The solar system has looked pretty much the same for billions of years: four inner rocky planets, four outer gas giant planets, plus a bunch of dwarf planets and moons and asteroids and comets zooming around the place.

But it wasn’t always like this. We already know that the outer planets, like Jupiter and Uranus, probably formed much closer to the Sun than they are now.

And the closer scientists look, the more it seems like we’re still missing some pieces of the planetary puzzle. Which is why some astronomers think that our solar system might have once had a whole extra planet that got kicked out as the other planets were shuffling around. Of course, without a TARDIS, we’ll never know the exact conditions as the solar system was coming together four and a half billion years ago.

But we can use computer simulations to study the past. Astronomers set up lots of different scenarios for the early solar system, then simulate what would have happened over time, based on things like how the gravity from one planet would have affected the others. If a simulation ends with a solar system that looks a lot like what we see today, that means the scenario’s original setup could be a good model for what the solar system used to look like.

By using this technique, astronomers developed what’s known as the Nice model. That’s the model that suggests that our outer planets originally formed closer in than they are today. The disk of gas and dust surrounding the young Sun would’ve been a great place for them to form.

Eventually, though, energy from the Sun blew all the gas away, leaving behind the planets embedded in a rocky disk. According to the Nice model, the planets then began to move, but astronomers still aren’t sure exactly what that movement looked like. One possibility is that they moved mostly through encounters with rocks in the disk called planetesimals.

When one of these rocks passed close to a giant planet, it would’ve been thrown into a new orbit by the planet’s powerful gravity. But at the same time, the planet would’ve moved a tiny bit in the other direction. Problem is, hurling planetesimals one by one is a pretty slow way to move through the solar system.

And while the planets were migrating, their gravity was probably wreaking havoc on the inner solar system. Researchers have a lot of trouble setting up simulations of this that end up actually looking like our solar system does today. One problem is that a lot of the time, the simulations end with Mars being ejected from the solar system entirely.

So that’s clearly wrong. You can avoid a scenario where Mars zooms away, if the gas giant planets migrated outward much more quickly. And that could have happened, if the planets were also slingshotting each other around, in addition to all the planetesimals.

But even though in those simulations, Mars tends to stick around, it’s not enough to entirely avoid the problem of losing a planet. A lot of the time, Jupiter just throws out one of its fellow gas giants instead. Physics-wise, it makes sense: the loss of a planet is what provides the energy that gets the other planets migrating outward more quickly.

But again, that’s obviously not what happened, because Saturn, Uranus, and Neptune are all still here. So astronomers realized that there had to be something missing in their picture of the early solar system, like another planet; one that did get ejected from the solar system, which is why we don’t see it anymore. In one study, when researchers ran thousands of simulations that started with an extra outer planet, they ended up with a normal-looking solar system about a quarter of the time.

Other investigations have been less conclusive, so we definitely can’t say for sure that our solar system once had an extra planet. But it is possible, and it would explain a lot. If it really did exist, this mysterious world was probably an ice giant with about as much mass as Uranus and Neptune.

And like those planets, it would’ve formed somewhere outside the orbits of Jupiter and Saturn. To be clear, though, wherever it is now, it’s not the Planet Nine you’ll sometimes hear people talk about. If it exists, Planet Nine probably got its current orbit hundreds of millions of years earlier.

Our lost giant would’ve joined the billions of other rogue planets drifting through the galaxy on their own, unconnected to any star. By now, it would’ve traveled so far that we’d never be able to identify it as one of our long-lost neighbors. In fact, we’ll probably never know for sure whether it even existed.

The best we can do is come up with better simulations, and look at other star systems for clues. If we’re still trying to figure out how many planets our solar system started out with, who knows what else we have left to discover? Thanks for watching this episode of SciShow Space, and thank you especially to our patrons on Patreon who make this show possible.

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