Wednesday, August 18, 2021

Solar system formation, a Detailed explanation

In this article we will discuss about the solar system formation, how our Sun was born, how it became stable, how the planets were formed, how the outer planets migrated from their position and many more things. So let us start.

The most widely accepted model describing the formation of our solar system is  “Nebular hypothesis”. It was proposed by Pierre- Simon Laplace, Emanuel Swedenborg and Immanuel Kant in the 18th century. Since then the basic model has undergone several changes or refinements but its basic assumptions are still considered to be valid.

According to the Nebular hypothesis the formation of our solar system began about 4.6 billion years ago with a giant molecular cloud spanning across about 65 light years. These giant molecular clouds mainly consist of hydrogen and helium and over a period of several millions of years, these giant molecular clouds tend to collapse and fragment. In the case of the cloud from which our solar system formed, each of these fragments had a size of about 3-3.5 light-years.

But the cloud of gas was floating in the galaxy then why did it fragment? Well, there are various possible ways. And the best plausible explanation is that imagine a nearby star is exploding in the form of a supernova. Then the shock waves originating from the supernova will propagate through the cloud and may cause local instabilities just like waves in the ocean.

Some parts in the cloud may become denser than others and thereby causing the initiation of fragmentation. And then one among these collapsing fragments formed the Solar System. The mass of such proto-stellar nebulae can range from few fractions to several times the mass of the Sun.

For a nebula with mass of our Sun, the process of collapsing will last about 100,000 years. After that the nebula further collapsed. It had a certain amount of angular momentum and because of the conservation of its angular momentum, the nebula spun faster as it collapsed. As the material within the nebula started to condense, the atoms within it began to collide with increasing frequency, hence converting their kinetic energy into heat.

At the center of the nebula the concentration of atoms and molecules was the highest compared to other parts. Also the frequency of collisions was also higher which eventually caused the center to become hotter than the surrounding disc.

After that, over a period of about 100,000 years the competing forces of gravity, magnetic fields, gas pressure and rotation caused the protostellar nebula to flatten into a protoplanetary disc. This disk extended up to about 200 AU and its center formed a hot, young protostar (A protostar is pre-stage to a normal star). Yet the fusion of hydrogen has not begun in the protostar.

Furthermore, the rotating protoplanetary disc provided material to the protostar and the contraction further continued. Due to this the protostar gained more mass and the temperature and pressure in it increased continuously. About 50 million years later, a turning point was reached, due to which the temperature and pressure at the protostar had become so high which caused the nuclear fusion to initiate, causing our Sun to born.

The nuclear fusion process created a source of internal heat. This heat increased the pressure within the young star (the Sun) that countered gravitational contraction until the outwards directed pressure and the inwards directed gravitation had the same strength. And finally our Sun became stable.

Formation of planets

The terrestrial planets mainly formed in the inner region of the protoplanetary disc. The dust grains in orbit around the Sun in this inner region agglomerated through direct contact to millimeter-sized objects and through further direct contact these tiny objects clumped together up to several hundred meters in diameter.

This process continued until these objects had accreted enough material to form objects of about 10-15 km in size and that was the point of time, when gravity prevailed. These planetesimals influenced each other gravitationally and collisions occurred. These collisions caused disruption but eventually lead to further accretion.[Read – How exoplanets are detected?]

In total, the accretion dominated the disruption by collisions and the planetesimals grew until only a few of them survived and formed planetary embryos of mass about 0.05 times of Earth. Subsequent collisions and mergers led to the formation of the terrestrial planets as we know them today.

Beyond the snow line, the formation of planets was the result of different processes. In that region, water ice and other form of ices were present. Yet, water ice dominated them all being the most abundant icy material in the protoplanetary disc and even in today’s solar system..

In general, the ices that formed the gas giants were more abundant than the metals and silicates that formed the terrestrial planets, thus allowing the giant planets to grow massive enough to capture hydrogen and helium. The planetesimals accumulated about four to five times the mass of Earth within a period of about 3 million years.

Further a runway process began during which the young gas giants accreted further material rapidly and thereby increased their sizes. Before the planetary migration, the early solar system consisted of the eight planets within which the Uranus and Neptune were in opposite locations. The outer planets were much closely spaced and more compact than in present days. The planets then migrated until they reached their current positions. 

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