This episode is supported by NordVPN.

Go to nordVPN.com/SPACE to learn more about virtual private networks, and internet security. [♪ INTRO]. For as long as you and I have been alive, in fact, for as long as humans have been looking at the sky, the planets have seemed more or less like a merry-go-round, reliably repeating the same orbits over and over.

But now that we've found hundreds of systems with multiple planets, there's reason to believe that solar systems are much more dynamic than we might have expected. Planets appear in orbits where they couldn't possibly have formed, and starless rogue planets float through space, many of them apparently ejected from their solar systems. Yet, among all this volatility, there are islands of stability.

Everywhere we look, we see groups of planets and other satellites that sync up their orbits and find safety by orbiting in gravitational lockstep. They can stably survive much of the turbulence of solar systems. Yet, their entire existence is like a fossil record of much more volatile times in the distant past.

It's one of the best ways we have of learning what happens during the birth of a solar system. Now, you might find it strange to think of a solar system as volatile in the first place. But planets and other objects around a star are constantly getting tugged around by anything with gravity.

So, no planet's trip around a star is exactly like the one before it. And even small nudges can add up in unpredictable ways. They can whisk a planet out of its orbit or send it into collisions, near misses, or some other gory end.

But throughout our galaxy, all sorts of objects have found relative safety by locking into a formation called orbital resonance. When two objects are in resonance, it means that their orbits are a ratio of two whole numbers. For example, maybe the first object completes two orbits every time the second object completes three.

When they lock into this formation, their gravitational influence on each other holds them in a stable orbit. It can keep them from migrating further or getting shaken up by other passing objects. It can also keep planets from getting too close and colliding.

In fact, that's probably why we still have Pluto. Pluto and Neptune's orbits cross paths but they never collide because they're in resonance;. Pluto makes two orbits every time Neptune makes three.

These bodies, bound to each other by gravity, can act kind of like a celestial gang that keeps each other safe. But resonances can only form in systems that are dynamic to begin with, meaning they're not just merry-go-rounds, endlessly repeating the same loops; they're systems where planets are actually migrating toward each other, something that often happens early in a solar system's life. While a planet drifts, it may wander close to another object, which gives it a pull every time it reaches a certain point in the orbit.

It's like when you're pushing a kid on a swing. If you give it a push every time the swing gets to the top of its arc, the kid will go higher and higher. That's because you're reinforcing the frequency of the swing.

And a similar thing happens to these two orbiting objects. If the first object keeps getting a tug that lines up with the frequency of its orbit, it gets stuck in that frequency. And the two objects keep going around the central star or planet, pulling periodically on each other in a way that reinforces their pace around the sun.

Once objects are trapped in these resonant orbits, they're much harder to disturb. Now, resonance is not always a good thing if you're worried about stability. For instance, in places such as planets' rings, objects can be destabilized when different resonant systems overlap.

In spite of its two-faced role, resonance seems to be a really important part of the story of how solar systems form. In our own solar system, many moon and ring systems are full of resonances. Jupiter's moons Io, Europa, and Ganymede are all in resonance.

The gaps and dense spots in Saturn's rings all formed from resonances, too. And in extrasolar systems, around a third of observed exoplanets seem to be in orbital resonance, or at least close to it. One of the most important things this tells us is that solar systems are not static places.

In general, planets and other objects don't just form in one place and stay in that orbit forever. Many of them migrate, and as they do, they get captured in resonances and get stuck in those orbits. Ironically, these resonances, these little islands of stability throughout the universe, are a memory of a past full of movement and change.

Next, scientists are working on figuring out why we don't actually see more resonances in a majority of systems. Maybe the answer is that some systems don't have a lot of migration, or maybe something happens to break up these resonances. Either way, it'll tell us something new about how solar systems form.

And since we can't look directly at any solar system's past, including our own, the fossil record in these resonant systems is one of the best ways we have to understand where we come from. One way to ensure stability a little closer to home is to protect yourself from people trying to steal your private information. NordVPN is a service that encrypts the data you send and receive online.

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