Inventory: one sun, 8 planets, about 200 moons, 200,000 asteroids, and a thousand billion comet nuclei.

Clues (Large-scale regularities of the objects in the solar system):

• Age constraints
  1. The ages (time since last melted) of rocks from asteroids (meteorites), the moon, and earth are all about 4.5 billion years (4500 million years). So, we think the solar system formed about 4.6 billion years ago.

• Dynamical constraints (regularities of motion):
  2. Everything goes around the sun in the same plane (except the comets, Pluto). This is also the plane of the sun's equator.
  3. Everything has an elliptical orbit of very low eccentricity (nearly circular) (except the comets, Pluto).
  4. Everything goes around the sun in the same direction (except the comets). This includes most moons and rotations.
  5. The spacing between planets increases greatly for those farther from the sun (Titius-Bode Rule, successive doubling).

• Chemical constraints (regularities of composition)
  6. Terrestrial (M, V, E, M) and Jovian (J, S, U, N) groups. The terrestrials have small orbits, short years, warm temperatures, small size, small mass, high density, made of metallic rocks not hydrogen-rich ices, slow spins, no rings, few moons.

Model A: Catastrophic Model: the Near-collision Scenario

• Narrative: The sun (with no planets yet) side-swipes another star. Huge tides rip blobs of gas from both stars. Some of the gas escapes, some falls back onto either star, and some cools and condenses into planets, moons, ...
  Note that the mass of the sun is ____________ percent of the mass of the whole solar system.
• Good points: 1, 2, 4, 5
• Bad points: 3, 6, hot gas would mostly escape, stars are so far apart that side-swipes are extremely unlikely to happen (and we know of at least 300 other solar systems).

Model B: Gradual Model: the Nebular Scenario (the presently preferred model)

• Narrative: A huge slowly rotating interstellar cloud (nebula) made of hydrogen, helium, and dust grains (carbon, iron, etc.) contracts gradually by its own gravity. Because of "conservation of angular momentum" the rotation speed of the cloud increases as its radius decreases. This makes the cloud shape flatter, becoming a dusty opaque disk. Solid bits are sorted by temperature (no ice grains near the protosun) and condense and accrete into planetesimals and finally planets, moons, etc. in about 10 million years. The sun lights up and blows away excess gas and dust.
• Good points: 1, 2, 3, 4, 5, 6
• Bad points: Model sun spins too fast unless the solar wind was much fiercer in past.