The Calvin and Hobbes Wiki

There were several places described only as "an uncharted planet" or something similar.

Screenshot 2017-02-27-20-19-17-329

Canyons on an uncharted planet.

One was a rocky, desert world, this planet was apparently the home of a Gurl. Spaceman Spiff came here when his Bizarrotron registered a 3.7 on the weirdness scale. Spiff left the planet thinking that his Bizarrotron needed to be recalibrated as the reading was too low.

Screenshot 2017-02-27-20-20-42-062

Another uncharted planet.

Another was the apparent home of the Reclining alien. In this case, Spiff landed on the alien by mistake, and all scenes of the surface are in fact parts of the alien.