I previously mentioned that some exoplanets do not orbit their stars in the same direction as the star itself is rotating. The number of these ‘wrong-way planets’ has now increased significantly. About a quarter of all extrasolar 'hot Jupiters' (Jupiter-sized planets that orbit very close to their stars) orbit in the opposite direction of their star’s spin. Smadar Naoz and her colleagues from Northwestern University may have found out why.
Hot Jupiters are thought to form far from their stars (about where our Jupiter is within the solar system) and then migrate inward due to forces from other planets. The Northwestern researchers modeled a system in which a Jovian-sized planet is joined with another planet even farther out. Over long periods of time, the Jovian planet is nudged closer to its star. In some cases, the tidal effects of the second planet can cause the Jupiter-like planet to first achieve a highly elongated eccentric orbit, and then to flip and start rotating in the opposite direction.
A retrograde hot Jupiter: the transiting giant planet orbits very close to the star and in a direction opposite to the stellar rotation. This peculiar configuration results from gravitational perturbations by another much more distant planet (upper left).
Credit: Lynette Cook
Prior to the discovery of hot Jupiters and retrograde orbits, we thought our solar system was the standard model for star system formation and appearance. Now we know that there’s a lot more variety out there than we thought. Which is all to the good, in my opinion.