Comparative planetary science or comparative planetology is a branch of space science and planetary science in which different natural processes and systems are studied by their effects and phenomena on and between multiple bodies. The planetary processes in question include geology, hydrology, atmospheric physics, and interactions such as impact cratering, space weathering, and magnetospheric physics in the solar wind, and possibly biology, via astrobiology.

Comparison of multiple bodies assists the researcher, if for no other reason than the Earth is far more accessible than any other body. Those distant bodies may then be evaluated in the context of processes already characterized on Earth. Conversely, other bodies (including extrasolar ones) may provide additional examples, edge cases, and counterexamples to earthbound processes; without a greater context, studying these phenomena in relation to Earth alone may result in low sample sizes and observational biases.

The term "comparative planetology" was coined by George Gamow, who reasoned that to fully understand our own planet, we must study others. Poldervaart focused on the Moon, stating "An adequate picture of this original planet and its development to the present earth is of great significance, is in fact the ultimate goal of geology as the science leading to knowledge and understanding of earth's history."

All terrestrial planets (and some satellites, such as the Moon) are essentially composed of silicates wrapped around iron cores. The large outer Solar System moons and Pluto have more ice, and less rock and metal, but still undergo analogous processes.

Volcanism on Earth is largely lava-based. Other terrestrial planets display volcanic features assumed to be lava-based, evaluated in the context of analogues readily studied on Earth. For example, Jupiter's moon Io displays extant volcanism, including lava flows. These flows were initially inferred to be composed mostly of various forms of molten elemental sulfur, based on analysis of imaging done by the Voyager probes. However, Earth-based infrared studies done in the 1980s and 1990s caused the consensus to shift in favor of a primarily silicate-based model, with sulfur playing a secondary role.

Much of the surface of Mars is composed of various basalts considered analogous to Hawaiian basalts, by their spectra and in situ chemical analyses (including Martian meteorites). Mercury and Earth's Moon similarly feature large areas of basalts, formed by ancient volcanic processes. Surfaces in the polar regions show polygonal morphologies, also seen on Earth.

In addition to basalt flows, Venus is home to a large number of pancake dome volcanoes created by highly viscous silica-rich lava flows. These domes lack a known Earth analogue. They do bear some morphological resemblance to terrestrial rhyolite-dacite lava domes, although the pancake domes are much flatter and uniformly round in nature.

Certain regions further out in the Solar System exhibit cryovolcanism, a process not seen anywhere on earth. Cryovolcanism is studied through laboratory experiments, conceptual and numerical modeling, and by cross-comparison to other examples in the field. Examples of bodies with cryovolcanic features include comets, some asteroids and Centaurs, Mars, Europa, Enceladus, Triton, and possibly Titan, Ceres, Pluto, and Eris.