ORIGIN AND EVOLUTION OF THE EARTH: THEORIES
THE BIG BANG THEORY
The Big Bang Theory is an astrophysical model of the universe that can be observed by human senses. The theory gives details about the origins of the universe from its early formations to its modern-day evolutions.
- Definition: It states that at some moment all of space was contained in a single point of very high-density and high-temperature state from which the universe has been expanding in all directions ever since.
- Events: There was a major cosmic explosion around 13 billion years ago from which all the matter of the universe was thrown out which eventually accreted to form stars, solar system, and celestial bodies.
- Key Features:
- According to this theory, the universe, ever since its birth, is expanding in all directions and the universe expanded from a very high density and high-temperature state.
- After the initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles, including photons, electrons, protons, and neutrons.
- Though simple atomic nuclei formed within the first three minutes after the Big Bang, thousands of years passed before the first electrically neutral atoms formed.
- The majority of atoms that were produced by the Big Bang are hydrogen, along with helium and traces of lithium.
- Credibility: In 1964, cosmic microwave background radiation was discovered, which was crucial evidence in favor of the Big Bang model. Other evidence such as cosmological redshift and gravitational waves have also been evidence of the theory.
NEBULAR HYPOTHESIS THEORY
The theory was developed by Immanuel Kant in 1755 and then modified in 1796 by Pierre Laplace. Originally applied to the Solar System, the process of planetary system formation is now thought to be at work throughout the universe.
- Definition: It suggests the Solar System is formed from gas and dust orbiting the Sun.
- Key Points:
- The Sun was surrounded by a solar nebula composed mostly of hydrogen and helium along with dust.
- Friction and collision of particles lead to the formation of a rotating disk-shaped cloud.
- Due to contraction, its speed of rotation increased. Consequently, the centrifugal force created a bulge in the equatorial region. When the centrifugal force exceeded the gravitational pull, a gaseous ring got separated. The repetition of the process led to the formation of successive rings.
- The material in the ring accreted to form planets while the central nebula resulted in the sun.
Limitations
- Origins of Nebula: The nebula’s origins are not explained by the nebula hypothesis notion.
- Source of Energy: It was unable to explain the nebula’s sources of heat and motion.
- State of Planets: According to the nebular hypothesis, because the nebula is in a gaseous condition, the planet should be in a gaseous state as well, however, this is not the case; planets are not in a gaseous state.
- Development of Solid Planets: The theory is unable to explain how the development of solid planets is triggered by gas nebulae.
- Planetary Rotation: The planets and satellites should all rotate in the same direction as the nebula, but this is not the case. Except for Venus and Uranus, all planets in our solar system rotate anti-clockwise.
- Angular Momentum: The hypothesis is unable to explain the fact that there is 98% angular momentum in the planets and only 2% is in the sun.
PLANETESIMAL THEORY
The Chamberlin-Moulton planetesimal hypothesis was proposed in 1905 by geologist Thomas Chowder Chamberlin and astronomer Forest Ray Moulton to describe the formation of the Solar System.
- Definition: According to the theory, tidal distortions on the sun’s surface were raised due to the close approach of a larger star. These tidal distortions, along with the sun’s eruptive powers, caused gaseous masses to shoot up to larger distances.
- Key Points:
- According to this theory, there was only one Sun before planet formation, and the Sun was frigid.
- Another introducing Sun passed close to our Sun, causing a cigar-shaped extension or bulge of material from our Sun due to gravitational pull.
- This bulging material was removed from the solar surface and given the name planetesimals, which refers to small-scale objects.
- The materials of planetesimals merged to form larger entities known as planets.
Limitations
- Limited Scope: Only nine planets were produced, according to this idea, it does not explain how this figure is arrived at.
- Planet Size: This idea proved unable to explain the disparities in planet sizes.
- Distance between sun: The distance between these two suns was enormous, and the gravitational force was not powerful enough to drag such massive masses away from the Sun.
- Condensation of Ejected Material: The greatest flaw in the theory is the assumption that the material drawn out of the stars would condense rather than dissipate.
GASEOUS TIDAL HYPOTHESIS
Sir James Jeans, a British scientist, propounded his ‘tidal hypothesis’ to explain the origin of the earth in the year 1919.
- Definition: The solar system was formed from the sun and another intruding star.
- Initially, the sun was a big incandescent gaseous mass of matter.
- Excluding the Sun there is another star termed as an ‘intruding star’ in the universe. This star was much ‘larger in size’ than the primitive sun.
- According to this theory, a hot star like the Sun and other intruding stars get attracted to each other while remaining in their orbits, resulting in ejected elliptical planetary formations like the Earth.
- The ejected matter which is dust, gasses, rock fragments eventually accrete forming planets & other celestial bodies that revolve around the proto sun.
Limitations
- Distance between stars: The universe is infinite in space and time and the stars are so distant from each other that such a close encounter between them is a remote possibility.
- Nature of Intruding Star: James Jeans did not explain the whereabouts and destiny of the intruding star which caused a tidal eruption on the surface of the primitive sun in the form of the filament.
- Distance between Sun and planets: N.N. Parisky has proved on the basis of mathematical calculation that the tidal hypothesis fails to explain the real distances between the sun and the planets in our present solar system.
- Composition of Planets: The theory claims that planets have been formed from the elements derived from the sun, but in reality, the sun consists mostly of hydrogen and helium whereas planets hold elements of higher atomic weight.
PROTOPLANET HYPOTHESIS
- H. McCrea proposed the protoplanet hypothesis, in which the Sun and planets individually coalesced from matter within the same cloud, with the smaller planets later captured by the Sun’s larger gravity.
- Definition: A Protoplanet, in astronomical theory, is a hypothetical eddy in a whirling cloud of gas or dust that becomes a planet by condensation during the formation of the solar system.
- Key Points
- According to this theory, rapidly rotating material (nebulae) developed large vortexes (rotating mass of gasses) at various places on the disc of the nebular material.
- Each of these vortexes accreted surrounding material by the gravitational attraction which led to the formation of protoplanets.
- Smaller vortexes developed inside the larger vortexes and gave rise to spinning discs that ultimately became satellites of planets.
Limitations
- Narrow Scope: It was assumed by many that other stars would have planetary systems that would, to a certain extent, mirror our solar system.
- Not applicable to outside: The theory does not hold any weight in solar systems outside our own or explain the nature of various discovered exoplanets.