In this tread on talk.origins, one of those useless little bits of trivia cropped up recently which caught my attention: The barycenter (or ‘center of mass’) of the Sun-Jupiter system is actually just outside the surface of the Sun. I’ve heard it said that one can consider that a planet has a moon only if the barycenter of the planet-moon system is inside the planet (as is the Earth-Moon system), but what if a planet has a barycenter outside the surface of the central star? It it still “in orbit“?
Well, in cases like this, common sense prevails, and of course, the Sun’s mass (1.981*10^30 kg) dominates the mass of Jupiter (1.899*10^27 kg), by a factor of a thousand to one. So, while technically, the Sun and Jupiter are orbiting a common barycenter outside the sun, the relative masses dictate that the planet is gravitationally bound to the Sun, and therefore ‘in orbit’ around it.
Another wikibit: if all of the planets are lined up on one side of the Sun, the common barycenter would be 500,000 kilometers above the surface.