Reid: It’s been more than twenty years since astronomers began detecting planets around other stars. They weren’t surprised to find planets in other star systems. But they were surprised by how those planets were arranged, and they ended up having to rethink what they thought they knew about how planets form.

We used to think that our Solar System was pretty standard, as star systems go. Our arrangement of four smaller inner rocky planets and four bigger outer gaseous ones makes a lot of physical sense. When you’re farther away from the Sun, space is colder, which allows more molecules to condense and build up larger planets. But then, we found our first exoplanet — the first planet we’d discovered outside of our solar system —  and it revealed a situation we used to think was impossible.

The first exoplanet was a world the size of Saturn, orbiting more than seven times closer to its star than Mercury does to the Sun! There’s almost no way it could have formed so close to its star, all the light gases would have boiled off before they could’ve formed a planet.

As astronomers collected more data, they started to find more and more of these big, gas planets, which we now call Hot Jupiters, close to their stars. They realized that Hot Jupiters migrated from somewhere else, and they started trying to figure out how. Since then, they’ve come to realize that planets can actually move around a lot in their early lives.

Planets form in regions called protoplanetary disks, made up of the leftover gas and dust after a star forms. We see these disks surrounding young stars all over our galaxy. They provide a place for solid matter to clump together, and eventually, the largest clumps become massive enough to pull in gas, giving rise to worlds like Jupiter and Saturn. Once they form, these new planets are still embedded in the protoplanetary disk. And that disk is the key to how they move around.

A new planet’s gravity affects the nearby disk material, bunching it up into giant waves called spiral density waves. They’re the same kinds of structures that give galaxies their arms and Saturn’s rings their details. There’s one density wave inside the planet’s orbit, closer to the star, and one outside the planet’s orbit, on the side farther from the star.

Now, the closer you are to a star, the harder its gravity pulls you and the faster you travel. So, over time, the wave towards the star starts to creep ahead, while the one outside the planet’s orbit starts to fall behind. But these density waves are made up of actual, physical stuff and therefore exert gravity of their own.

So that wave ahead of the planet is pulling it forward in its orbit, which gives the planet a little boost in energy, sending it a bit farther from the star. Meanwhile, the wave behind the planet is doing the exact opposite. Its gravity tugs the planet backwards, reducing its energy and forcing it to fall into a smaller orbit.

If these forces exactly balanced out, the planet wouldn’t move at all, but usually the protoplanetary disk is less dense in some places than others. That difference makes the pull of one wave more powerful than the other, so the planet moves. In simulations, this process is so effective at moving planets around that some scientists aren’t really sure why so many survive at all.

So, finally an explanation for how those strange Hot Jupiters could exist where they do! They probably formed much farther away from their stars before migrating in to where they are now. Even the planets in our own Solar System probably migrated long ago. Astronomers call this idea the Nice Model, after the city in France where it was developed.

The Nice Model says that when the outer planets were forming four and a half billion years ago, they were much more closely spaced than they are today. Over time, interactions with the disk drove them to where they are now, but they might have made some major detours along the way. Jupiter, for example, might have spent some time in what today we’d call the inner Solar System.

If it wandered close to the orbit of Mars as both planets were forming, it could have dramatically reduced the amount of material left to make the Red Planet. This would help explain why many models of planet formation suggest that Mars should be roughly Earth-sized, when really, it only has about a tenth of our mass!

And Neptune may have started its life closer to the Sun than Uranus. Then, once they began to move, they would have swapped places. It might have taken hundreds of millions of years for things to settle down to how we see them today. By that time, the disk of gas and dust would have blown away, leaving behind only the scattered, rocky remains of planet formation.

As those rocks encountered a world like Jupiter, the planet’s gravity could have flung them towards the Sun — and in the process, propelled the gas giant just a little bit farther. Slowly, the Solar System we know today took shape — one rock at a time.

And if our search for planets around other stars has taught us anything, it’s that this is only one possible way things might have played out. We now know that planets don’t just stay put after they’re formed — there are all kinds of processes that can move them around.

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