A new theoretical study by CfA astronomer Hagai Perets and his colleague proposes a possible answer: the distant planets are not part of the original stellar system - they were captured by the star. Astronomers know that there are many so-called "free floating planets" in space - planets that have been tossed out of their original solar system by a random gravitational encounter with another planet. Some of these orphan planets have recently been detected.
The scientists calculated that it would be possible for a star to capture one of these orphans if the conditions were right; namely, if the star and planet happen to pass close to each other with only a small velocity difference, and if there are no other massive bodies nearby to interfere with the "adoption." They ran a series of computer simulations to test all these and other possibilities, and they found not only that such a capture was possible, but that a star could even capture several orphan planets.
In fact, they found that sometimes two free floating planets could capture one another and form a binary planet. None of these binaries has yet been seen, although some astronomers think that since Pluto and its moon Charon have such similar masses they are a binary system, although not necessarily one that was captured.
The new results seem to offer a reasonable, if exotic, explanation for some of the complex planetary configurations that have been discovered, and they remind us that nature is full of surprises.