A. Comparison between the bodies in the solar system. 

All the trillions of bodies in the solar system range in size from minute dust particles to large bodies such as the sun. For convenience, they can be broken down into four main classes. 1. a star, the sun, which we have already discussed under stars. 2. the large outer or Jovian Planets 3. the smaller inner or Terrestrial Planets 4. the small bodies including asteroids, the smaller moons, comets, and dust and gas. 

B. The Planets:  

When we look at the solar system it is apparent that planets fall into two distinct groups, controlled by where they developed in relation to the protostar, the small dense inner planets and the large gas giants. This is quite apparent when we compare the characteristics of the two sets of planets. 
 

Inner Planets
Outer Planets
Solid Gaseous
Craters No Craters
Denser Less Dense
Massive molecule atmosphere (relatively) H, He atmosphere
Small Large
Nearly spherical Oblate
Rapid Revolution Slower Revolution
Slower Rotation Rapid Rotation
No rings Rings
Few moons (satellites) Many moons
C. The outer planets

Still resembling the original planets in many ways, the giant, massive, outer planets condensed in an area of the solar system were temperatures were low and allowed Hydrogen, Helium and other light materials to accumulate. Based on data received from the Voyager probes and calculations, we know they share common characteristics. 

They all contain dense, rocky cores surrounded by a liquid layer of either Hydrogen or water and are enveloped in thick atmospheres of Hydrogen and Helium, (and methane also, in the case of Uranus and Neptune). Compared to the inner planets, they are rapidly rotating giants, with rings and are accompanied by many moons. Their composition gives us insight in what the original nebular material must have been like. 

D. The inner planets 

Like the outer planets, the inner planets represent variations on a basic theme. Depleted in light materials because of higher nebular temperatures, they are richer in heavier elements, dense, rocky, slowly rotating, small and warm, and have few moons. Depending on the intensity of their gravitational field they may have atmospheres. Except for Mercury and the moon, they have geologically active surfaces. 

Geology is the search for understanding of the largest inner planet, the earth. The detailed explorations of our solar system in the last quarter of a century have amply demonstrated how knowledge begets knowledge. Because all the planets formed from the same materials and in the same way, a thorough understanding of the earth has given us insight into all the other bodies around us. Conversely, information gleaned from other planets has given us insight into the earth's history, especially its early history.