Observations of the Solar System
| Observation | Explanation |
|---|---|
| The planets all rotate in the same direction, on the same plane. | All of the planets in our solar system were formed from a spinning solar nebula, and conserve the momentum of the original spinning. Just like pizza, everything flattens out as it spins more and more, constraining everything to a flat plane. |
| There are four inner terrestrial (rocky) planets and four outer jovian (gaseous) planets. | Planets closer to the Sun are hotter and form more tightly-packed, hot surfaces than those further away, where it is cold enough for hydrogen and other gasses to come together and form planetary objects. |
| Comets and asteroids exist. | Comets and asteroids are left over debris from the creation of planets and the sun. They are found in the Asteroid and Kuiper belts, and often remain there because of the gravitational pull of the objects surrounding them. |
| Il y’a des exceptions aux règles | Things like the Earth’s moons, Uranus’ axis tilt, et cetera, are caused by separate physics phenomena acting on them as opposed to planetary concepts. |
Solar System Formation
The Solar System probably formed from a cloud of gas in space.
Solar Nebulae
Materials in a Solar Nebula
| - | Examples | Can condense @ temperature below | Relative abundance by Mass |
|---|---|---|---|
| Hydrogen and Helium Gas | Hydrogen, helium | Do not condense in nebula | 98% |
| Hydrogen compounds | Water (), methane (), ammonia () | 150K | 1.4% |
| Rock | Various minerals | 500-1300 K | 0.4% |
| Metal | 0.2 |
Solar Nebula Consolidation
Heating
As the cloud collapses, gravitational potential energy is converted to kinetic energy. The acceleration of the particles in the cloud generates that kinetic energy, which then makes the cloud hotter and hotter. A star finally forms at the center of the cloud, where both the temperature and density peak due to gravity pulling everything inward.
Spinning
The rotation speed of the cloud from which our solar system formed must have increased as the cloud contracted.
Flattening
Collisions between particles in the cloud ended up causing it to flatten into a disk. These collisions between gas particles both gradually reduced random motions in other directions and up and down motions. You can think of this like spinning a ball of pizza dough— eventually, it flattens into a pizza. The cloud kept getting flatter and flatter as it shrunk.
Heavy Bombardment
There was a period of heavy bombardment in the late stages of solar system formation. This brought materials from other parts of the solar system to our planets, and formed many crators.
Age of the Solar System
Measuring the Age of a Rock
Like most matter we study, rocks are made of atoms. But you can’t distinguish a ‘young atom’ from an ‘old atom,’ so how can we date it? Well, some atoms undergo changes over time, that we can then track. For rocks, we can measure when the atoms of a rock became ‘locked together’ or solidified.
One notable isotope that decays over time is carbon-14; in 5730 years, half of all carbon-14 atoms in a sample decay to nitrogen-14. A half-life is the time for half the nuclei in a substance to decay.