Astronomers Scott Kenyon of the Smithsonian Astrophysical Observatory and Ben Bromley of the University of Utah theorized in their 2004 Nature article that some solar system objects with unusual, highly inclined orbits might have been captured when another star system passed by the Sun early in its history — a stellar encounter. The recent discovery of large Kuiper Belt object “Buffy,” designated 2004 XR 190 by the International Astronomical Union, appears to confirm the astronomers’ “capture” theory.
Kenyon tells Astronomy, “We think of most of the objects in our solar system as being formed here. It’s not clear that it [Buffy] is captured, but there is the possibility that it was captured from another star.” He believes these objects “tell us about the history of the solar system better than the Sun or the Moon” because not much has happened to them since their capture. If Buffy is a captured planet from another star, then the search for extrasolar planets just got a whole lot closer to home.