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 What We Know 1. The planets are spaced relatively far from each other (no clumps or groups) 2. _ 3. The orbits of the planets around the Sun all lie nearly in the same plane 4. _ 5. The rotation of the planets on their axis is usually the same direction (counter-clockwise) as their orbit around the Sun What We Know 6. Most of the moons in the Solar System orbit their planets in a counter-clockwise fashion 7. _ 8. Asteroids orbit the Sun similar to the planets, but do not share characteristics with either the terrestrial or jovian planets 9. _ 10. The Oort Cloud contains small icy fragments that are distributed in a shell around the Solar System 1
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 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 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 2
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 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 Eventually, objects grow to the size of small protoplanets Collisions Occasionally, two protoplanets can collide to form a new larger body 3
Formation of the Inner Planets 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 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 4
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 Formation of the Outer Planets 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 5
Kuiper Belt and the Oort Cloud Early after Gas Giant Formation After Smaller Bodies Ejected How Did We Do? 1. The planets are spaced relative far from each other (no clumps or groups) 2. _ 3. The orbits of the planets around the Sun all lie nearly in the same plane 4. _ 5. The rotation of the planets on their axis is usually the same direction (counter-clockwise) as their orbit around the Sun How Did We Do? 6. Most of the moons in the Solar System orbit their planets in a counter-clockwise fashion 7. _ 8. Asteroids orbit the Sun similar to the planets, but do not share characteristics with either the terrestrial or jovian planets 9. The Kuiper belt contains a large collection of icy objects outside the orbit of Neptune 10. _ 6
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 90 o A glancing collision early in its history could have knocked Uranus on its side 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 7