I don't know based on the rulebook. The rulebook states on page 11 that "some planets have one or more moons. You cannot land on these unless some effect or tech allows you to do so. When you land on a moon, gain the depicted rewards. Each moon has space for only 1 lander." It also says, "From now on you can land on a planet's moon instead of the planet itself. The cost is the same as landing on the planet" (page 17, Probe Techs).

However, the rulebook does not specify whether a player with the tech to land on moons must always choose a moon if one is available, or if they may choose to land on the planet instead. It also does not clarify what happens if all moons are occupied—whether the player may then land on the planet, or if they are prevented from landing entirely.

To answer this question definitively, the rulebook would need to specify the priority or choice between landing on moons versus the planet when both are options, and what happens when all moon spaces are full.

Source: Core Rulebook (pp. 11, 17, 25, 9)