Comets are usually thought of as icy, dusty emissaries from the deepest reaches of the solar system. But according to a new simulation, many of them could have originated somewhere even more exotic—in budding planetary systems around other stars.



Like most stars, the sun may very well have been created in a tightly nestled birth cluster, a stellar nursery with tens, hundreds or possibly even thousands of stars. During millions of years of intimate infancy, the newborn stars could have exchanged vast numbers of comets from the fringes of their disks, each of them winding up with an ensemble of hand-me-downs from their stellar siblings.



According to a study published online June 10 in Science, the capture mechanism could account for a substantial share of the comets in the Oort Cloud, a spherical flock of hundreds of billions of comets, and perhaps more, at the solar system's outer fringes. The new model could resolve the confoundingly large population of the Oort Cloud, says Harold Levison, a planetary scientist at the Southwest Research Institute in Boulder, Colo.



Prevailing theories of solar system formation, Levison explains, hold that early in the system's history there were plenty of icy objects left over from planetary formation. But interactions with the newly formed giant planets ejected many of those comets into interstellar space, flung others out into what would become the Oort Cloud, and knocked some into elongated, somewhat shorter orbits in what is known as the scattered disk. Scattered disk objects occupy elliptical orbits and may venture as close to the sun as Neptune's orbit before looping back out to the deeper reaches of the solar system. Models of planetesimal dispersion predict that roughly 10 times as many objects wound up in the Oort Cloud as in the scattered disk, according to the new study.



But observations do not match theory: By one estimate, the Oort Cloud contains some 700 times as many comets as the scattered disk. So Levison's group set out to test a theory floating around since at least 1990—that star-mingling billions of years ago could provide an additional cometary reservoir to populate the Oort Cloud.



In the new simulation the researchers modeled interactions among 30 to 300 stars packed in a birth cluster just a few light-years across. They followed the evolution of the clusters until the stars dispersed, which took anywhere from 10 million to 52 million years. Two mechanisms for comet exchange emerged: In the more common scenario gravitational effects among neighboring stars stripped off comets that then roamed free through the cluster before hitching a ride with another star when the cluster dispersed. The other, less common, mechanism showed that comets could migrate directly and in large numbers from one star to another during a close approach.



A rough estimation, assuming that our solar system is fairly typical, finds that one third to two thirds of the Oort Cloud comets originated in the grip of another star. A more dramatic assessment, which the researchers assert is more realistic, comes from a simple comparison of observation and theory. The capture process, Levison says, "is the only mechanism that we can think of that could contribute to the Oort Cloud" beyond the standard prediction, which appears to provide a woefully insufficient number of comets. If the capture of comets from other stars indeed provided the rest, that would mean that more than 90 percent of the Oort Cloud sprang from extrasolar beginnings.



But estimates for the cloud's size are indirect, based on observations of the few comets that venture inward to the planetary region; the cloud itself is too distant to be directly observed. And population estimates for the scattered disk also vary. The contents of both reservoirs "are uncertain by an order of magnitude," says Paul Weissman, a senior research scientist at the NASA Jet Propulsion Laboratory in Pasadena, Calif. "So I'm not entirely certain that they have a problem that needs solving."



Faced with these uncertainties, Levison and his group did choose somewhat conservatively from the available population estimates, picking a relatively small figure for the size of the Oort Cloud and a large one for the scattered disk. "I think the numbers are pretty solid," Levison says.



Weissman takes issue, however, with the larger estimate of the contribution of extrasolar comets to the Oort Cloud. "The claim that over 90 percent of the cloud is captured is a substantial overstatement," Weissman says. "What they have shown is that the number can be about one third to two thirds." Even that, he notes, is "a very important result."



Regardless of whether the sun nabbed one third, two thirds or nearly all of its comets from its birth-cluster companions, why should it have made out so well? In the standard model of solar system formation, Levison notes, the bulk of the comets originally available in the sun's disk wound up not in the Oort Cloud but flung clear into interstellar space. The capture mechanism provides a way that some of those losses could have been offset. "It's not that our sun got more at the expense of somebody else," Levison says. "Everybody gets more."