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One of the most-asked questions of astronomers is: how did our Sun and planets get here? It's a good question and one that researchers are answering as they explore the solar system. There has been no shortage of theories about the birth of the planets over the years. 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 misevaluation of our origins. Some early theories suggested that the planets were spat out of the Sun and solidified. Others, less scientific, suggested that some deity simply created the solar system out of nothing in just a few "days". The truth, however, is far more exciting and is still a story being filled out with observational data.
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 counterclockwise 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 densest, 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 supplied the fundamental building blocks for the planets, asteroids, and comets that would eventually form. About 50 million years after the initial shock wave instigated the collapse, the core of the central star became hot enough to ignite nuclear fusion. The fusion supplied enough heat and pressure that it balanced out the mass and gravity of the outer layers. At that point, the infant star was in hydrostatic equilibrium, and the object was officially a star, our Sun.
In the region surrounding the newborn star, small, hot globs of material collided together to form larger and larger "worldlets" called planetesimals. Eventually, they became large enough and had enough "self-gravity" to assume spherical shapes.
As they grew larger and larger, these planetesimals formed planets. The inner worlds remained rocky as the strong solar wind from the new star swept much of the nebular gas out to colder regions, where it was captured by the emerging Jovian planets. Today, some remnants of those planetesimals remain, some as Trojan asteroids that orbit along the same path of a planet or moon.
Eventually, this accretion of matter through collisions slowed down. The newly formed collection of planets assumed stable orbits, and some of them migrated out toward the outer solar system.
Does the Solar Nebula Theory Apply to Other Systems?
Planetary scientists have spent years developing a theory that matched the observational data for our solar system. The balance of temperature and mass in the inner solar system explains the arrangement of worlds that we see. The action of planet formation also affects how planets settle into their final orbits, and how worlds are built and then modified by ongoing collisions and bombardment.
However, as we observe other solar systems, we find that their structures vary wildly. The presence of large gas giants near their central star doesn't agree with the solar nebula theory. It probably means that there are some more dynamical actions scientists haven't accounted for in the theory.
Some think 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.