1. CH 25.5 Solar System Formation
Objectives:
How did our solar system form?
What evidence supports our ideas?

2. Nebular Theory
The solar system formed from a cloud of dust and gas (nebula). 4.6 billion years ago Nebula produced by the explosion of a previous star.
National Geographic’s: Birth of the Solar System

3. Protoplanetary Disk
Protoplanetary disk forms when gravity pulls particles and gas to the center. Disk begins to spin (possibly because of a shock wave from a “nearby” supernova. Gravity continues to contract the cloud to the center – making it spin faster (angular momentum … think ice skaters spinning) Protosun at center  Sun when temperatures reach millions of degrees and NUCLEAR FUSION BEGINS. Nuclear fusion combines hydrogen atoms together to make helium and releases LOTS of ENERGY! Stars begin to shine!

4. Planets Grow by Accretion
Accretion: Growth by colliding and sticking together
Particles must be liquid or solid to stick together. They must COOL DOWN and CONDENSE from gas  liquid!
Planetessimals (asteroid size) Protoplanets (moon size) Planets

5. Inner vs. Outer Planets
Inner Solar System = “Rocky” Planets Hotter  Metals & Rock condense (gas  higher temps.
Not made of H or He b/c …
Solar wind blows light gases to outer s.sys.
Smaller planets do not have enough gravity to hold on to H, He atmospheres.
Outer Solar System = “Gas” Giants
1. Solar wind blows H/He to outer solar system
2. Outer solar system is COLD ENOUGH for gases like H, He, methane to CONDENSE (gas  liquid). For accretion to occur, materials need to be liquid or solid.

6. Evidence to Support
1. Sun is 99.9% of mass (b/c gravity pulls nebula to center) 2. Planets all orbit in same plane (b/c started off in spinning disk) 3. Planets orbit in same direction (b/c started off in spinning disk)

7. 4. We’ve observed protoplanetary disks around other stars.

8. Evidence that Earth is 4.6 billion yrs. old Meteorites
All meteorites (asteroids, space rocks, etc.) are the same age when radioactively dated: 4.6 billion yrs.
Meteorites do not change (no volcanism, plate tectonics, weathering).
We assume all objects formed at the same time.
There are metals found in meteorites that can only be made during supernova explosions.
Our solar system formed from a nebula produced by a supernova.

9. Summary