Early Theories
The Nebular Hypothesis, originally proposed by Immanuel Kant was the first scientific hypothesis for the origin of the Solar System. The ‘nebula’ referred to is the vast cloudy interstellar medium that consists of gas and dust, in which new stars are born.
This hypothesis considered that the planets were formed out of a cloud of material associated with a youthful Sun, which was slowly rotating.
The hypothesis considered that the planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating. Later in 1900, Chamberlain and Moulton considered that a wandering star approached the sun.
As the passing star moved away, the material separated from the solar surface continued to revolve around the sun and it slowly condensed into planets
Revised Nebular Hypothesis- By Otto Schmidt in Russia and Carl Weizascar in Germany. They considered that the sun was surrounded by solar nebula containing mostly the hydrogen and helium along with what may be termed as dust.
Modern Theories
Origin of the Universe:
When we apply the scientific laws of physics, according to the theory of Big Bang, it predicts that all matter existed more than ten billion years before the time of the initial explosion of the “tiny ball”.
An atom was formed which began to divide up into smaller atoms forming a sphere with outward pressure. It is interesting to note that during this process of formation of different elements in this sphere more than ten billion years before the explosion of this huge sphere, electrons were formed.
That is why electrons were able to travel through space without colliding into protons because electrons were not yet created. Big Bang Theory- also called expanding universe hypothesis.
At the Big Bang the “tiny ball” exploded violently This led to a huge expansion It is now generally accepted that the event of big bang took place 13.7 billion years before the present. The expansion continues even to the present day. As it grew, some energy was converted into matter.
There was particularly rapid expansion within fractions of a second after the bang.
The expansion of universe means increase in space between the galaxies. An alternative to this was Hoyle’s concept of steady state It considered the universe to be roughly the same at any point of time.
The Star Formation
The universe encompasses everything in existence, from the smallest atom to the largest galaxy; since forming some 13.7 billion years ago in the Big Bang, it has been expanding and may be infinite in its scope.
The part of the universe of which we have knowledge is called the observable universe, the region around Earth from which light has had time to reach us.
In the case of the universe, there may be raisins out there that we can’t see any more because they have moved away so fast that their light has never reached Earth. Fortunately, gravity is in control of things at the local level and keeps our raisins together.
These initial density differences gave rise to differences in gravitational forces and it caused the matter to get drawn together. These formed the bases for development of galaxies.
A galaxy contains a large number of stars. Galaxies spread over vast distances that are measured in thousands of light-years.
A galaxy starts to form by accumulation of hydrogen gas in the form of a very large cloud called nebula. A light year is a measure of distance and not of time.
Light travels at a speed of 300,000 km/second. Considering this, the distances the light will travel in one year is taken to be one light year. This equals to 9.461
1012 km. The mean distance between the sun and the earth is 149,598,000 km. In terms of light years, it is 8.31lminutes.
Formation of Planets
Planetary development, or planetary evolution, is the hypothesis that all of the planets in our solar system were created from a nebula and therefore have similarities in their makeup and structure.
Each of the planets has specific characteristics revealing how they formed and what materials they contain. From Mercury to Mars each planet seems to share similar traits and was presumably created in a similar manner.
The following are considered to be the stages in the development of planets:
- The stars are localised lumps of gas within a nebula. The gravitational force within the lumps leads to the formation of a core to the peas cloud and a huge rotating disc of gas and dust develops around the gas core.
- The gas cloud starts getting condensed and the matter around the core develops into small- rounded objects. These small-rounded objects by the process of cohesion develop in the same period sometime about 4.6 billion years ago.
- Planetesimals are a large number of smaller bodies.
