For most of the 20th century, using own solar system as a guide, we thought we had a rough but plausible idea of how planets might be distributed. Since light gases like hydrogen and helium would tend to get pushed away from the nascent star by the various solar geysers as it cranked up to official starhood, it should as come as no surprise that the inner solar system would feature rocky planets and moons while the frigid, outer solar system would be festooned with lighter ices and gases. So the end result would be planets with lots of rock and metal like Mercury, Venus, Earth, and Mars huddled up close to the sun and larger, gas giants like Jupiter and Saturn with their attendant icy moons in the suburbs of the solar system.
That idea, at least as far as formation, still holds water. Then again, we only had the one solar system to go by! But starting in the ‘70s we had another tool—computer models. And that’s where a big mystery reared its icy head: Uranus and Neptune couldn’t have accrued where we see them today, way past the orbit of lovely ringed Saturn. And there were more mysteries: The physics of planetary formation being modeled suggested that dozens of sizable proto-planets might form. Some would surely be gobbled up, but with that many, some of them should have been favored by statistics and hung around.
One solution to those mysteries is now in better focus, and it comes with some startling conclusions: There may be lots of planets floating around free of our sun; the inner solar system was probably pummeled by a bunch more; and there might even be at least one lone survivor still out there, undiscovered, until now ...
To understand what may have happened shortly after the major planets formed, we have to understand orbital resonance. A simple example is when one planet or moon orbits its primary body twice while the next one out orbits exactly once. In that event, they both line up at the same place time and time again. We see this with some of the moons of Jupiter.
But what if, early on, Jupiter and Saturn slipped into a two-to-one resonance? In that case, any objects near them would receive a periodic gravitational kick. Objects between them would really get an assist, especially when the two planets passed one another, which would happen in the same place, orbit after orbit after orbit …
It might take a few hundred orbits, it might take several million, but eventually an object in between Jupiter and Saturn would get slung out of it’s orbit like a bat out of hell. This almost certainly happened to a bunch of smaller bodies early on. A good number of them would have formed a disk of icy, rocky objects—and we see something like that today called the Kuiper Belt. And if Neptune and Uranus were both in between Jupiter and Saturn, and they finally got loaded into the double-whammy slingshot provided by that resonance, they might get flung out to where they are now. Where they would wreak havoc on the established orbits of that belt of much smaller bodies that preceded them. Sending many of them careening toward the sun, where an occasional inner planet got in the way.
The evidence? We see enough impact craters on just about every solid body in the inner solar system, from Mercury to Mars, to infer that the inner solar system was pummeled for several hundred million years starting a little over 4 billion years ago in what astronomers call Late Heavy Bombardment. But there’s another side to this story.
If there was another planet in there with Neptune and Uranus, a sizable world mind you, it could have gotten slung out too. But there’s nothing in the laws of these physics that says the slung planet can only go so far. Under the right conditions, over time, the combined masses of Jupiter and Saturn could send that sucker clear out past Pluto. It would be out there, so distant and so dim it probably wouldn’t show up in any telescope made in the last 300 years. But there still might be signs of it:
The planet has yet to be seen. So why is it thought to be out there? And how can we know so much about it? Planet Nine is the best fit to explain the orbits of six distant objects. What’s odd about these six objects is that they have peculiar but remarkably similar orbits. These objects have been nudged off kilter and yet they are all shepherded together in the same region of space.
That’s basically all we have at this point. No one’s seen Planet Nine, and there have been lots of predictions in the past that have proven wrong. But another neat thing about the orbital resonance theory and the sling shot effect it may produce on Uranus and Neptune is that it doesn’t stop with Planet Nine. It’s possible—indeed, likely—given dozens of proto-planets to work with that some of these objects would get thrown completely out of the solar system. If so, they’re now condemned to wander the corridors of interstellar space basically forever, alone and cold and dark with no star to call their own. And since similar dynamics can play out in other solar systems, it may be that our galaxy is littered with these so-called rogue worlds, that there are many more loose planets wandering around than there are worlds orbiting stars in a proper solar system. That would make then the norm, and planets like our Earth the statistical exception!