The goal of comparative planetology is to discover the commonalities among the ways that planets form and evolve. The planets that have been discovered differ substantially from one another when viewed as individual, isolated systems, but astronomers believe that there are universal principles that apply to them. By comparing the properties of different planets, scientists hope to spot these universal principles and gain greater insights.
There are many factors that separate each planet in Earth's solar system from one another, and that is only within one solar system. Scientists have discovered many planets orbiting other stars, some quite different from the ones near Earth. The biggest division in comparative planetology is between the rocky planets, like Mercury, Venus, Earth and Mars, and the gas giants, Jupiter, Saturn, Uranus and Neptune. In comparative planetology, scientists look for commonalities both within and between these major groups.
Even within this solar system, comparative planetology is in its infancy. The ability to send probes beneath the outer atmospheres of other planets is relatively new and still extremely limited. Even relatively close planets with known solid surfaces, like Venus, contain conditions that are incredibly hostile to most equipment. Even worse are the gas giants, with their immense gravity, thick atmospheres and extreme weather.