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Solar System Formation

Translated in Romanian as well.

Everything has a beginning, and our story begins when the cloud that was the Solar Nebula began to contract. All stars exist in islands called galaxies, and galaxies contain old and new stars as well as clumps of dust clouds. These clouds contain mostly hydrogen and some heavier metals (any elements that are heavier than helium is considered a metal by Astronomers). As we will learn the the Sun section, stars create their energy by a process called fusion. When a star ends its life, it explodes in a tremendous phenomenon called a supernova. A supernova has so much energy that heavy metals are formed - metals like iron and gold. These elements "seed" surrounding hydrogen clouds so that newer stars will contain more heavy elements in their atmospheres.

It is believed that for a system of planets to form around a star during cloud contraction the cloud must contain heavy elements.

The image below demonstrates how our Solar System was formed:

For the cloud to begin contraction, two conditions must be met:

  • Jean's Mass - the cloud must contain a certain amount of mass that depends on...
  • Jean's Length - if the cloud of a certain mass is also a certain diameter, contraction will begin
Click the image on the left for a video of a simulated formation of a system of planets.

This particular simulation demonstrates the Solar Nebula Hypothesis - the formation of our Solar System (video care of Swinburne Astronomy Online).

Jean's Mass and Jean's Length are an advanced type of Stellar Astrophysics that will not be covered here, but a simple definition is that when a cloud contains a certain amount of debris and is a certain size, the likelihood of cloud contraction is high. But if a cloud exists happily at a certain size, how can it begin contraction? The answer is outside influence:

  • Surrounding supernova can generate shockwaves that will affect the debris in a nearby cloud - if the cloud is shocked enough and debris is forced inward, contraction can begin
  • The natural rotation of the galaxy can slowly affect the structure of a cloud
  • Density Waves - a theory that explains the spiral structure of galaxies - can also stimulate these clouds
This image from the Hubble Space Telescope shows a solar nebula, evidence to support the Solar Nebula theory. This is called a protoplanetary disk, or proplyd. This proplyd lies at the heart of the Orion Nebula along with dozens of other proplyds. The darker area is the dense molecular cloud while the proto-star (a star that is hot, but not hot enough to initiate fusion) is the glow in the center.

While the above image shows the early stages of a system of planets, there are stars that already have formed planetesimals. The image below is of Beta Pictoris:

This image clearly shows that this star has 4 distinct rings. These rings will eventually coalesce into solid bodies called planets.

Our Solar System has three distinct features as a direct result of its formation:

During formation, these bodies that would form planets are called planetesimals. They begin their life as very small clumps of debris, but as the Solar Nebula contracts, a natural disk shape forms (a result of the conservation of energy - a natural phenomenon). These planetesimals assume their orbits and gather surrounding debris in a process called sweeping. Because the terrestrial planets form close to the proto-sun (the Sun at this point has not initiated fusion) the warmth melts away any ices so rocky planets form. The Gas Giants are at a greater distance and much of the ice and gas remains.

During the sweeping process, the protoplanets undergo another natural phenomenon called chemical differentiation, a process in which heavy elements sink towards the center of the object while the light elements remain closer to the surface. This is the reason for the internal structures of the planets are dense, rocky and heavy.

During all of this planet evolution, the proto-star in continuing to contract until it reaches a magic temperature resulting from a process called Kelvin-Helmholtz contraction (as an object of mass is compressed, it generates heat as a result of the compression), the proto-sun will "ignite." The temperature required for fusion to occur is 106 K - or 999,726.85 C.

When the Sun ignites, the result is a shock wave called T Tauri Wind. This wind is strong enough to blow away any gas in the inner Solar System, but not strong enough to strip away the thick atmospheres from the Gas Giants.

The asteroid belt creates a natural boundary between the terrestrial and gaseous planets, but it could have been a planet. The gravity influence of Jupiter is believed to be enough to prevent to asteroids from coalescing into a planet. Instead, individual asteroids are knocked out of orbit and either put into elliptical orbits near Earth - called Near Earth Objects - or captured by Jupiter to become one of its many moons.

The residual debris of our Solar System is locked in two regions:

  • The Kuiper Belt - are area containing rock/ice bodies beyond the orbit of Pluto
  • The Oort cloud - a cloud containing "dirty ice" well beyond the orbit of the Kuiper Belt (comets are from the Oort Cloud)

It takes millions of years for this process to occur. And this was a long time ago:

  • The Earth is about 4.56 billion years old - that is the Earth as it was already formed
  • Our Sun is also only 4.5 to 5 billion years old

The Sun is considered an average, middle-aged star. Its safe to say that there is still at least another 4 billion years before the Sun enters the Red Giant phase. Details on a stars evolution will be covered in the stars section.

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