One who studies a planet, whether a naturalist or as part of a surveying or scientific research team.
A planetologist will record or measure the type, age, and geological history of the world; the effects of stellar primaries and planetary companions, and resulting tides and radiation, the nature of the moons if any, the frequency of cometary impacts, weather conditions (if there is an atmosphere), tectonic activity if any, and so on, survey local landforms, and so on. Or e may specialize in a particular world type, such as Gaian, Europan, gas giant, etc.
A planetologist may also assess the suitability for mining, development, megascale engineering, terraforming, and colonization. While this is generally done with remotes and instrumentation from orbit, extended visits to the surface to study the local conditions and hidden hazards may also be necessary.
Although being a planetologist is seen as a romantic profession that many bionts and vec aspire to, in fact most planetological work nowadays is done by neumann probes and remote surveying ai, usually over extended periods lasting decades or centuries, with little contact (beyond transmitted reports) with the rest of the galaxy.