Planet 5 must be a very small planet when one considers it is even smaller than Planet 6, which itself is doubtless minuscule. It is a mineral planet, as evidenced by the noises it made upon impact with the rocky Planet 6.
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 5 the "receiving end" of the collision. In real life, Calvin was trying to solve the problem "5+6" in a math test, inspiring the scenario.