Anybody Missing a Gas Giant?

Now, you might think the post title is a Barney Frank joke, and I admit it would be worth a snicker even if it's a little on the cheap side. Of course, I just made the joke anyway, so apparently I'm a little cheap. Who knew?

Anyway, the post is actually about a theory an astronomer in Texas has about the planets of the solar system. Our sun has four interior rocky planets -- Mercury, Venus, li'l ol' us and Mars -- and four gas giants -- Jupiter, Saturn, Uranus and Neptune -- and one demoted mini-planet -- Pluto. But David Nesvorny from the Southwest Research Institute suspects there may have been a fifth gas giant planet at one time.

Dr. Nesvorny studied the Kuiper Belt, an area of asteroids outside the orbit of Neptune. He put that data into computer models and projected the orbits of the various planets back into the early history of the solar system. About 600 million years after it came into being, some kind of orbital click-clackery occurred and some of the planets apparently migrated from their slots into the ones they have now. Dr. Nesvorny's problem is that when he looks at what used to be and what currently is, he finds out he can't get there from here.

The problem is Jupiter, which could not have moved slowly from its earlier position to its current one without affecting the orbits of the inner planets, including us. We might have smacked into one of our neighbors and at the very least, our orbit would not have been as steady as it is now, meaning there probably wouldn't be an us to wonder about this. But if Jupiter had moved quickly from one position to another, it would have spared us but probably knocked Uranus or Neptune out of the solar system, so clearly Dr. Nesvorny could not choose the cup in front of him...er, the swift-Jupiter model.

Ah, but he could, if there had been a fifth gas giant planet similar in size to Uranus or Neptune. Then Jupiter could spring from there to here and wreak its havoc on an outer planet which we wouldn't see because it would be gone.

So far, there's not much empirical evidence to support Dr. Nevsorny's idea, just his mathematical models and projections. Ironically, Neptune was found based on mathematical models and measurements of Uranus' orbit, but it had the advantage of being someplace it could be found. Our possible evictee enjoys no such privilege, meaning it remains just a theory for now.