This solar system just doesn't work. According to a new computer simulation, the planets could never have come together in their current configuration. The only explanation is that we once had a fifth gas giant...and it's still out there somewhere.
That's a pretty big claim to make, so let's see how David Nesvorny of Colorado's Southwest Research Institute reached that particular conclusion. The key idea here is that our solar system wouldn't have formed in its present configuration — the planets would have tugged on each other as they formed, pulling each other out of their original orbits and into new ones. The solar system doesn't begin fully-formed, even once the various planets are complete. It's still a long evolutionary process.
We can't know exactly how the planets first fit together, so Nesvorny simply tried a bunch of different possible starting positions and ran simulations to find out which could conceivably result in the present solar system. The problem is that Jupiter, Saturn, Uranus, and Neptune are just so damn big that they can barely move around without violently disrupting each other's orbits.
In the vast majority of simulations, one gas giant would rip another apart. Even in simulations where all four gas giants survived, it was often at the expense of the rocky planets. According to Nesvorny's simulation, the current solar system is of astoundingly low probability, assuming we're only starting with four inner rocky planets and four outer gas planets. It's when you alter that assumption that things start to get interesting.
By adding a fifth gas planet into the mix, Nesvorny found that the odds of our current solar system emerging increased dramatically. That's obviously not proof of this fifth gas planet's existence, but if this computer simulation holds up, then it's actually more likely than not.
Nesvorny speculates that this fifth planet was an ice giant, similar in mass and composition to Neptune and Uranus. This planet couldn't survive the gravitational force of Jupiter, and it was ultimately ejected from the solar system. If we're going to give this planet an appropriately mythological name, then I really think we have to go with the son of Jupiter who was angrily thrown out of Mount Olympus as a baby for being too ugly. That would mean the fifth gas giant was the planet Vulcan... uh, no one else is using that name, right?
Anyway, the recent discovery of countless rogue planets supports the basic theory that this gas giant was ejected from our solar system, never to be seen again. Presumably, it's still out there somewhere in the Milky Way, the long lost sibling of Earth on its cold, lonely journey, destined never to make it back home. Now, if you'll excuse me, I think I have to go write at least ten short stories about this planet. A concept this awesome pretty much demands it.