Hal Levinson of the Southwest Research Institute (SWRI) in Boulder, Colorado reports that computer simulations show that the early sun could have stolen comets from neighboring newborn stars 4.5 billion years ago. Given the close proximity of young stars - as we see in the nearby Orion Nebula - "It's hard to imagine it not happening," says Levison.
Conventional wisdom has been that comets accompanied our own solar system's planetary formation and then got gravitationally booted out to huge distances, like storing junk in the attic. But this kind of mayhem was happing around neighboring star systems too. Therefore, interstellar space got cluttered with a snowstorm of comets orphaned from their birthing star.
Levinson's model estimates that there should be 400 billion comets out there that are loosely bound to the sun. But in those simulations where the comets only originate in the newborn solar system, the predicted population is a paltry 6 billion - one for nearly every person now living on Earth.