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The Madness of the Planets

Our home in the universe continues to rock out of control.

I am a staunch believer in leading with the bad news, so let me get straight to the point. Earth, our anchor and our solitary haven in a hostile universe, is in a precarious situation. The solar system around us is rife with instability.

Residents of Chelyabinsk, Russia, experienced this firsthand at 9:20 a.m. local time last Feb. 15, when a 50-foot-wide asteroid slammed into Earth’s atmosphere and exploded above the town, shattering windows, collapsing the roof of a local zinc factory, and sending more than a thousand people to the hospital with glass cuts and other injuries. Millions of people saw the videos of Comet ISON meeting a different but related cataclysmic fate as it took a swan dive past the sun on Thanksgiving Day. In the space of a few hours, the 4.5-billion-year-old comet was reduced to a cloud of sputtering rubble.

But these incidents are mere pixels in the sweeping picture emerging from the latest theories of how our solar system formed and evolved. Collisions and dislocations are not occasional anomalies; they are a fundamental cosmic condition.

“Things are not as simple as they were supposed to be, with the planets staying quiet forever,” says Alessandro Morbidelli, a planetary dynamics expert at the Nice Observatory in France. “When the planets form they don’t know they have to form on good orbits to be stable for billions of years! So they are stable temporarily, but are not stable for the lifetime of the star.”

Translation: Earth was forged in chaos, lives in chaos, and may well end in chaos.

While Morbidelli is explaining all this to me in a cheery Italian accent, I cannot help fixating on the grim connotations of his last name. He and his scientific compatriots are amplifying a recent realization about our celestial home: Instability is our natural state. For centuries, Isaac Newton and his followers envisioned a solar system that runs like divine clockwork. Only in the past decade have high-precision mathematical simulations shown just how wrong he was. Carl Sagan famously declared that “we’re made of star stuff.” Morbidelli has an equally profound message: We are made of cosmic chaos.

“You might take a trip around the galaxy, come back in 5 billion years, and say, hey, there is no Mercury anymore and Earth is now on an eccentric orbit that is catastrophic for life,” Morbidelli continues in his musical voice. It’s easy to be cheery when talking about events that nobody alive will ever experience, I suppose, but his tone doesn’t change a bit as he starts ticking off the parts of the solar system in flux.

“There are unstable populations in the asteroid belt, Kuiper belt, and Oort Cloud,” Morbidelli says. These instabilities are relics of the chaos in which Earth and the other planets formed. The Chelyabinsk meteor emerged from the asteroid belt, perhaps from a smashup that took place about 30,000 years ago. The Kuiper Belt, just beyond Neptune, and the Oort Cloud, a vast hive of dormant comets extending halfway out to the next star, can also send objects careening our way. That is where Comet ISON came from.

After the Chelyabinsk disaster, Morbidelli and a group of colleagues huddled to figure out the implications of all that instability. Meteor impacts seemed to be happening a lot more often than their models predicted. When they presented their updated results last month, they concluded that the true rate of Chelyabinsk-scale events is probably ten times as high as they had previously estimated. There is no clean separation between the unsettled past and the present.

Morbidelli’s view replaces Newtonian clockwork with something more akin to quantum uncertainty, with everything defined by probabilities of survival. Over time, every object in the solar system that can be destroyed, scattered, or ejected will eventually be destroyed, scattered, or ejected. That is how Earth came to be. That is how it exists today. 

The belief that Earth and the rest of the solar system were born in largely their present form—the arrangement and characteristics of the planets almost preordained—has deep, clinging roots in the history of science. It extends further than Newton, back to the influential writings of the 13th-century monk Johannes de Sacrobosco, who described the universe in terms of clean, geometric patterns. You could plausibly draw a line all the way to the perfect heavenly spheres of Aristotle. Amazingly, the same basic philosophy (built atop a very different scientific foundation) persisted well into the Space Age.

In retrospect, that simple and comforting view had begun to unravel long before most scientists recognized what was happening. As far back as the 1970s, theoretical models trying to simulate the formation of the solar system kept coming up with an unwanted result: The planets migrated wildly, toward and away from the sun, making a royal mess of things in the process. “It would come out of pen and paper theory work, but it was immediately ignored,” says Kevin Walsh, a leading researcher in solar system dynamics at Southwest Research Institute in Boulder, Colo. “It showed up as like throwaway comments in some papers, but no one seriously proposed that planetary migration was an important process.”

The idea languished for a couple decades until a series of scene-shifting discoveries forced the theorists to reconsider. In 1995, two Swiss astronomers detected 51 Pegasi b, a planet orbiting a dim yellow star located 50 light years away in the constellation Pegasus. It was the first world found orbiting another star similar to the sun. The planet is a gassy, Jupiter-size giant, the kind of world that theoretically can form only in the cool regions far from its parent star. But there was 51 Pegasi b, hugging close in a searing-hot orbit. The only sensible explanation astronomers could come up with was that the planet had formed far out and then somehow shifted sharply inward. They dubbed this puzzling world a “hot Jupiter” and waited to see if it was a fluke.

It was not. Within a year, a competing American team discovered two more hot Jupiters; several dozen similar worlds are now catalogued. As astronomers got better at searching, they started to find a number of other improbable planets. Some were in highly oval orbits; some revolved around their stars at a steep angle, or even backward. Such arrangements did not seem physically possible unless the planets had migrated dramatically at some point. If that process happened around other stars, it could have happened here as well. “And that’s when it really started to kick off,” Walsh says.

Collisions and dislocations are not occasional anomalies; they are a fundamental cosmic condition.

In 2009, Walsh was a postdoctoral fellow at the Nice Observatory, where he collaborated with Morbidelli. Walsh was already an expert in solar system dynamics; now he became fascinated by the concept of migrating planets and buried himself in the details of how the process would work. He focused on the earliest stages of solar system formation while Morbidelli tackled a later, secondary instability.

Getting planets to move is extremely easy in mathematical models of a newborn solar system. The challenge—as those pen-and-paper theorists of the 1970s had discovered—was finding ways for planets not to move. Data from the Hubble Space Telescope and other great observatories show that, in the big picture, infant planets emerge from a swirling disk of gas and dust around a just-formed star, known as a protoplanetary nebula. For the first few million years, planets are little more than debris bobbing on the waves in the disk.

“That nebula outweighs the planets about a thousand to one, so the gas can push the planets around really dramatically,” Walsh says. As a result, he realized, the early solar system must have been more like bumper cars than clockwork. He also saw that if he fully embraced the idea of instability and took it to its logical conclusions, he could account for many aspects of the solar system that had previously defied easy explanation: Why is Mars so small? How did the asteroid belt form? And above all, why is Earth’s chemical makeup so different than was predicted by the original formation models?

Walsh knit his ideas into a theory he calls the Grand Tack, which creates a startlingly new narrative of how the Earth and other planets formed. At present, Jupiter’s orbit is 5.2 times wider than Earth’s. It is also sticking to its 11.8-year orbit like a metronome. But according to Walsh, Jupiter actually formed quite a bit farther out and then, during the solar system’s initial 5 million years, executed a series of dramatic swoops. First it spiraled inward to the place where Mars is now (about 1.5 times the Earth-sun distance), as the dense gas in the nebula dragged it toward the sun. Then it migrated out past its current location, yanked by the gravitational influence of the newly formed planet Saturn. The whole process took about 500,000 years—an eternity in human terms, but blazingly fast for the solar system, which is 4.6 billion years old.

So what happens, I ask, when a planet that size goes on the prowl? “Oh, it raises hell!” Walsh replies. “That’s a really big planet and it’s moving all over the place. It acts like a giant snowplow and essentially wipes out everything in its way.”

Fortunately for us, Earth had not yet formed when Jupiter was on the move; if it had, our planet might have plunged into the sun or spun off into dark oblivion. The giant planet’s influence on the inner solar system, where we live, was more indirect. Most of the action happened on the outbound track, when Jupiter rammed through thick swarms of icy comets and asteroids. That snowplow effect sent those water-rich objects raining down on Earth just as it was beginning to grow. “The bulk of the water that we see on Earth is a result of the scattering from Jupiter’s outward migration,” Walsh says. Whenever you take a swim, or just take a drink, you are benefitting from the solar system’s foundational instability.

The migration of Jupiter reshaped the solar system in many other ways. It cleared out the original asteroid belt on the way in and filled it with new objects on the way out. It reorganized the distribution of comets. It stunted the growth of Mars, making it into the cold and nearly airless world it is today—bad luck for prospective Martians. At the same time, Jupiter deposited enough material closer to the Earth that our planet ended up colliding with one of the leftover planetary cores. The moon is thought to have formed from the wreckage of that cataclysm.

After Jupiter finished its wanderings the solar system looked stable, but the appearance was superficial. Instead, it had set the stage for a second great upheaval, one that has been baffling scientists for half a century.


NASA’s Apollo missions achieved many notable things, but for the purposes of this story their greatest legacy was bringing back 842 pounds of moon rocks. On Earth, almost all evidence of the solar system’s unruly early history has been worn away by erosion, biological activity, and the slow dancing of the continents. On the moon, there is no erasure. The moon never forgets.

During the 1970s, even as NASA was retreating from the majestic Apollo missions to a rickety Skylab, several groups of researchers set out to decode the lunar samples brought back by the astronauts. The moon’s surface contains an intact chemical record of all the asteroids that have pummeled it over the eons. Planetary scientists expected to find a steady progression from disorder to order: lots of impacts right after the formation of the solar system, then a rapid tapering off as the moon (like the Earth and other planets) mopped up the last bits of debris. That is not at all the story written in the rocks.

When three geochemists—Fouad Tera, Dimitri Papanastassiou, and Gerald Wasserburg—sifted thoroughly through the lunar material, they saw instead that most of the material created by impacts had an age of about 3.9 billion years. The moon apparently experienced another intense barrage of asteroids at that time, a full 700 million years after the formation of the solar system. The researchers called it the “terminal lunar cataclysm”; now it’s known as the Late Heavy Bombardment. Either way, it stayed on the books for decades as one of the solar system’s biggest mysteries.

Whenever you take a swim, or just take a drink, you are benefitting from the solar system’s foundational instability.

Around 2005, Morbidelli decided to take a crack at solving it. In conjunction with three other researchers, including Harold Levison, Walsh’s neighbor and collaborator at the Southwest Research Institute, he wrote up a series of papers in which he connected the Late Heavy Bombardment to a previously unknown, belated instability in the formation of the solar system. The resulting “Nice model” (that’s Nice as in the French town where Morbidelli works) is now the most widely accepted explanation for the solar system’s second wave of devastating impacts.

According to this model, the solar system never quite found its steady groove after Jupiter migrated back out and the sun blew out its birth nebula. An enormous cloud of comets orbiting the sun beyond Neptune—the region that now marks the Kuiper Belt—was slowly but inexorably working gravitational mischief.

At first, Neptune’s orbit was synchronized with Jupiter’s, a pattern called a resonance. Jupiter circled the sun three times, perhaps, for each one orbit of Neptune. Resonances tend to keep things stable. Over hundreds of millions of years, however, that cometary cloud dragged Neptune into a new orbit. “When Neptune gets off resonance with Jupiter the system goes ‘boom,’ completely unstable. Then the violent evolution starts,” Morbidelli says. Neptune migrated out, flinging comets inward; those comets reached Jupiter, which batted them even farther out; Jupiter, in response, migrated inward.

In the end, Saturn and Uranus, as well as Neptune, moved into more distant orbits. Jupiter settled into its present, closer one. In one version of the theory, developed by Morbidelli’s colleague David Nesvorny at the Southwest Research Institute, our solar system originally had a fifth giant planet that got ejected entirely during this commotion; if so, it is currently wandering alone among the stars. Most comets got exiled to the Oort Cloud, far beyond the planets. Many other comets and asteroids went careening closer to the sun, where great numbers of them smacked into moon, Earth, and other inner planets.

Although almost all traces of this hellish era have vanished from Earth, some fleeting bits remain. Bruce Simonson of Oberlin College is tracking down extremely ancient impacts from the most distinctive evidence they leave behind: glass spherules the size of BBs (created from rock melted by the asteroid or comet) and elevated concentrations of the element iridium (much more common in meteorites than in Earth’s surface). Earth’s two oldest rock beds, one in western Australia and the other in South Africa, preserve records going back at least 3.4 billion years. For the past two decades, Simonson has been prospecting there for signs of what the Late Heavy Bombardment did to our planet.

One of his most intriguing results is that the rain of asteroids may have continued for a staggeringly long time, until 2.5 billion years ago or even more recently. “There’s evidence it was a more gradual ramp down, and we think it’s convincing evidence, but of course it’s ours,” Simonson says. If he’s right, regular asteroid blows were occurring well into the era of life on Earth, which began around 3.5 billion years ago. He also sees signs of what he technically calls “big-ass impacts,” many times bigger than the impact that helped kill off the dinosaurs.

Surprisingly, Simonson thinks life soldiered on just fine amid all the falling rocks. “I’m not a big fan of impacts and extinction,” he confides. “The only one where we have clear coincidence between impact and a change is the end Cretaceous [when the dinosaurs went extinct].” He thinks that, overall, extinctions are more likely to have been caused by enormous volcanic eruptions and by the changing configuration of the continents and oceans. That’s a little comfort, at least, considering that everywhere on Earth where he looks, he finds evidence of ancient asteroid blasts.


At the time of the Late Heavy Bombardment, asteroids were hitting Earth at least a thousand times as often as they do now. Could something like that happen again? No, both Morbidelli and Walsh answer without hesitation. The first two planetary rearrangements cleared out 99.9 percent of the asteroid belt and Kuiper Belt; there is just not enough left to recreate the kind of chaos that ruled 3.9 billion years ago.

So are we home free? No again. “The terrestrial planets, they are not totally stable,” Morbidelli says. That instantly captures my attention: Earth is one of the four terrestrial planets. “Mercury is on the edge of the instability, and it could go nuts, start to encounter Venus, then the orbits of Venus and the Earth could become unstable themselves.” From there, Venus could collide with Earth, or Earth could go careening off on a totally new orbit, sterilizing the planet. The odds are not great, but they’re not all that small either—about 1 percent over the next few billion years.

I question Morbidelli to make sure I’m understating him correctly. A 1 percent chance of disaster is surprisingly high odds in the cosmic-doomsday business. He sets down the phone for a moment and I hear him in the distance, double-checking with someone else in his office (“Do you know the probability that Mercury gets crazy?”). Then he’s back on the line: “Yes, 1 percent.” And he warns that the subtle divergences that would set the whole cataclysm in motion are like the weather, chaotic and impossible to forecast far in advance. They could be building up right now.

We are back to a probabilistic view of the solar system, in which nature builds some inherent uncertainty into the system. “It may be that instabilities are just a natural part of life for planetary systems as complicated as ours, and chaos keeps us from really understanding it,” Walsh says.

“Mercury could go nuts, start to encounter Venus, then the orbits of Venus and the Earth could become unstable themselves.”

There are definitely other, less catastrophic, and more comprehensible forms of instability at work right now. Comets still leak out of the region around the Kuiper Belt, a lingering hangover from the Late Heavy Bombardment. Comets in the Oort Cloud get sent flying by passing stars. In addition to gravitational mischief by the planets, a slight pressure created by sunlight, known as the Yarkovsky Effect, keeps shifting the paths of the asteroids, guaranteeing that the risk of impact will never go away. A year ago, the standard line was that events like Chelyabinsk happen once every 200 or 300 years. The updated estimate is a couple times a century, maybe more often.

But Morbidelli is not at all gloomy or apocalyptic about his work. The more I speak to him, the more I absorb his perspective. Instability is a mechanism that transforms things from generic and boring into particular and interesting—whether those things are people or planetary systems. “If you want to describe the global evolution of a person, well, he’s born and then he dies. That’s it,” Morbidelli says. “If you want to describe a specific person in detail, I cannot do it in a general scheme. There is a general scheme, but there are plenty of specific ramifications that drive you to be the person that you are. For a planetary system it is the same thing. That’s chaos: extreme sensitivity to tiny changes.”

To Morbidelli, we’re not at war with a hostile universe, we’re part of it. The astronomer has clearly spent a lot of time contemplating the personal implications of his work. “Planetary systems evolve by instable steps until they find their final peace; it is almost a Buddhist view of the universe,” he concludes, lyrical as ever. “Everything evolves to wisdom and peace—and stability—through big revolutionary events.”


Corey S. Powell is an editor at large at Discover magazine, acting editor of American Scientist, and writer of the blog Out There.

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