processes. 3. Capture Theory: The Capture Theory, proposed by Thomas Jefferson Jackson See in the early 20th century, posits that the Earth was initially a rogue planet wandering through space. It suggests that the Earth was not formed in our solar system but was captured by the Sun's gravitational pull. According to this theory, a passing star or another celestial body approached our solar system and the Sun's gravity captured the Earth, causing it to become one of its planets. However, this theory is less favoured compared to the Nebular Hypothesis and Planetesimal Hypothesis due to the complex dynamics involved in such a capture event. The "Capture Theory," also known as the "Capture Hypothesis," is an alternative theory proposed to explain the origin of celestial bodies, including planets, moons, and asteroids, within our solar system. Unlike the widely accepted Nebular Hypothesis, which suggests that planets formed from a rotating disk of gas and dust around the young Sun, the Capture Theory proposes that some celestial objects, such as planets like Earth, were not formed in our solar system but instead were captured from interstellar space. History and Background: The idea of the Capture Theory has been around for many years, and it was first proposed by the American astronomer Thomas Jefferson Jackson See in the early 20th century. See suggested that stars passing close to our solar system could capture planets and other objects through gravitational interactions, incorporating them into the solar system as new members. The Capture Theory gained some attention in the scientific community, as it provided an alternative explanation for the origins of celestial bodies. Key Concepts and Mechanisms of the Capture Theory: The Capture Theory suggests that the Earth was initially a free-floating celestial object, unattached to any star, wandering through interstellar space. At some point, this rogue planet came close to the Sun and was gravitationally captured, becoming a part of our solar system. There are two main scenarios proposed for how the capture event could have occurred: Gravitational Capture: In this scenario, the rogue planet's trajectory brought it close to the Sun. As it passed by, the Sun's gravitational pull was strong enough to capture the planet into its orbit. The capture process involved a complex interplay between the planet's velocity, direction, and the gravitational forces acting upon it. Once captured, the planet began to orbit the Sun like any other planet in the solar system. Exchange Capture: The exchange capture scenario involves a more dynamic interaction between the rogue planet and an existing planet in the solar system. As the rogue planet approached the solar system, it interacted closely with an existing planet. During this encounter, gravitational forces acted upon both planets, and the rogue planet effectively exchanged its position and orbital energy with the planet it encountered. This exchange allowed the rogue planet to be captured by the Sun's gravitational pull and become a part of the solar system. Challenges and Limitations: While the Capture Theory offers an intriguing alternative to the Nebular Hypothesis, it faces several significant challenges and limitations: Low Probability: The probability of a rogue planet coming close enough to the Sun to be captured is considered to be extremely low. The vast distances between stars and the rarity of close encounters make this scenario less likely compared to other theories. Orbital Stability: The captured planet must acquire a stable orbit within the solar system to remain in a long-term stable relationship with the Sun. Achieving such stability is not always guaranteed, as the complex gravitational interactions could lead to ejection from the solar system or collisions with other bodies. Lack of Evidence: To date, there is no direct observational evidence of the Capture Theory in action within our solar system or elsewhere in the universe. As a result of these challenges and the success of other theories, such as the Nebular Hypothesis, the Capture Theory is not widely accepted by the scientific community as the primary explanation for the origin of planets in our solar system, including the Earth. However, it remains an interesting concept that contributes to the broader understanding of the diversity of planetary systems in the universe. 4. Colliding Planets Theory (Giant Impact Hypothesis): The Giant Impact Hypothesis proposes that the Earth was formed as a result of a massive collision between a Mars-sized protoplanet called "Theia" and the young Earth, around 4.5 billion years ago. This collision was extremely violent and led to the ejection of a significant amount of debris into space. The debris eventually accreted to form the Moon. This theory explains some of the Moon's unique characteristics and the similarities between its composition and certain rocks found on Earth. While the Giant Impact Hypothesis is widely accepted for the Moon's origin, ongoing research continues to refine the details of the impact and its consequences for Earth's formation. The "Colliding Planets Theory," also known as the "Giant Impact Hypothesis," is a widely accepted scientific explanation for the origin of the Moon. According to this theory, the Moon formed as a result of a massive collision between a Mars-sized protoplanet called "Theia" and the young Earth, approximately 4.5 billion years ago, during a time when the solar system was still in its early stages of formation. Key Elements of the Giant Impact Hypothesis: Formation of the Protostellar Disk: The solar system began as a cloud of gas and dust, known as the solar nebula. Gravitational forces caused the nebula to collapse, resulting in the formation of the early Sun at the center. As the Sun formed, it generated a strong solar wind, clearing the surrounding region of dust and gas and forming a protostellar disk around it. Theia's Formation: