In Greco-Roman mythology Jupiter is the king of the gods, a deity who destroyed an older race of titans to become the jealous and vengeful lord of heaven and Earth.
Strange though it may seem, scientific theory lends credence to this historical fiction. As the largest, heaviest object orbiting our sun, Jupiter’s namesake world is the lord of planets, a dominant force in the solar system. Eons ago, while flinging leftover debris from planetary formation out of our solar system, Jupiter probably also tossed some down toward our primordial globe, delivering some of the water that now fills our oceans. Jupiter still shepherds swarms of asteroids, occasionally sending some whizzing harmlessly into interstellar space or on destructive collision courses with Earth and other planets. Jupiter may have even played a role in the asteroid-linked extinction of the dinosaurs about 66 million years ago, an event that ushered in the reign of our mammalian ancestors. Without Jupiter, humans might not exist.
A new study, however, suggests that without Jupiter, Earth itself might not exist either. Where this and the other rocky planets now orbit there may have first been a previous generation of worlds destined to be bigger, gas-shrouded, utterly uninhabitable orbs. But Jupiter came swinging in, clearing the way for small worlds like Earth by destroying those older planets. The study, co-authored by California Institute of Technology planetary scientist Konstantin Batygin and University of California, Santa Cruz, astrophysicist Greg Laughlin, appeared in the March 23 Proceedings of the National Academy of Sciences.
A hole in the solar system
There are hundreds of reasons to suspect that our solar system used to have more and bigger inner planets—the hundreds of multiplanetary systems discovered by planet-hunting projects such as NASA’s Kepler mission. Although our solar system is essentially empty inward of Mercury, equivalent regions around most other stars appear to be packed with close-in, intermediate-mass planets—those between the size of Earth and Neptune. Hopeful astronomers have dubbed these worlds “super-Earths” but most of them seem to be more like hydrogen-rich, gas-shrouded mini-Neptunes—very unearthly indeed. “Now that we can look at our own solar system in the context of all these other planetary systems,” Laughlin says, “the standard-issue planetary system in our galaxy seems to be a set of super-Earths with alarmingly short orbital periods. Our solar system is looking increasingly like an oddball.”
If so, the obvious question is how it got that way. According to Batygin, there’s no reason to suspect that the actual process of planet formation occurred very differently around our sun than around other stars. Instead, the explanation for our solar system’s outlier status may be found in the details of its subsequent evolution—controlled to a remarkable degree by Jupiter.
Migrating worlds
Astronomers used to consider planetary systems reasonably static and stable. Planets would coalesce out of the swirling disks of gas and dust around young stars, a bit like trees springing up from dirt, putting down roots and scarcely budging from where they were born. Small, rocky planets would form in the intense light and heat close to stars, whereas gas-giant planets would form farther out, where cold temperatures preserved more gassy feedstock. Small or large, gassy or rocky, most planets would move about their stars in pristine, near-circular orbits. All this cohered with our understanding of our own solar system. But we may have been wildly mistaken about what is the norm.
Twenty years ago when astronomers found the first planets orbiting other stars, they also began realizing that planetary systems are chaotic places. Some planets did not orbit in near-circles but in oblong “eccentric” paths that took them swinging close and then far from their stars—almost as if they had been thrown off-kilter by the gravitational influence of other worlds. And most of the newfound giant planets were very different than Jupiter—in scorching, star-hugging orbits far inward from the cold outer regions where they must have formed. Planets could migrate, too, propelled by gentle interactions with their formative disks or by close encounters with their planetary siblings.
Ever since those discoveries researchers have been grappling with the idea of planetary migration to better understand not only the features of other planetary systems, but our own. One example is the “grand tack” scenario, which posits that in the first few million years of our solar system’s existence Jupiter migrated into and then back out of the inner solar system, following a course similar to a sailboat’s when it tacks around a buoy. Back then Jupiter would have still been embedded in a gas-rich disk. Much of that gas was spiraling down toward the sun—so much that the action would have sapped some of Jupiter’s angular momentum, too, causing the giant planet itself to spiral in to the vicinity of where Mars is today. Jupiter would have kept falling in toward the sun if not for being caught by the subsequent formation of Saturn, which began drifting in as well. As the two giant planets came closer together, they were caught  in an orbital resonance. This resonance expelled all the gas between them, gradually reversing their death spirals and causing them to “tack” back out to the outer solar system.
As outlandish as it seems, the physical mechanisms underlying the grand tack hypothesis are sound and there are good reasons to suspect it took place. The scenario neatly explains Mars’s anomalously small size, which theorists believe should be larger, given how much planet-forming material should have existed long ago in its orbit. In the grand tack Jupiter would have ejected most of that material, leaving behind just enough for Mars to form. The hypothesis also helps explain the distribution of icy and rocky bodies in the Asteroid Belt and various other features of the solar system.
The grand attack
In their study Batygin and Laughlin investigated whether Jupiter’s grand tack could explain the gaping hole at the heart of our solar system, too. Using numerical simulations, the duo examined what the grand tack would do to a hypothetical embryonic population of super-Earths caught in mid-formation. The simulations suggested that Jupiter’s inward spiral would send swarms of 100-kilometer-wide planetary building blocks cascading into the inner solar system. The giant planet’s gravity would also sling those building blocks and the inner planets themselves into overlapping, elliptical orbits, creating an interplanetary demolition derby of whirling, colliding fragmenting worlds. “It’s the same thing we worry about if satellites were to be destroyed in low Earth orbit,” Laughlin says. “Their fragments would start smashing into other satellites and you’d risk a chain reaction of collisions. Our work indicates that Jupiter would have created just such a collisional cascade in the inner solar system.”
Although these collisions would have been spectacularly violent, they could not by themselves entirely destroy the coalescing super-Earths. Instead, the avalanche of debris from the collisions would have raised powerful aerodynamic headwinds in the surrounding solar system disk, forming spiraling swirls of gas that then swept the first generation of inner rocky planets down into the sun. “It’s a very effective physical process,” Batygin says. “You only need a few Earth masses worth of material to drive tens of Earth masses worth of planets into the sun.”
Beyond observations of other planetary systems suggesting that ours is an outlier, there is scant evidence that our sun formed and lost an earlier generation of inner worlds. But Laughlin finds the technical strength and sweetness of the idea compelling. “This kind of theory, where first this happened and then that happened, is almost always wrong, so I was initially skeptical,” he says. “But it actually involves generic processes that have been extensively studied by other researchers…. Jupiter’s ‘grand tack’ may well have been a ‘grand attack’ on the original inner solar system.”
A lonelier planet
After Jupiter’s grand attack, only whiffs of volatile gas and dregs of shattered rock would remain, but Batygin notes that only about 10 percent of the total material Jupiter may have injected into the inner solar system would have been required to form Mercury, Venus, Earth and Mars. As Jupiter reversed its course and spiraled back to the outer solar system, its passage could have settled a fraction of the dregs into more circular orbits. Across a span of one hundred million to two hundred million years those meager, volatile-depleted dregs would then glom together to make the relatively small and arid inner planets we know today. All this is consistent with a wealth of other evidence suggesting the inner rocky planets formed significantly later than the outer giants, and explains why the sun’s inner worlds are smaller and have thinner atmospheres than those observed around other stars.
The picture that emerges is that we may be even more cosmically alone than previously appreciated. “One of the predictions of our theory is that truly Earth-like planets, with solid surfaces and modest atmospheric pressures, are rare,” Laughlin says.
If true, Batygin and Laughlin’s study would mean that the vast majority of close-in, potentially rocky and habitable planets we now observe around so many other stars may not turn out to be rocky or habitable at all. Instead, visiting them you’d be crushed, cooked and smothered beneath their thick hydrogen-rich atmospheres. The study also suggests that far-out Jupiters are very uncommon around other stars; rather than only briefly visiting inner systems, most giant planets would migrate there to stay, potentially precluding the formation of Earth-like worlds.
In this view, it may really be Saturn that we must thank for being here, because the Ringed Planet’s existence may have kept Jupiter from settling closer to the sun. Which, with poetic license, brings us back to mythology—where Saturn was Jupiter’s father as well as the god responsible for Earth’s wealth, pleasure and plenty. Next time you look up at the heavens, uncrushed and uncooked beneath a clear, hydrogen-free sky, don’t thank your lucky stars—thank Jupiter and Saturn.