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ss evol

(courtesy of NASA, California Institute of Technology)

History and Evolution

Our solar system is located about 30,000 light years from the center of our Miky Way galaxy in a spiral arm of the galaxy known as the Orion arm. If you could look at a photograph of our galaxy, our solar system would be far too small to see.

Our solar system is 4.6 billion years old, the same age as our sun, because our solar system formed at the same time. The formation of our solar system is described in the Nebular Model. Consider a large, slowly rotating nebula (cloud of gas and a little dust) in space. If it is cool enough and dense enough, it's own gravity will cause it to contract and get smaller and denser. When this object contracts it's rotation speeds up. This is a principle in physics known as the "conservation of angular motion" (for example, when a figure skater closes her arms, she spins faster). As the rotating cloud gets faster, it flattens out, like the shape of our galaxy and our solar system. The rotating clouds of gas and dust accrete into planets. The original cloud of gas and dust contracted and became the hottest and densest so most of the mass ended up forming our sun (when a gas is compressed, it heats up). The more mass - the more gravity, so it can "hold on" to lighter elements like hydrogen and helium that have a higher escape velocity. This is also how the gas giants, Jupiter and Saturn, formed.

ss_evol_art

(courtesy of NASA, California Institute of Technology)

In summary, the Nebular Model can account for the characteristics we see today in our solar system. Some of these characteristics include:
All planets revolving around the sun in the same direction the sun rotates and in approximately the same plane. The orbits of the planets all have low eccentricities (the orbits are fairly circular). The rotation of most planets is in the same direction as the sun's rotation. Most moons revolve around their planets close to the planet's equatorial plane and in the same direction as their planets rotate. The planets of the solar system have differentiated (seperated into different layers from an earlier molten state).

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