Usually a phrase like this is in the context of a debate or other more formal speaking situation, in which case the higher formality and specification of "the Earth" make sense.
I would agree with IAmABaka if it were a positive statement such as "I think people can make earth a peaceful place" which would appropriately be said in an informal situation. The paired "don't" and "cannot" are, otherwise than within formal speech, an atrocious affectation...