Retrograde planets

The field of exoplanets is moving so fast it can be hard to keep up. A truly surprising result in the last few days is the apparent misalignment of a significant fraction of planetary orbits with the spin-axis of their parent stars and the observation that 20% of a particular sample of planets are actually orbiting in the opposite sense to their star's spin - in retrograde motion.

The general consensus (actually harking back to Laplace and Kant in the late 1700's) is that planets and stars form together as dense regions of nebula or molecular clouds are contracted by self-gravity. Since angular momentum (spin) is a generally conserved quantity these clouds form rotating disks of gas and dust - with the star forming at the higher density center and planets forming in stages from the disk material. In this picture planets should always orbit in the same sense as the star spins. So what's going on out in the real universe ?

The bottom line is that we don't yet know. A pretty good candidate explanation is that planetary systems can get rearranged over time. Planets within a system exert gravitational pull on each other, and this can result in a variety of behavior - from gradual orbital change to chaotic behavior. Stars are also more often than not part of binary or multiple systems - in orbit about a common center of mass. These distant heavyweights can also, over hundreds of millions of years, cause planetary orbits to vary in ellipticity and in inclination - sometimes causing a planet to pass close enough to the parent star that tidal friction eventually circularizes it. Although snared by the star, the planetary orbital inclination may still reflect that earlier jostling.  The end result is that some fraction of planets look to be askew - possibly even retrograde.

It raises all sorts of interesting questions. I'm still scratching my head over the retrograde planets - in these systems then tidal forces will, eventually, pull the planets into their stars. So either these are young systems, where the planets have not yet suffered orbital decay, or they're still being tugged at by unseen companions. Or the physical model for putting them there is wrong in some way.  I dare say that there will be a flurry of new papers to address this. If the mechanism for setting these planets into their current places is as proposed then it also means that those systems probably couldn't harbor terrestrial type worlds - at least not in their inner regions. It's fabulous stuff, nature is always surprising us.