The Asteroids Might Remember a Forgotten Giant Planet

The Asteroids Might Remember a Forgotten Giant Planet

We have much to learn from the rocks of the asteroid belt.


The formation of the solar system is a deeply perplexing puzzle. We’re left with clues all over the place: the positions and sizes of the planets, the members of the asteroid belt, Kuiper Belt, and Oort Cloud and the populations of moons around the planets. But how did we get to this from a vague disk of gas and dust billions of years ago? 

Simulations and models lead the way, and recently researchers have turned to the asteroid belt for more help: the asteroids living closest to the sun actually preserve a memory from when the solar system was still evolving, and might even offer clues to the hypothesis that we once had five giant planets.

Baby solar systems

Billions of years ago, our solar system was just a bunch of random gas and dust floating around as a nebula. As it collapsed, it formed a rapidly spinning merry-go-round of a flat disk around the young and hungry proto-sun. Over the course of 100 million years, that disk somehow became the planets and other smaller denizens of our home system.

Computer simulations of the disk-to-planet process are fantastically difficult, due to all the rich and complex physics involved, but they have a few general features. The innermost worlds tend to be small and rocky, while the outermost planets tend to be big and gassy and/or icy. Plus the process of formation leads to a bunch of random junk floating around.

Another general feature is that newborn planets tend to move quickly into resonant motion, meaning that orbits become integer multiples of each other. For example, Mars might orbit four times for every Jupiter orbit, and Jupiter might orbit twice for every turn around the sun that Saturn gets. 

And when it comes to our solar system in particular, in simulations the giant planets tend to form much closer together, and much closer to the sun, than they are today.

So then the question becomes: Once we’ve got a batch of baby planets formed from our protostellar disk, how do we get those planets in their modern-day positions?

It’s Nice out there

Enter the Nice Model, named after the city in southern France where a few sun-baked researchers cooked up the idea in 2005. In the bare-bones version of the model, the too-close-for-comfort giant planets are surrounded by a disk of leftovers: tiny planetesimals that never got to play the planet game and had to hang out in the outskirts of the solar system.

But not for long. Every so slowly, over the course of 100 million years, the outermost giant planet (usually thought to be Neptune, but in some versions of the model it’s Uranus) drifts close to one of those leftovers. Close enough to interact gravitationally, doing a little orbital dance where the planet pulls the bit of rock inward to a smaller orbit, and in exchange sends itself farther out. 

And then that little scattered rock encounters the next planet in and does the same thing. And then it approaches Saturn and repeats the process again, going ever sunward and spreading out three of the giant planets.

And then that plucky little planetesimal finds Jupiter, who is generally in no mood for games and doesn’t like to be told what to do. Instead of nudging the rock inward, the massive bulk of our system’s largest planet just sends that unlucky bit of debris out of the solar system altogether. That doesn’t come without a price, however; the energy needed to eject the planetesimal reduces Jupiter’s own orbit, sending it slightly closer to the sun.

This model is able to explain in large part the modern-day positions of the planets, and how they were able to get there from their birthplaces. And since 2005, more sophisticated versions of the Nice Model have appeared, trying to explain finer details of our system’s makeup, including the possibility that we once were home to a fifth giant planet that got lost in all the gravitational reshuffling.

Look to the asteroids

But all versions of the Nice Model have a particular problem with the asteroid belt. All that orbital dancing in the outer system can have big impacts on the inner worlds and their own population of planetary leftovers. The on-again-off-again gravitational resonances that the outer planets experience as they migrate to and fro in the outer reaches destabilize members of the nascent asteroid belt, scattering them into all sorts of crazy orbits.

In particular, the various versions of the Nice Model tend to send the innermost belt members (the chunks of rock within 2.5 astronomical units) into orbits with high inclination, meaning that they’re angled with respect to the rest of the solar system. (One astronomical unit, or AU, is the average Earth-sun distance — about 93 million miles, or 150 million kilometers.) And yet, we find most asteroids are on an even keel with the major planets, so we must be getting something wrong in our models.

Recently, a team of researchers took a more refined approach to the simulations, looking especially at the interactions of Jupiter and Saturn as they waltzed together in the early days of the solar system. The scientists found that during the process of planetary migration, Jupiter and Saturn approach a 5:2 resonance, meaning that Jupiter orbits five times for every two orbits of Saturn. 

They don’t stay in that resonance for long. But the details of Saturn’s orbit while near the resonance give it just the right gravitational effect on the inner system to clear away any high-inclination wannabes in the asteroid belt.

And what about the more exotic models, like early solar systems including a fifth giant planet? It too has an effect on all the resonances, which means that the modern-day asteroid belt may actually be a fossil record, remembering what the young system was like. And the more we study those little leftover asteroids, the more we can learn about our own origins.


David Aragorn

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