1. Formation of the Solar System
Many of the characteristics of the planets we discussed last week are a direct result of how the Solar System formed
Until recently, theories for solar system formation were solely based on observations of our own system of the Sun and planets
New observations of extra-solar planets are expanding our knowledge on the processes involved in solar system formation

2. What We Know
The planets are spaced relatively far from each other (no clumps or groups) _
The orbits of the planets around the Sun all lie nearly in the same plane
The rotation of the planets on their axis is usually the same direction (counter-clockwise) as their orbit around the Sun

3. What We Know
Most of the moons in the Solar System orbit their planets in a counter-clockwise fashion _
Asteroids orbit the Sun similar to the planets, but do not share characteristics with either the terrestrial or jovian planets
The Oort Cloud contains small icy fragments that are distributed in a shell around the Solar System

4. The Explanation
Astronomers have spent a lot of time and effort seeking a theory to explain all ten of the previous observations
The theory begins with a cloud of gas and dust from which the Solar System can form
The galaxy is full of many such clouds, detectable by the light they obscure
Dark gas cloud
New young stars

5. Nebular Contraction
Something must first trigger the gas cloud to collapse
Nearby supernova
Collision of two clouds _
In addition, as the cloud shrinks, it will begin to rotate faster and faster, due to conservation of angular momentum

6. Conservation of Angular Momentum
Many physical quantities in nature must be conserved
Mass, energy, and angular momentum cannot be created or destroyed, but can be transferred from one form to another _

7. A Spinning Disk As the cloud contracts, it spins faster and faster _
Eventually, the majority of the gas and dust is concentrated into a disk

8. Condensation and Accretion
The central portion of the disk with continue to shrink until a star is formed _
Small particles of dust begin to collide with each other, becoming larger and larger

9. Collisions Eventually, objects grow to the size of small proto-planets_
Occasionally, two proto-planets can collide to form a new larger body

10. Formation of the Inner Planets

11. The Role of Temperature
As the planets were forming, there was a wide range of temperatures in the solar nebula _
Closer toward the Sun, only metals were able to survive the high temperatures

12. The Role of Temperature
Mercury, forming very close to the Sun, is comprised mainly of metals, like iron _
In addition to hydrogen and helium gas, the outer planets have high amounts of water and ammonia ice

13. Where Did the Gas Come From?
The solar nebula was full of gas, so why did only the four outer planets capture huge amounts? _
These cores began to sweep up nearby gas (mainly hydrogen and helium) very early

14. Formation of the Outer Planets

15. Solar System Debris
While small bodies were colliding and accreting in the Solar System, many objects were also ejected to more distant orbits _
In the inner Solar System, small bodies were ejected out to an orbit between Mars and Jupiter, forming the asteroid belt

16. Kuiper Belt and the Oort Cloud
Early after Gas Giant Formation
After Smaller Bodies Ejected

17. How Did We Do? 
The planets are spaced relative far from each other (no clumps or groups) _
The orbits of the planets around the Sun all lie nearly in the same plane
The rotation of the planets on their axis is usually the same direction (counter-clockwise) as their orbit around the Sun    

18. How Did We Do? 
Most of the moons in the Solar System orbit their planets in a counter-clockwise fashion _
Asteroids orbit the Sun similar to the planets, but do not share characteristics with either the terrestrial or jovian planets
The Kuiper belt contains a large collection of icy objects outside the orbit of Neptune    

19. What About the Exceptions?
There are a few objects in the Solar System which do not follow the 10 observations we listed _
Uranus' axis is tilted by more than 90o
A glancing collision early in its history could have knocked Uranus on its side

20. Formation of Earth's Moon
It is also believed that a collision between proto-Earth and a Mars-sized proto-planet created our Moon
Computer models can reproduced this situation _