1. 2. THE SOLAR SYSTEM’S EARLY HISTORY
Collapse of the solar nebula
Planet formation: the frost line
The age of the solar system

2. The Nebular Theory

3. Collapse: Galactic Recycling
Our solar system formed from gas which had already been cycled through many generations of stars.
Each cycle increased the amount of “heavy” elements.

4. Collapse: Birthplace of the Solar System Step 1. A star is born
Hubble’s sharpest image of the Orion Nebula

5. Collapse: From Cloud to Disk
A gas cloud starts to collapse due to its own gravity.
It spins faster and heats up as it collapses.
Vertical motions die out, leaving a spinning disk.
The solar system still spins in the same direction.

6. Collapse: Angular Momentum and Energy
Angular momentum conservation causes the cloud to spin faster as it contracts:
Collapse stops when the cloud spins at orbital speed. ( 7,780m/s or miles per hour)
Energy conservation causes the cloud to heat up:
potential energy
kinetic energy
thermal energy
gravitational collapse
gas shocks

7. Collapse: Disks Around Other Stars
We can see disks around other stars, as expected if these stars formed from collapsing gas clouds.

8. Break

9. Collapse: Birthplace of the Solar System
Step 2. Planets and other bodies are formed
Hubble’s sharpest image of the Orion Nebula

10. Planet Formation
At the end of the collapse phase, the solar nebula was a uniform mixture of different materials.
Some of these materials began condensing out of the gas.

11. Planet Formation: The Frost Line
The disk was hot at the center, and cool further out.
Inside the frost line, only rocks & metals can condense.
Outside, hydrogen compounds can also condense.
The frost line was between the present orbits of Mars and Jupiter — roughly 4 AU from the Sun.

12. Planet Formation: Terrestrial Planets
Within the frost line, bits of rock and metal clumped together to make planetesimals.
As the planetesimals grew, they became large enough to attract each other.
Finally, only a few planets were left.

13. Planet Formation: Jovian (Gas) Planets
1. Outside the frost line, icy planetesimals were very common, forming planets about 10 times the mass of Earth.
These planets attracted nearby gas, building up giant
planets composed mostly of H and He.
The disks around these planets produced moons.

14. Planet Formation: Asteroids and Comets
“Leftovers” from early stages of planet formation
Asteroids form inside frost line, comets outside
Scattered by jovian planets into present orbits

15. Planet Formation: Explaining the Exceptions
Giant impacts in early solar system:
explain rotation of Venus, Uranus
form Moon from collision debris
Satellite capture after near-miss:
moons of Mars captured from asteroid belt
Triton captured from Kuiper belt

16. Nebula gas cloud starts to collapse due to gravity
Nebula begins to rotate
Flattens into disk shape
Planetesimals appear
Planets begin to form
Our solar system