Astronomers have gained an important clue to guide their hunt for extrasolar worlds. And that clue points to the unlikeliest of places — our own backyard. “It’s possible that some of the objects in our solar system actually formed around another star,” says astronomer Scott Kenyon (Smithsonian Astrophysical Observatory). How did these adopted worlds join our solar family? They arrived through an interstellar trade that took place more than 4 billion years ago when a wayward star brushed past our solar system. According to calculations made by Kenyon and astronomer Benjamin Bromley (University of Utah) and published in the Dec. 2, 2004, Nature, the Sun’s gravity plucked asteroid-sized objects from the visiting star. At the same time, the star pulled material from the outer reaches of our solar system into its grasp. “There may not have been an equal exchange, but there was certainly an exchange,” says Bromley. Kenyon and Bromley reached this surprising conclusion while working to explain the mystery object Sedna, a world almost as large as Pluto but located much farther from the Sun. Sedna’s discovery in 2003 puzzled astronomers because of its unusual orbit – a 10,000-year-long oval whose closest approach to the Sun is well beyond the orbit of Neptune.