That Time Jupiter Almost Killed Earth

Article May 10, 2025

I confess, I was feeling uninspired when it came time to write this week’s blog post. I blame the weather, which has been gray and damp here in the Boston area for many days. So I did what I always do when I need a pick-me-up: I think about my favorite solar system theories.

What, I’m a raging space nerd, you thought I was going to count my blessings or something? Nah. I get my kicks thinking about things like the Grand Tack Hypothesis. And there it was, my inspiration!

Since you are unlikely to know what the Grand Tack Hypothesis is unless you also happen to be a raging space nerd (hi friend!), let’s find out about the time Baby Jupiter may have tried to murder the heck out of the inner solar system—and why it wound up not doing so.

Wandering Feet

A simulation showing the interaction between a large planet and the materials of a protoplanetary disk in a still-forming solar system. Credit: Philip Armitage

First things first: baby planets go a-wandering. It’s highly unlikely that any of the planets of our solar system formed exactly where they are now. More likely they spent some of their misspent youths while they were still building themselves up into proper planets meandering around. This is known as “planetary migration”.

You might be wondering how a planet can just go for a stroll, but it generally only happens in young systems that are still forming. Most of it depends on there still being something left of the protoplanetary disk from which the solar system formed in the first place. The growing planets can interact with the material of the disk, either through direct contact or gravitationally, and gain or lose momentum. This will cause them to increase or decrease the size of their orbits.

Once larger objects exist in the solar system, gravitational interactions between them can also cause migration. For instance, Pluto probably wound up where it is today through its gravitational interactions with Neptune, which also probably played a role in making Neptune move outward from where it originally formed.

We know for sure that this happens, though some of our best evidence for it comes from other solar systems. For instance, the very first world we ever found around another star is 51 Pegasi b, a gas giant bigger than Jupiter but orbiting closer to its star than Mercury is to the Sun. We’re pretty sure the pressures from a young star would prevent such a big gas giant from forming so close, so it must have formed farther out and then moved inward.

Close to Home

Okay, so we can accept that young planets move about. What evidence is there that something like this happened in our own solar system? Well we’ve already mentioned Pluto and Neptune and the likelihood that they helped pull each other to where they are today.

Mars is smaller than it should be, unless something like the Grand Tack occurred in the solar system’s history. Credit: Kevin Gill

Then there’s the asteroid belt. Although it’s got bajillions of rocks in it (no, that’s not the technical term, but you get it), and is usually depicted as being a crowded debris field, it’s actually pretty empty. The average distance between rocks in the belt is something around 600,000 miles.

We know a lot of asteroid-ish objects struck the inner solar system during the early days, so clearly there used to be a lot more asteroids. Evidence suggests that the migration of the outer worlds, especially the massive-and-still-growing Jupiter, gravitationally drove them to scatter, peppering the inner worlds with shrapnel. Today’s belt is what’s left.

Then there’s what we like to call “the Mars Problem”. Essentially formation models of the solar system say Mars should be bigger than it is, probably more like Earth or Venus-sized. If a similar sort of scattering took place in the region Mars was forming as seems to have taken place in the asteroid belt, it would have left Mars with significantly less stuff to build itself up from, leaving us with the smaller (yet still loved!) planet we see today.

Beware the Big Guy

Jupiter’s wanderings may have helped shape the modern solar system. Credit: NASA/STScI

Okay, so there’s plenty of evidence to suggest that the outer planets were moving about in their youths. Specifically, there is evidence that Jupiter was moving inward towards the Sun, and throwing its mass (read: gravity) around as it did.

But here’s the thing: Jupiter didn’t stop on its own. If it was left on its own, Jupiter likely would have kept moving inward. Ultimately it would probably have wound up like 51 Pegasi b, snug up against the Sun. And that would have been bad news for the inner solar system, Earth included. We’ve already seen evidence that Jupiter’s inward motion caused other things to scatter inward. Had this continued unchecked, Jupiter probably would have scattered the entire inner solar system right into the Sun.

51 Pegasi b’s system may have once had small, rocky worlds. It doesn’t anymore. That could have been us. It would have been, if something hadn’t made Jupiter reverse direction.

Grand Tack

Here’s where the Grand Tack Hypothesis comes in. “Tack” is, apparently, a sailing term (I grew up not far from the ocean, but I’ve never been a sailor. I learned this term because of Jupiter), meaning when the boat changes directions. So Jupiter’s theoretical change of direction is called the Grand Tack.

According to this hypothesis, Jupiter began forming kind of in the middle of where the asteroid belt is now, about 3.5 AU from the Sun, and then started its potentially fatal (if you happened to be an inner rocky planet) movement inward. Then, at about 1.5 AU (more or less where Mars lives now), it stopped. It then “tacked”, turning itself around and heading back out. And we think it’s because of Saturn.

A simulation of the Grand Tack Hypothesis, showing Jupiter plowing the inner solar system before it before it gets stopped by Saturn catching up to it. Credit: Walsh et al (2011)

Jupiter wasn’t the only one moving Sunward at this point—Baby Saturn was following behind it. Being smaller than Jupiter it actually was able to migrate faster. Right around the time Jupiter got to about 1.5 AU from the Sun, Saturn had caught up enough to reach a critical point where it entered a 3:2 resonance with Jupiter.

Resonance, as I’ve written before, can have powerful effects on objects and their surroundings. In this case the resonance between the two baby gas giants cleared the protoplanetary disk remnants from between them, forming a big ol’ gap. With no more disk materials to rub up against, Jupiter stopped its inward wander.

Eventually gravitational interactions with the inner, remaining part of the disk caused both Jupiter and Saturn to gain momentum, and they began to move back outward until all the interacting forces deposited Jupiter into a gravitationally stable orbit more or less where it is now, about 5.2 AU from the Sun. Its “Grand Tack” was complete.

We suspect Saturn still had some wandering to do. Simulations suggest the Grand Tack would have left it about 7 AU from the Sun, while today it’s closer to 10. Clearly it still had itchy feet after Jupiter had decided to settle down.

Hail Saturn!

This is, of course, just a theory based on simulations to help explain things like the composition of the asteroid belt and why Mars is so darned small. Nobody was around back then to actually see Jupiter going full solar system bully.

Saturn may have saved the inner solar system from fiery doom. Credit: NASA/JPL

But if it’s correct, then Saturn is the reason the inner solar system still exists. This has implications for when we look at other solar systems. The Grand Tack Hypothesis suggests that when we find systems that have inner rocky worlds and something like Jupiter in it as well, then it probably will also have something like Saturn. It’s possible that a Saturn is needed to put the brakes on the big planet and save the little ones.

And it means we likely owe our very existence to Saturn. All hail the Lord of the Rings (and my sincerest sorry-not-sorry to Professor J.R.R. Tolkien)!