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

The Nebular Theory

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.

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

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.

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 17403.36 miles per hour) Energy conservation causes the cloud to heat up: potential energy kinetic energy thermal energy gravitational collapse gas shocks

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

Break

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

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.

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.

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.

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.

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

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

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