LinR: Origins of the Solar System

Cosmogony (origin of solar system)
Age ∼ 4.5 x 10⁹ years

Executive Summary of Observations

1. All planets lie in almost the same plane, and directions of rotation are same(except Venus).
   
2. Spin of sun lies in ecliptic, all planets (except Uranus) close to it.
   
3. Orbit shapes are nearly circular (little eccentricity)
   
4. System repeated as
   • Sun => Planets and Asteroids,
   • Planet => Moons and Rings
5. Comets and meteors have very eccentric orbits, at random angles ("inclinations")to the ecliptic.
   
6. Most of mass the of solar system is in the sun. Most of the angular momentum of the solar system in the planets.
   
7. Inner planets are terrestrial, outer planets are gas giants (jovian).
   
8. Inner planets are closely spaced, outer planets are widely spaced.

The old theories:

Close encounter of a star with the sun pulls out a tongue of material

Companion star explodes, material captured by sun condenses into planets

Both dynamically impossible. No angular momentum!

Nebular Hypothesis. A rotating gas cloud, probably compressed by a nearby supernova shock wave, starts to collapse. The central part collapses to the sun.

Conservation of angular momentum causes the outer part to speed up.

The outer planets condense first. Gas and dust particles moving at an angle to ecliptic are more likely to interact and hence collapse into the plane normal to the rotation axis. Majority rule (less effective at the limits, hence Mercury & Pluto orbit further out of the plane.)

Intense solar winds remove hydrogen and helium from the inner part of the solar system. Terrestrial planets form from the left over refractory materials.

The orbits are circularized by collisions and tidal effects.

e.g. υ (say Upsilon) Andromeda

Where is Upsilon Andromedae? Credit and Copyright: Till Credner & Sven Kohle

has 3 planets, one very close

(New York Times)

so it might look like this

Drawing Credit & Copyright: Lynette Cook

• Total planets discovered: 134  
• Total planetary systems: 98

(as of today!). Special conditions required for the discoveries (can only see large planets, fairly close and orbit must lie close to our line of sight) suggest they are very common: probably 50% of stars have them.