Staff at Leicester University in the UK have uncovered a terrible secret: Super Mario Galaxy couldn’t exist because the gravity involved would cause the planets to explode.
Writing in the Journal of Physics Special Topics, they explain the problem in great detail. The quick and dirty version is that Mario is able to walk and jump on every planet in much the same way as he does on Earth, despite the fact that they are small enough for him to circumnavigate on foot in a couple of minutes. They explore the possibility of a planet with a 50 meter radius but Earth-like gravity, concluding:
Clearly, the degeneracy pressure far outstrips the gravitational pressure by eleven orders of magnitude. The outcome of this discrepancy is that if constructed, the planet would survive for only a very brief moment before violently destroying itself and any short plumbers who happen to be running about on its surface.