Not all adventures have to take place at a specific place; often times they occur in the spaces in between. These areas may be sparsely inhabited by sapient beings and rife with wildlife, or perhaps completely devoid of life altogether. Adventures that take place on a planet (or moon) in areas of wilderness (between sites and communities) are known as planetary wilderness adventures.
Planetary wilderness adventures get special treatment in WCRPG. Technically, a whole sub-Chapter of these rules (namely Chapter 8.2) is devoted to the purpose of creating this type of adventure, though (as mentioned) that set of rules is primarily meant to augment an adventure. They do this by generating random encounters for each hour of transit on a planet's surface, making travel a little more challenging. In the process, they require the players to play out the entire transit; some player groups may just want to get to where they're going without worrying about what happens in between, which is why those rules are considered optional.
Planetary wilderness adventures can be treated a lot like site-based adventures; all that's needed encounters at particular points. These encounters may be generated at random by the planetary exploration, they may be tailored to occur at pre-determined points, or they may be a mixture of both. Each encounter point can be treated as a site in its own right, using natural phenomena (trees, rock formations, rivers, shorelines, storm fronts, etc.) to form the boundaries of the encounter area (similarly to the "natural sites" mentioned in the previous sub-Chapter). Once that's been done, all that is left is to populate the encounter with whatever other features the GM has in mind.
Wilderness adventures can easily be incorporated into a larger adventure. Such an adventure may begin in a community, include a wilderness portion to travel to a specific site, include some events at the site, and then include another wilderness portion to return to the place of origin once events at the site are brought to a close. Playing out the areas in between communities to frame the other sequences of an adventure is perfectly acceptable and certainly makes the in-between places a lot more interesting for the players. Not all wilderness adventures will involve the characters simply going from place to place; sometimes the wilderness itself will be the destination and setting for the bulk of the adventure.
Characters tend to be more susceptible to the environment in the wild than they would be in urbanized areas or sites. That's not to say that the environment can't affect the characters in those areas, it's just that usually sites located in particularly hostile environments will tend to have some features designed to make the area habitable; characters in the wild won't have those benefits. Rarely used equipment such as oxygen tanks and anti-radiation medication may become essential for survival out in the open. For more on environmental effects and how they may affect characters, see Chapter 12.3.