Does Jupiter deserve its reputation as the protector of Earth?
Perhaps you’ve heard of this idea: There’s a lot of junk floating around in the outer solar system, icy and/or rocky chunks out past Jupiter, that sometimes fall in to the inner solar system, becoming comets. There used to be a lot more of that material, but over the eons Jupiter’s immense gravity either drew them into Jupiter itself, or flung them away, ejecting them from the solar system altogether.
In this way, Jupiter prevents them from coming toward us, impacting our fair planet, and causing one mass extinction after another.
Or so the story goes. That’s always bugged me, though. It’s always seemed to me that it should send as much junk flying down into the inner solar system as it does ejecting it from our neighborhood (I even mention this at the very end of my Crash Course Astronomy episode about Jupiter).
It’s bugged my friend Kevin Grazier for a long time, too.* He’s an astronomer who worked on the Cassini mission, using plotted trajectories for the spacecraft as it flies among the moons of Saturn to help plan observations, so he’s very familiar with how the gravity of an object can affect another object.
He heard the story of Jupiter saving us from periodic mass extinctions, and decided to dive more deeply into it. He actually got his degree working on it, and has continued his research since. He wrote about his latest results in a paper for the journal Astrobiology called “Jupiter: Cosmic Jekyll and Hyde,” where he traces the roots of this idea and then tests it.
First, the origin story: In 1994, astronomer George Wetherill used computer modeling to see how the outer planets affected all these proto-comets when the solar system was first forming. He found that Jupiter did sweep this material up, slingshotting (slingshooting? Slangshotting?) it out of the solar system. This idea got picked up by astronomers/authors Peter Ward and Donald Brownlee in their book Rare Earth, cementing it into the popular mythology.
However, as Kevin points out, the computer facilities and models Wetherill used were not as good as they are now. Kevin decided to redo the models and see what’s what. The basic idea is fairly simple: use the computer to simulate tens of thousands of small “test particles” in the outer solar system, allowing them to be affected by the gravity of the outer planets (Jupiter, Saturn, Uranus, and Neptune). The test particles start out orbiting the Sun between the outer planets, but as the gravity of the planets affects them, their orbits change. What happens over time?
Kevin ran several versions of these models, including a solar system that just has Jupiter but no Saturn, Saturn but no Jupiter, and also using much smaller masses for both planets to represent Jupiter and Saturn as they were still forming and growing larger. These different scenarios will change how the particles are affected, and can be compared for more insight.
What Kevin found was fairly complex, but it can be boiled down to a few basic conclusions. The most important one: In the early solar system, Jupiter and Saturn together flung a lot of material into the inner solar system. Of all the material orbiting between the two, roughly half was sent our way, while the other half was ejected from the solar system. Virtually none of the material was absorbed by Jupiter itself.
So right away we see that Jupiter’s reputation of planetary protector isn’t earned. It threw a lot of comets at us! Interestingly, the model runs showed that Saturn was complicit in this as well. Saturn sent material toward Jupiter, and then the big guy sent it down toward us. As Kevin writes, “To use an analogy from sports: Jupiter may score the goal, but Saturn earns an assist.” The models that had Jupiter but no Saturn still resulted in material bombarding us, but far less.
Mind you, this material started out in completely nonthreatening orbits. This stuff was no danger to us because it was a billion or so kilometers out from the Sun on circular orbits. If neither Jupiter nor Saturn were there, that stuff would possibly orbit the Sun that far out forever, never hitting us. But put the big gas giants there, and suddenly that benign material becomes ammunition.
Even the lower-mass models of Jupiter and Saturn (to represent them as planetary embryos, if you will) were pretty good at shooting material into the inner solar system, but not as efficiently as the present-day planets. That makes sense; lower mass means lower gravity, so the embryonic planets weren’t able to move the material around as much.
So, far from being a planetary protector, Jupiter (with its ringed partner) actually dropped a lot of cosmic bombs on our world back in the day.
However, let’s not be too hasty in hanging the black hat on Jupiter. That icy material may have been important to life on Earth. First, we have a lot of water on our planet, and it’s still not clear how much of it formed locally, along with the Earth, and how much was brought from comet impacts. But certainly comet impacts early on did provide some water, and from Kevin’s models we have Jupiter to thank for that (incidentally, some asteroids orbiting between Mars and Jupiter, like Ceres, have lots of ice in them, and it’s possible that came from impacts of comets tossed that way by Jupiter as well).
But there’s more. We’ve seen that meteorites (which come from asteroids) and comets have a lot of the building blocks of life in them; organic carbon-based molecules like amino acids. Life on Earth uses these to build proteins, and they are critically important. It’s possible these molecules formed here, but it’s also possible impacts brought them here as well.
So yeah, we got a pounding from Jupiter (and Saturn) billions of years ago, but that may have also brought the ingredients for life to our planet. We’re here because of that!
So what does this mean for Jupiter’s rep?
If Kevin's right, then billions of years ago Jupiter sent as much stuff our way as it cleaned up. That's probably still true today, but there's far less material out there, and so there are far fewer objects to be sent toward the inner solar system. The ratio of material ejected versus aimed at us is probably the same, but with the total number so much lower, Earth impacts from such objects are incredibly rare.
In the end, assigning a label of “friend” or “foe” to Jupiter is far too black and white. Like anything else in the Universe, it’s capable of doing things that can be a net benefit or a net harm. Usually it’s a mix of both. That’s why I like Kevin’s title for his paper: Robert Louis Stevenson’s tale of a man tortured by his inner beast showed us that we all have the potential for good and evil. As a dork, I’ll add that the Star Trek episode “The Enemy Within” (where Kirk gets split by a transporter accident into a Good Kirk and Evil Kirk) probed this idea further, showing we need both sides of our personae to make us who we are.
Should we be surprised it’s any different for planets? To push the dorkiness further, we need to embrace both the dark and the light side of the force (of gravity) of Jupiter. It made things rough for a while, but it also paved the way for life to begin on Earth. In the end, that’s probably a fair trade.
Postscript: I have to add that Kevin’s work is not the last word on this. We know the planets affected each other in the early solar system, causing their orbits to shift as well (current thinking is that the total mass of the smaller objects moved around may have been significant enough to affect the planets, too); though by how much we don't know. This effect wasn't included in his simulations, and a few other difficult-to-model effects were not included either. There may yet be more to this story, and there’s much work left to be done to find it. In other words: expect sequels.
*Full disclosure: Kevin and I have been friends for a long time; we’ve done TV science show consulting together, and panels at DragonCon and San Diego Comic-Con over the years about the science in science fiction. I also wrote a foreword to a book he did on that topic as well.