Star hoppers. During a close stellar encounter, some Kuiper belt objects from one solar system (purple) may end up in another system (blue), and vice versa.

Aliens Invade Kuiper Belt

Like fleas jumping from dog to dog, frozen miniplanets may hop from one star to another, astronomers have proposed. Computer simulations show that some of the icy objects in the outermost reaches of our solar system may actually be alien invaders that originally orbited another star. The exchange would have occurred when the other star had a close encounter with our newborn sun.

Beyond the orbit of Neptune, tens of thousands of icy bodies, with diameters between 100 and 1500 kilometers, constitute the so-called Kuiper belt. These Kuiper belt objects are thought to be leftovers from the formation of our solar system. However, the remote and extremely elongated orbits of some of these objects are hard to explain. Take Sedna, for example. The mysterious miniplanet, whose discovery was announced earlier this year (ScienceNOW, 15 March ), takes 11,000 years to complete one orbit and never gets closer to the sun than 10.5 billion kilometers--almost twice the average distance of Pluto.

Now, Scott Kenyon of the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, and Benjamin Bromley of the University of Utah in Salt Lake City suggest that Sedna-like objects may have crossed over from another star to our sun during a close encounter. This likely took place when both stars were only a few tens of millions of years old and still resided in a densely populated cluster, the pair reports in the 2 December issue of Nature.

Alessandro Morbidelli of the Côte d'Azur Observatory in France and Harold Levison of the Southwest Research Institute in Boulder, Colorado, arrive at similar conclusions in a paper in the November issue of The Astronomical Journal. "We developed the same idea independently," says Morbidelli. However, their simulations imply more than one encounter that happened much earlier than suggested by Kenyon and Bromley.

The case for interlopers is not yet clinched. An alternative and more likely scenario, according to Kenyon and Bromley's computer simulations, is that a passing star's gravity stirred up the Kuiper belt and flung some objects into large, eccentric orbits. To rule this out, Kenyon and Bromley say, astronomers must detect distant Kuiper belt objects with very steep orbits relative to the other planets. Morbidelli adds that finding large numbers of Sedna-like objects, even with low inclinations, would also indicate that they originally belonged to another star.

Related sites
Background on the work by Kenyon and Bromley, with computer simulations of the stellar encounters
Paper by Morbidelli and Levison (PDF file)
More on Sedna and its discovery