A new study has come out conjecturing that there used to be five giant planets in our solar system, and one got ejected by Jupiter through its gravitational influence. It's an interesting bit of research, based on computer modeling.
It's pretty well established that the outer planets have moved around a bit since the solar system formed, with a possibility that Uranus and Neptune even swapped places! But the models have a hard time explaining how this could've happened without Jupiter totally messing up the inner solar system. The models seem to indicate the orbits of Mars and Earth would not look at all as they do today if this were the case.
Using the known math of physics and gravity, the author of this new study set about trying to figure out how this might be the case, and got the idea that maybe there was a fifth big planet back then, billions of years ago, with perhaps the mass of Uranus. He found that under certain circumstances, he could show that the addition of this planet explains the way both the outer and inner solar system look today, and why we don't see this planet any more, since it was tossed out of the solar system through interactions with Jupiter.
All in all, a nice result. I want to stress, though, that it's not set in stone. Basically, it's a cool idea supported by models, but we don't know how real it is. Maybe there's something else we don't know: maybe there were two extra planets, or maybe Jupiter interacted with the three other giant planets in a different way. I got email from some friends about all this, and I stressed that this is a model, not evidence. We do know that rogue planets roam the galaxy, and they were almost certainly ejected in this manner, so the idea of a lost solar system planet isn't crazy! But it's only one possible scenario. There are lots of other possibilities.
I was pleased to see Alan Boyle MSNBC's Cosmic Log made the same case. A story like this is a bit sexy, and I was wondering if some venues might run away with it, so it's good to see some solid reporting on it!
And it's interesting to think that somewhere out there, light years away, a lonely, dark, and slowly freezing planet may be bulleting through the galaxy. If we ever do develop interstellar travel, will our descendants stumble on that planet? It's a romantic thought.