An early maelstrom shaped our Solar System. Sometime after the planets took shape from primordial gas and dust, resonant tugs between the giant planets threw their orbits out of kilter. The gravity of the errant giants blasted Pluto and its many icy neighbors into the far-out Kuiper belt. The instability also scattered oddball moons and asteroids and triggered smaller bodies to pummel the inner planets.
Now, that scenario is experiencing some upheaval of its own.
Scars on the Moon had convinced many planetary scientists that the storm hit about 3.95 billion years ago, 650 million years after the Solar System formed. But this model has long had a flaw: Mercury, Venus, Earth, and Mars would likely not have survived such a late assault. And over the past few years, a new timeline has begun to emerge, one that shifts the chaos earlier, to less than 100 million years after the system’s creation—and perhaps as few as 10 million years. “The tides are moving and people are now more and more convinced that the instability happened early,” says David
Nesvorny, a planetary scientist at the Southwest Research Institute (SwRI) in Boulder, Colorado. Several new papers explore what triggered this early instability and how it can explain a host of Solar System quirks.
Two decades ago, scientists recognized that planets must have migrated to create the modern Solar System. A group including Alessandro Morbidelli, a planetary scientist at the University of C?te d’Azur, gathered in Nice, France, for 1 year to hash out the idea, creating what’s known as the Nice model. As the model now goes, after the giant planets formed out of the gas disk, Jupiter drew its fellow giants into a resonant chain of orbits where, for example, Saturn orbited the Sun three times for two turns of Jupiter. The surrounding gas acted as a damping agent, calming any instability like an air conditioner in a room of irritable siblings. But once the gas dissipated, the collective push and pull of giant planets’ masses, agitated by nearby planetary building blocks, unleashed chaos.
The turmoil came relatively late, suggested lunar rocks collected from impact craters by the Apollo astronauts. The ages of the rocks seemed to indicate that the Moon suffered a cataclysmic assault, dubbed the Late Heavy Bombardment (LHB), 3.95 billion years ago, sandwiched by hundreds of millions of years of quiet. But over the past few years this story has evaporated, says Nicolle Zellner, a lunar geochemist at Albion College. New work suggests rocks collected by astronauts at multiple craters, once believed to represent simultaneous strikes, are instead debris from a single impact, 3.95 billion years ago, that created Imbrium Basin. More precise dating of meteorites ejected from the Moon shows that the impacts responsible took place as many as 4.3 billion years ago—or well after the supposed LHB. “The idea of a very strong cataclysm has gone away,” Zellner says.
Planetary dynamicists have welcomed the LHB’s vanishing act. Their models had highlighted a puzzle: A late catastrophe would have either destroyed the rocky planets of the inner solar system or disrupted their stately, nearly circular orbits, flat with the Solar System’s plane. “This was my first red flashing light,” says Kevin Walsh, a planetary scientist at SwRI.
Now, in a paper accepted for publication in Icarus, Morbidelli and co-authors show that such a late instability wouldn’t work in any case. Their computer modeling indicates that, for a late instability to have created the current Solar System, a large gap would have had to exist between Neptune and the encircling disk of planetary building blocks outside its orbit. But the gap rarely appears in the models. And without the gap, it’s impossible to delay the catastrophe, Morbidelli says.
Freed from the late constraint, planetary scientists are now exploring how an earlier cataclysm could explain odd features of the Solar System. Over the past few years, Matthew Clement of the Carnegie Institution for Science, Walsh, and others have shown in computer simulations that an instability less than 10 million years after Solar System formation would allow the inner planets to coalesce in peace. An early instability would also scour away planet-forming material near Mars and the asteroid belt, explaining their weirdly low masses. And in a paper published last month in the Monthly Notices of the Royal Astronomical Society: Letters, they show that as Saturn moved away from Jupiter near the end of the instability, a final tug between them might have flung away asteroids in orbits far removed from the orbital plane, giving the asteroid belt its current compact structure. “We kind of simplify the whole story,” Clement says. “We can have one event explain all these problems.”
Still, “The details are strongly debated,” says Thomas Kruijer, a geochemist at Lawrence Livermore National Laboratory. There’s little direct evidence for such an early instability, and at least two other scenarios that could explain how the rocky planets survived. Clement also has yet to reconcile a similarity between noble gases measured by the Rosetta spacecraft around the comet 67P and features of Earth’s atmosphere, which suggests the instability likely caused Earth to be bombarded with a hail of comets after it was solid—not before.
But Kruijer says a bombardment within the first 100 million years of the Solar System is plausible. Perhaps the best evidence for it is now found near Jupiter, Nesvorny adds. There, following Jupiter in its orbit, spins a binary asteroid named Patroclus-Menoetius. The icy composition of its two bodies indicates they formed in the far reaches of the Solar System and were implanted into Jupiter’s wake during the instability. In a 2018 paper, Nesvorny and co-authors showed there’s no way the binary would have survived 600 million years in the outer Solar System—collisions would have ground it up after only 100 million years. “That’s a very solid constraint” supporting an early instability, Morbidelli says.
The hunt is on for more observations that can parse what happened during those first 100 million years, whether from asteroid samples, clusters of primordial asteroid families, or craters on the Moon and Mars. “Now, the question is, was it a few million years after or 80 million years?” Morbidelli says. “Honestly we don’t know.”