The Solar System

Our solar family

Orbits and tilts of the planets

Chemical condensation sequence in the solar nebula

Mass of Members

ObjectPercentage of Total Mass of Solar System
Cometsapprox. 0.0005-0.03%
All other planets and dwarf planets0.04%
Moons and rings0.00005%
Asteroidsapprox. 0.000002%
Cosmic dustapprox. 0.0000001%


  • Orderly motion
    • Nearly circular orbits in the same plane
    • Everything is moving counter clockwise
    • Almost all spinning counter clockwise
  • Asteroids and comets
  • Exceptions


  • Took ~100 million years
  • We have the rules
  • Forces and interactions
    • Gravity
    • Angular momentum (Kepler's second law)
    • Particles heating up, moving faster, bumping into one another
  • Can learn about formation by looking at patterns
    • All the planets lie in nearly the same plane
    • All the planets revolve around the Sun in the same direction
    • The Sun spins in the same direction about its own axis
    • It seems as though the Sun and planets formed together from the solar nebula
  • Planet composition
    • Spectroscopic analysis --> which elements are present
    • Sun, Jupiter, Saturn --> similar elements
      • Hydrogen-dominated composition
    • Terrestrial planets and the Moon --> similar elements
      • Deficient in light gases and various ices that form from elements like oxygen, carbon, and nitrogen
      • Rarer heavy elements
    • Inner planets are made of elements that can survive heat
    • Outer solar system is much cooler so ice/gas can exist
    • Planets must form before materials get blown into outer space by solar wind


Steps in forming the Solar System

  • Start with a blob or cloud of gas and dust
    • Must have rotation
  • Begins to contract because of its own gravity
    • Or could have been hit by a shockwave
  • Proto-sun forms
  • Disk flattens and rotates faster
    • Accretion disk
      • Rotation slows collapse in the plane of rotation forming a disk
      • Smaller particles run into larger particles forming larger and large chunks
        • Planetesimals
      • Disk also heats up through gravitational contraction
        • Up to hundreds of Kelvin
      • Material near the centre is moving faster so temperature --> higher
  • Sun turns 'on'
    • Starts shining
    • Removes gases from inner areas --> leaving dust and debris
  • Gas planets can form farther Outer

Space Astronomy