This picture is changing rapidly with the discovery of planets around nearby Solar-type stars. Use surface features or Doppler shift in spectral lines. B. Empirical classifications of planets in our particular planetary system. The newly discovered planets around nearby stars are mostly giant planets and are much closer to the parent star, so it appears that our situation is not general. This result is significant because the effects are systematic and about 60 planets have been found.
L has to be constant for a closed system.
Revolutions, rotations of planets, satellites. Terrestrial planets are all slow rotators, with few satellites, whereas Giants are fast rotators with many satellites. Poynting-Robertson Effect are dominant forces. Gives concentration of heavy elements toward centre. Use earthquakes to study discontinuities.