1. Solar Nebula Hypothesis

2. Recall…
Stars form within a large cloud of dust and gas called a nebula.
The core of the nebula collapses inwards due to gravity, after some kind of “disturbance” (ie energy shockwave from nearby supernova).
As the pressure and density increases eventually the temperature is hot enough for hydrogen fusion to occur.

3. Why do all the planets revolve in the same direction?
The initial nebula rotates slightly, after being disturbed, in a uniform direction
As nebula contracts, the speed of rotation increases (gravitational potential energy converted to kinetic energy). This is similar to a figure skater drawing their arms in to rotate faster. Important because with no velocity to revolve around star, all matter falls into star. Therefore NO PLANETS.

4. Mmmmm Pizza…
The nebula continues to rotate and due to centrifugal forces, flattens out (like pizza dough flattening out when being spun in the air)

5. How did planets form? Early theory #1: Rapid Spinning.
Large chunks of sun broke off of sun due to rapid spinning (like you on merry-go-round as a kid)
NOT ACCEPTED because of composition of sun vs. planet
Early theory #2: Capture.
Planets/objects came within Sun’s gravitational pull and they were not able to escape orbit of sun. Formation of planets the occurred from these objects.
NOT ACCEPTED because material of that size/magnitude not just floating around in interstellar space

6. Accretion!
temperatures within the rotating disk dropped and small particles such as iron and nickel rock started to form
these particles collided for millions of years and accreted to form the planets, moons, and other small bodies

7. How does accretion explain the Characteristics of our solar system?
Accretion occurs in a very predictable pattern ruled by physics and chemistry
All matter in a nebula has different condensation temperatures ( at 0 degrees molecules of water vapour turning into a snow crystal in a cloud)
Temperature in space is much more extreme than on earth where even rock, mineral and elements take on gaseous forms at high temperatures.

8. On earth we are only used to seeing a few compounds condense before our eyes (water)
The nebula that created our solar system was very hot as matter contracted. As contraction slowed, temperature dropped so metals and minerals were the first to condense. This formed the matter that accreted into the rocky inner planets. ( brought together by electrostatic attraction and gravity)
See slide 11/12 for more detail!!
In order for compounds like water, methane or ammonia to condense, far lower temperatures were are required.
Only areas away from the Sun were cool enough to condense the ICES which is why the gas giants are all located in the outer solar system, past the “frost line”. The majority of a nebula is ICE/gases, thus huge Jovian planets were able to form. (ex. the amount of sand you have determines how big your sand castle can be !)

9. Clean-Up
Finally, with enough gravitational pull, planets were able to clean up their orbital neighborhood. Space in between planets starts to clear!
The only location where cleanup was disturbed is the area between Mars and Jupiter. Jupiter’s huge gravitational pull disrupted accretion and continued to spread the accreted pieces apart even when gravity attempted to bring them together, leaving us with the Mars/Jupiter Asteroid Belt.
Once Fusion begins in sun/star, a blast of energy is emitted from sun and further cleans up solar system of debris

10. The 4 steps in the Solar Nebular Theory are…
1) Nebula Collapse, proto-star formation
2) Rotation in same direction around star
3) Accretion
4) Clean-up

11. How does Accretion occur?
Particle of rock and or metals collect together due to gravity.
As more particles are attracted to proto planet, its gravity increases
Radioactive decay of elements AND energy released due to objects collisions with other objects increases temperature and differentiation of densities can occur (heavy elements drawn inwards; lighter element move outwards. Iron and other metals make core, silicon, carbon, nitrogen make up crust.
Outgassing of oxygen and other gasses eventually forms atmosphere