Comparative planetology in its broadest sense is the systematic study of similarities and differences among the planets. The goal is is to understand the planets as individual worlds and as part of a larger family of the Solar System by studying their characteristics. By studying other planets, we might even learn about the past and future of our own planet, the Earth. Some terrestrial planets, such as Mars, Mercury and the Moon, have preserved a record of the history of the solar system at their surfaces. Mars, for example, is large enough to have been geologically active throughout its history, yet small enough to have sufficiently cooled not to destroy this complete surface record by modern processes (as the Earth does by plate tectonics). On the other hand, Venus provides an example how the Earth might evolve in the future, should the "greenhouse effect" become significant here. Even the giant planets, different though they seem, offer insight into the formation of the solar system, its stability, and its history, and therefore are important to a complete understanding of the Earth's history. In particular, the moons of the outer planets are in some cases comparable to inner solar system objects.
In planetary geology, the emphasis is on the comparison between surface processes on different bodies in the Solar System that have solid surfaces. The only process that affects all bodies is probably impact cratering, but other endogenic and exogenic processes are also widespread. Volcanism, for example, is known from all terrestrial planets, the Moon, and Io. To understand the universal physics of eruption processes, geologic studies are indispensable, as they provide observational evidence for volcanic products from different environmental conditions.
The main comparison that can be made in comparative planetology is to features on the Earth, as it is much more accessible and allows a much greater range of measurements to be made. Earth analogue studies are particularly common in planetary geology, geomorphology, and also in atmospheric science.