Astronomers have long thought the eight planets orbit in nearly perfect circles because they once formed within the swirling disk of dust and gas that surrounded the young sun. But in 2003 scientists discovered something strange: a dwarf planet known as Sedna whose elongated orbit takes it from twice Pluto’s distance to more than 20 times its distance from the sun. And it is not alone. In the years since astronomers have uncovered nearly two dozen distant icy objects whose orbits are oblong and strangely tilted compared to the plane of the solar system. To explain such oddities, scientists speculated that maybe these worlds are scars from a violent past, a sign something—perhaps a passing star—knocked them off course in our solar system’s infancy. Or maybe there is a distant ninth planet whose gravity sculpts their peculiar orbits.
The latter hypothesis has gained traction over the past several years, leaving the first in the dust, says Susanne Pfalzner, an astronomer at the Max Planck Institute for Radio Astronomy in Germany. Anomalies in the orbits of some small outer solar system objects have amassed evidence for a “Planet Nine” roughly 10 times Earth’s mass. Meanwhile a stellar interloper has been considered too unlikely—until now. Pfalzner and her colleagues recently published a paper to the preprint server arXiv that has been accepted by The Astrophysical Journal showing stars might buzz our solar system far more often than previously thought. Not only do the results lend credibility to a stellar flyby but they just might also explain how the elusive Planet Nine would have landed in its odd orbit in the first place.
Astronomers know the sun has not always been so solitary. It was born within a cluster of hundreds to perhaps tens of thousands of stars that dispersed only 10 million years later. So while the sun was still entombed within that cluster, stars would have rocked to and fro in a dizzying dance that easily could have brought one waltzing into our nascent solar system. But after the cluster broke apart the likelihood of such an encounter dropped nearly to zero, or so the thinking went. But Pfalzner and her colleagues now argue the odds of an encounter remained quite high after the cluster had started to disperse. After many long computer simulations they found there is a 20 to 30 percent chance a star perhaps as massive as the sun would swing nearly as close as Pluto at 50 to 150 astronomical units. (One AU is the mean distance from Earth to the sun, or 93 million miles.) And there is no doubt such a close approach would surely shake our young solar system.
Although the large planets would remain unbothered (much like the sun is only slightly jostled by the minor gravities of the eight planets), the encounter would perturb the solar system’s smaller objects—tossing them around and placing them in odd orbits in the distant reaches of the solar system. What is more: the simulations also re-created a second trend astronomers have observed in the solar system, that outer objects tend to cluster together in space. They travel together in tight-knit groups that all cross the plane of the solar system at roughly the same spot before swinging outward to the same distant point. In short, simulations including a stellar interloper can perfectly re-create the observations to date. “But whether they’ll last for 4.5 billion years” or over the solar system’s entire life span, “is the million-dollar question,” says Scott Kenyon, an astronomer at Harvard–Smithsonian Center for Astrophysics who was not involved in the research. And Pfalzner agrees. She would like to model the long-term behavior next to see whether those changes will hold over the solar system’s entire lifetime. It could be that a flyby clusters objects for a cosmic moment before they randomize again. If that is the case, then a planet is the best explanation for the observations.
Scientists are eagerly tracking down more data with a number of different observing campaigns. A handful of teams, for example, are already scouring large chunks of the heavens in search of more oddities in the outer solar system. Scott Sheppard, an astronomer at the Carnegie Institution for Science who was not involved in the study, cannot contain his excitement over the upcoming Large Synoptic Survey Telescope—an 8.4-meter-wide scope that will likely uncover hundreds of new solar system rocks. “That’s really going to open up the floodgates for trying to discover these distant objects,” he says.
Meanwhile Kenyon is hopeful the Gaia spacecraft, which is in the process of charting one billion stars to unprecedented accuracy, will help find our sun’s long-lost siblings. That will allow scientists to better understand the stellar cluster in which our young solar system formed, along with the likelihood another star zoomed too close. “Gaia is the new savior on the block,” he says. A recent Gaia study even traced the paths of nearby stars into the past and projected those paths into the future, only to find that 25 stars speed dangerously close to home over a 10-million-year time period. That tally is seven times as much nearby stellar traffic as previously thought. Then, of course, there are a number of surveys searching for the elusive Planet Nine itself.
But Pfalzner argues the discovery of another major member of the solar system will not rule out a stellar flyby. “It’s not an either–or scenario,” she says. “If Planet Nine exists, this would not be in any way a contradiction to the flyby model, but possibly even a point in favor for it.” Her team argues Planet Nine’s predicted orbit, which is also both eccentric (stretched out) and inclined (tilted from the solar system’s plane), was likely shaped by the stellar interloper itself. So she and others will continue to hunt for both Planet Nine and further oddities.
And although astronomers might disagree over the specifics of our solar system’s origin story, they are all certain the treasure trove of objects already discovered in the outer solar system is only the beginning. Sedna was the tip of the iceberg, Sheppard says. “There’s just so much sky we haven’t covered to date that it’s more likely than not there’s something pretty big out there.”