Major Observations Included in Every Theory
Every solar system formation theory has to explain some facts we know about our solar system. Listed below are some of the major observations.
- More than 99 percent of the mass of the entire solar system, consisting of the Sun, planets, moons, meteors, asteroids, comets etc. is contained inside the Sun. However, the Sun only contains about 0.5 percent of the angular momentum.
- The presence of huge amounts of light metals on Earth like beryllium, boron, etc., points out that this matter could not have come from the core of an exploded star.
- The solar system is extremely flat, i.e the orbits of most planets lie in the same plane. However, the orbits of Mercury and Pluto (not a planet anymore) deviate from this plane considerably.
- There is a clear demarcation between the terrestrial and giant planets.
- The Sun’s rotation axis has a tilt of 6o.
Famous Historical Theories
1. Laplace’s nebula theory (1796): This was a monistic theory - i.e. it involved a single system. The theory states that it all began from a huge cloud of gas which was spinning slowly. This cloud collapsed under gravity. As it collapsed, it started spinning more quickly and began to flatten to conserve angular momentum. Laplace also proposed that planets were formed by the condensation of annular rings around the Sun which was one of the major drawbacks of this theory.
2. Schmidt-Lyttleton accretion theory (1944): This was a dualistic theory - i.e. it involved interaction of two systems. Schmidt proposed that the Sun passed through a huge cloud of gas and acquired matter to form planets. However, for his theory to be consistent, he postulated the presence of third body in the vicinity when the Sun passed through this cloud. However, the need for the third body’s presence was removed when Lyttleton gave his modification of the theory involving line accretion. In this form of accretion, mathematically described by Bondi and Hoyle (1944), the material from the cloud get focussed gravitationally. The velocity of the material perpendicular to its orbital axis around the central object get destroyed and it is left with less than escape speed.
There were many other theories during those times like Chamberlin and Moulton’s planetesimal theory, Jeans’ tidal theory, Von Weizsacker’s vortex theory etc. which were all more or less rejected.
There are four major theories that are still considered as plausible for the formation of the solar system.
1. Proto-planet Theory: McCrea, in 1960, gave a monistic theory trying to explain simultaneously the formation of Sun and the planets. He started with a cloud of dust and gas in a state of hypersonic turbulence. Thus, rapid internal collisions took place. This resulted in aggregation of masses at various places. He experimented with parameters like mass and radius of cloud to explain the current state of the solar system. He gave a revised theory in 1988.
2. Capture Theory: The theory, given by Woolfson in 1964, considers an interaction between a condensed solar mass star (SUN) and a protostar of lesser mass. The protostar entered the Roche limit and was disintegrated to form bodies like planets. This would give a planar structure but highly elliptical orbits.
3. The Solar Nebula Theory: This theory, given by Cameron in 1973, begins with a slowly rotating nebula very similar to the Laplacian theory. The planet formation starts with a disc of 0.01 to 0.1 solar masses. This is also a monistic theory dealing with mass and angular momentum distribution.
4. The Modern Laplacian Theory: This theory is also derived from the Laplacian theory and was given by Prentice in 1974. He started with a cool gas cloud which condensed in such a way that the angular momentum of the central body was only 1 percent of the system. His contribution was mainly giving a mathematical explanation of the process of formation. The theory also explains formation of planets in concentric rings.
1. The Graduate Series in Astronomy: The Origin and Evolution of the Solar System - M. M. Woolfson, IOP Publishing Ltd 2000
4. Solar System Evolution: A New Perspective, Second Edition - Stuart Ross Taylor, Cambridge University Press 1992; Stuart Ross Taylor, 2001
Image Credit: Pierre-Simon Laplace. https://en.wikipedia.org/wiki/File:Pierre-Simon_Laplace.jpg