Planet 6 is presumably a very small planet as its curve can be very distinctly made out from a low altitude. It is a mineral planet, and its landscapes are no doubt the product of considerable erosion. Its surface is covered in some spots by a thick cloud cover which obscures the ground from space. Although the atmosphere of the planet appears breathable, it contains no life-forms.
Spaceman Spiff visited planets 5 and 6 in order to collide them and document the results. Planet 6 was pulled out of orbit toward the unmoved Planet 5, making 6 the "distributing end" of the collision. In real life, Calvin was trying to solve the problem "6+5" in a math test, inspiring the scenario. The larger Planet 6 destroyed Planet 5 upon impact, resulting in Calvin writing "6+5=6."