@ARP: That is mentioned more than once during the series. The terraforming authorities acknowledge that they most likely will always have to replenish the atmosphere (most likely with comets).
Man, more people around here need to get subscriptions to Sky and Telescope.
A way more plausible theory is that every 20 million years or so the solar system passes though denser regions of the galactic arms. Interaction between stars and larger concentrations of matter in the local interstellar medium perturb comet orbits in the theoretical Oort Cloud. The orbits change for the comets, and boom - you have comet showers into the solar system on a regular cycle.
That - or non-bound stars pass by every once in awhile that stir up the mix, so to speak.
So! This brings me to another thing: we're not even sure if the Oort Cloud is even there. Proof comes from observation, and not once have we EVER seen a comet at the distance requred to prove the Oort Cloud is really there. We know the Kuiper Belt is there, because we can directly observe Kuiper Belt bodies. Not so with the Oort Cloud.
Not to rag on my own posse, but it blows my mind how little sci-fi nerds actually know about the SCIENCE. Doesn't anybody actually keep tabs on the newest news? Nemesis was a hot topic...in the 80's.
@chakrabs: One thing - The Oort Cloud is only a theory. Not a single body has ever been observed in the Oort cloud, mainly because not even WISE is sensitive enough to detect bodies that far away.