Earlier start for a solar system shuffle

The solar system was once a more tumultuous place than it is now. According to dynamical simulations, the outer planets drastically shifted their positions, with Jupiter migrating inward and Saturn, Uranus, and Neptune scooting outward. Closer to home, in an episode known as the late heavy bombardment, Earth’s moon and the inner planets were pelted with rocky and icy bodies 400 million to 700 million years after the solar system’s formation, as evidenced by the age of rocks and craters on the Moon, Mercury, and Mars (see the article by Brett Denevi, Physics Today, June 2017, page 38 ). In multiple papers published over the past dozen years, researchers have hypothesized that those events were linked: Perhaps the shuffling of the outer planets caused material to be flung at the inner planets. But in a new study, researchers strongly reject that hypothesis and instead conclude that the planetary migration occurred far too early in the solar system’s history to trigger the bombardment.

David Nesvorný of the Southwest Research Institute and colleagues performed a set of dynamical simulations that focused not on the solar system’s largest objects but rather on two minor planets: a gravitationally bound pair of Jupiter’s trojans, Patroclus and Menoetius (illustrated here). The trojans, which share the giant planet’s orbit, are thought to have accreted in a dense outer region of the Sun’s protoplanetary disk before getting dislodged by the migrating Neptune and then captured by Jupiter. Patroclus and Menoetius made a particularly good test case because of the fragility of their mutual gravitational attraction. Through the simulations, Nesvorný and colleagues explored how long the pair could have endured in their birthplace before getting set loose by Neptune toward Jupiter. They found that given the frequency of collisions in the disk of planetesimals, the binary’s odds of remaining intact plummeted quickly. To have a 10% chance of survival, the trojans would have had to get jostled by Neptune and captured by Jupiter no more than 100 million years after the solar system’s formation, long before the era of the late heavy bombardment.

If the simulations prove valid, astronomers will need to find an alternative cause of the late heavy bombardment. What’s more, the planetary shuffle still likely resulted in Earth’s getting pummeled. That first barrage would have predated the late heavy bombardment by at least 300 million years. The researchers may be able to test their model with evidence straight from the source: NASA’s Lucy mission is slated to fly past Patroclus and Menoetius in 2033. (D. Nesvorný et al., Nat. Astron., in press, doi:10.1038/s41550-018-0564-3 .)