Fasten your seatbelts, folks — I'm about to take you on a wild ride!
Compared with the systems of planets being found around other stars, our solar system is an orderly place, with each planet tracking around the Sun in a stable, roughly circular orbit. For centuries, the planets' long-term stability has been taken as evidence that they formed where they are now, sucking up gas, dust, and larger building blocks from the protoplanetary disk around them until reaching their final sizes.
But dig a little deeper, and you find serious problems with that simplistic view. For example, Uranus and Neptune should have ended up much smaller and less massive, because billions of miles from the infant Sun the protoplanetary pickings were slim and the assembly process too slow. Conversely, Mars formed in the fat of the disk and should have ended up at least 10 times more massive than it is today. And no one really understands the asteroid belt's existence — particularly why it's crudely divided into rocky bodies (called S types) nearer the Sun and dark, carbon-dominated hunks (C types) farther out.
Dynamicists solved the Uranus-Neptune dilemma several years ago by positing that the four giant planets were initially a much closer-knit family, coming together in a cozy zone 5 to 12 astronomical units from the Sun.
The Big Four coexisted peacefully at first, but after a couple of million years things got ugly. Jupiter's gravity jostled Saturn into an unstable, wide-swinging orbit, triggering a chain reaction of close encounters that ultimately threw Neptune and Uranus out to the distant depths of interplanetary space they now occupy.
Theorists now have computer models that get the outer solar system to come out right, more or less, but they're still vexed by the inner planets. The thorny problems of a too-small Mars and a compositionally stratified asteroid belt remain.
Worse, discoveries of other solar systems were revealing radically different inner-planet architectures: "hot Jupiters" whirling so close to their suns that a year for them is just days long, and massive planets in orbits so wildly out of round that any lesser worlds they encountered would have been tossed out. Given all the disorder so common among the exoplanets, it's remarkable that the Sun ended up with any small, close-in worlds at all.
But there's been a breakthrough in modeling our solar system's formation, details of which emerged at last week's meeting of the American Astronomical Society's Division for Planetary Sciences. It turns out that getting four right-size terrestrial planets and the right kind of asteroid belt is a snap — but it requires dramatic new thinking about the path Jupiter (and Saturn) took getting to their current locations.
Solving for Mars
The stage for this revolution was actually set last year, when Brad Hansen (University of California, Los Angeles) tried assembling the inner solar system an entirely new way. He took a cue from the one other place known to have close-in, Earth-size planets: the system surrounding the millisecond pulsar B1257+12. Discovered in 1991, these pulsar planets are often overlooked because their host "star" is so extreme.
Prior computer simulations assumed that the inner planets accreted from a dense, massive belt of mile-wide planetesimals extending almost out to Jupiter. But invariably the outcome was a too-massive Mars and jumbled mess in the asteroid belt. However, Hansen realized that PSR B1257+12's planets must have assembled from a limited disk of hot material closely surrounding the pulsar.
When he tried that approach with our solar system, starting with a disk confined to just 0.7 to 1.0 astronomical unit from the Sun, voilà! — his computer runs routinely coughed up sets of planets with bigger ones (think "Earth" and "Venus") in the middle and smaller ones ("Mercury" and "Mars") near the inner and outer edges.
So why should Earth and its immediate neighbors have formed from such a limited disk? Hansen had no clue when he published his results last year. "In my paper I freely admit the choice was ad hoc," he allows. But it worked — far better, in fact, than any of the previous trials.
Meanwhile, the outer-planet crowd had wondered how Jupiter managed to avoid becoming a close-in captive of the Sun, as so many other beefy exoplanets had. On paper, tidal interactions between the King of Planets and the Sun's protoplanetary disk should have drawn Jupiter inward to its doom, or nearly so.
As early as 1999, however, theorists Frederic Masset and Mark Snellgrove (then at Queen Mary College) showed that Jupiter would have indeed migrated inward — but only until it linked up with Saturn in a 3:2 resonance, that is, with the two spaced such that Jupiter completed three orbits for every two of Saturn's. At that point the pair would have reversed direction and headed outward. (The mechanics of this coupled migration are a little involved; if interested, you can get the details here.)
Hansen's shot-in-the-dark simulations, combined with the realization that the gas giants could have migrated both inward and outward, gave solar-system modelers a "Eureka!" moment. What would have happened, they wondered, if young Jupiter had ventured much closer to the Sun than where it finally ended up?
The amazing answers came to light at last week's meeting. Kevin Walsh, who'd worked this problem with Alessandro Morbidelli while post-docing at Côte d'Azur Observatory in France, ran computer simulations that put Jupiter initially 3½ a.u. from the Sun and allowed it to creep inward to 1½ a.u. (about where Mars orbits now). The results were remarkable in their breadth and significance.
First, Jupiter's gravity would have forced the small stuff in its path inward too, creating a perturbation-driven snowplow that piled all the rocky planetesimals into a mini-disk with an outer edge 1 a.u. from the Sun. According to presenter David O'Brien (Planetary Science Institute), a member of Walsh's team, Jupiter took only 100,000 years to drive inward to 1½ a.u.and another 500,000 years to reach its current orbit, 5.2 a.u. from the Sun.
Second, the new computer runs confirmed what Hansen had already shown: a mini-disk of rocky material extending only to 1 a.u. provided just what's needed to assemble four terrestrial planets — and a Mars that's not too big.
At the meeting, David Minton and Hal Levison (Southwest Research Institute) described their own simulations using a truncated mini-disk, and they come to much the same conclusions. One key variation is that, in the Minton-Levison runs, Mars forms well within the disk and migrates to its outer edge and beyond.
This could be a good thing, because a moving Mars would provide the gravitational perturbations needed to kick iron-rich planetesimals out of the disk and into the inner asteroid belt, where they're commonly found today. "The original locations of Mars in the [disks] I calculated were quite variable," Hansen comments. "The outward migration was driven by scattering, so things shake up quite a bit."
Third, Jupiter probably would likely have come in even closer, perhaps sliding all the way into the Sun, had not Saturn (already in tow via the 3:2 resonance) grown massive enough to hit the tidal brakes and reverse both planets' movement. In this sense, the formation and survival of the terrestrial planets hinged not on Jupiter's existence but on Saturn's.
Fourth, Jupiter's inward trek would have completely swept clear the asteroidal region from 2 to 4 a.u. Most of the objects there were lost completely, but roughly 15% ended up scattered into a disk beyond Saturn. After reversing course and moving outward, the two planets scattered some of those previously displaced objects again, this time inward, returning them to what's now the inner asteroid belt.
Fifth, as Saturn and Jupiter continued outward to their final orbits, they encountered another group of asteroids. Unlike the rocky bodies that had boomeranged out and back, these were carbon- and water-rich objects that had formed 6 to 9 a.u. from the Sun. Tossed inward by perturbations from the dynamic duo, they formed most of what's now the outer asteroid belt.
A New Paradigm?
To recap: in one sweeping narrative, these theorists propose solutions for both a minimalist Mars and a stratified asteroid belt with a rock-rich inner region and a carbonaceous, water-harboring outer belt. As a bonus, the new mindset leads naturally to a set of four inner planets (correct sizes, correct orbits) that assembled on the right time scale (within about 30 million years of the Sun's formation). It even provides a source of water for Earth (C-type asteroids) and a near-Earth environment conducive to the presumed giant impact that formed the Moon.
This radical scenario represents "a paradigm shift in our understanding of the evolution of the inner solar system," says Walsh. That's an understatement! It all seems hauntingly Velikovskian to me, except that these folks have clearly done their homework.
Will "Jupiter's Grand Tack" (as Morbidelli dubs it) hold up to further scrutiny? Walsh and his team have submitted a fuller treatment to Nature for publication, but other dynamicists are already weighing in based on the presentations heard last week. "Many aspects of their model look good to me," observes SwRI officemate William Bottke, "but lots of first-order things have to be tested before they can declare victory on all fronts."
For example, it's now widely accepted that most of Earth's water was imported from the outer asteroid belt. Yet Bottke thinks the scenario envisioned by Walsh, Morbidelli, O'Brien, and others would require a vast reservoir of water-rich (C-type) bodies, totaling hundreds of times the mass of the current asteroid belt. "We need to vet these models with more physics and more cosmochemistry," he says. Also, the depth of Jupiter and Saturn's inward penetration would have depended critically on how fast Saturn grew to nearly full size and when. The broader the range of initial conditions that "work," the more confidence there'll be that this scenario is the right one.
Morbidelli remains confident that they're onto something profound. "We consider ourselves celestial geologists," he quips. "We're now able to 'read' the current solar-system arrangement well enough to figure out what the early planets did."