How contained is the system, really? I guess it is pretty contained because we can't really jump at escape velocity.
Lets say we built something that could exert enough force on the planet to actually alter the orbit of the planet. Naturally, this would likely result in that object skyrocketing off into space itself as you'd need an ass load of force to do that.
Now, hypothetically (and I'm talking hypothetically...like really hypothetically), if the population of the planet, were able to put a combined net force that is similar to our big object earlier by jumping, what would happen?