Topic Solar systems: Discussion JOURNAL: 1. Describe the protoplanet nebular model of the origin of the solar system. Which part or parts of this model seem least credible? What information could you look for today that would cause you to accept or modify this least credible part of the model? 2. What are the basic differences between the terrestrial planets and the giant planets? Describe how the protoplanet nebular model accounts for these differences.