There have been no shortage of theories as to how our solar system formed. This is not surprising considering that for centuries the Earth was believed to be the center of the entire Universe, not to mention our solar system. Naturally this led to a miss-evalutation of our origins.
As our understanding of our place in the galaxy has grown, we have re-evaluated the question of our beginnings. But in order to identify the true origin of the solar system, we must first identify the conditions that such a theory would have to meet.
Properties of Our Solar System
Any convincing theory of the origins of our solar system should be able to adequately explain the various properties therein. The primary conditions that must be explained include:
- The placement of the Sun at the center of the solar system.
- The procession of the planets around the Sun in a counter clockwise direction (as viewed from above the north pole of Earth).
- The placement of the small rocky worlds (the Terrestrial planets) nearest to the Sun, with the large gas giants (the Jovian planets) further out.
- The fact that all the planets appear to have formed around the same time as the Sun.
- The chemical composition of the Sun and planets.
- The Existence of comets and asteroids.
Identifying a Theory
The only theory to date that meets all of the requirements stated above is known as the solar nebula theory. This suggests that the solar system arrived at its current form after collapsing from a molecular gas cloud some 4.568 billion years ago.
In essence, a large molecular gas cloud, several light-years in diameter, was disturbed by a nearby event -- either a supernova explosion or a passing star creating a gravitational disturbance. This event caused regions of the cloud to begin clumping together, with the center part of the nebula, being the most dense, collapsing into a singular object.
Containing more than 99.9% of the mass, this object began its journey to star-hood by first becoming a protostar. Specifically, it is believed that it belonged to a class of stars known as T Tauri stars. These pre-stars are characterized by surrounding gas clouds containing pre-planetary matter with most of the mass contained in the star itself.
The rest of the matter out in the surrounding disk supplies the fundamental building blocks for the planets, asteroids and comets that would eventually form. About 50 million after the initial shock wave instigated the collapse, the core of the central star became hot enough to ignite nuclear fusion at a high enough rate to balance the immense gravitational force trying to collapse the object upon itself. At this point the conditions for hydrostatic equilibrium are met, and the object is officially a star -- our Sun.
The region of space near the Sun still contain high enough temperatures that the lighter elements can not yet precipitate, meaning that the old solid matter available is the denser metals and rocky material. While further out, where the temperatures are cooler, the lighter elements will dominate.
As the small, hot globs of material collide together, they form larger and larger blocks of material. Eventually, these objects will become large enough to become spherical under the force of their own gravity.
Eventually most of the mass will be pulled into these growing spheres, and eventually form planets. The inner worlds remain rocky as the strong solar wind from the new star sweeps much of the nebular gas into the outer solar system, where it is captured by the emerging Jovian planets.
Once most of the mass and gas in the nebula is occupied in one of the orbiting bodies, the accretion of matter through collisions slows and you are left with planets in stable orbits. They will all orbit in the same direction due to the angular momentum of the collapsing gas cloud.
The Problem with the Solar Nebula Theory
Much effort went into developing a theory that matched the observational data of our solar system. The balance of temperature and mass in the inner solar system explains the structure that we see.
However, as we observe other solar systems, we find that their structures vary wildly. The presence of large gas giants near their central star is in direct conflict with the solar nebula theory. Perhaps there are several solutions to solar system formation, or perhaps our theory is wrong.
Some point to evidence of this in that it appears that the structure of our solar system is the one that is unique, containing a much more rigid structure than others. Ultimately this means that perhaps the evolution of solar systems is not as strictly defined as we once believed.