Via Sky & Telescope, news that planetary systems Out There are even more of a chaotic mess than we thought before. Several newly discovered transiting extrasolar planets turn out to be orbiting retrograde with respect to the rotation of their parent stars.
This, to put it mildly, was not expected. Planets discovered by transiting, passing in front of their stars, tend to be 'hot Jupiters,' orbiting rather close in, since such tight orbits are more likely to have the needed edge-on positioning.
If, as generally assumed, planets form from a protoplanetary disk of gas and dusk, they should all be born prograde, and the subsequent encounters that might leave a planet in a retrograde orbit would be expected to produce large orbits, not small tight ones.
'The new results really challenge the conventional wisdom that planets should always orbit in the same direction as their stars spin,' said Andrew Cameron, who reported the findings to the Royal Astronomical Society. (How cool is that?)
An earlier Sky & Telescope article, which I missed at the time, gave hints of these findings, but also made a rather curious statement, from MIT exoplanet specialist Joshua Winn: 'Now we've gotten a glimpse of the weird, wild systems we've been hoping to find all along. Theorists will have a lot of fun trying to get the planets in these orbits.'
The observational study of exoplanets is only 17 years old, but this is a discipline that has already forgotten its origins. Its entire history has been one big joke on the theorists, with punch lines that continue to unroll. When haven't observed extrasolar planetary systems been weird and wild?
It's somewhat forgotten now, but the first discovered extrasolar planets do not orbit a sunlike star but a pulsar, a post-supernova remnant that was about the last place anyone expected to find planets. The first planet of a sunlike star, 51 Pegasi b, discovered in 1995, is a hot Jupiter, something so unexpected at the time that there was an argument over whether it was a 'planet' at all, or belonged to some other class of substellar object. (No one knew how soon planetary status even in the Solar System would become a matter of controversy.)
From a science fiction perspective, if anyone before 1995 had set a story in a planetary system typical of those we've discovered since (currently about 48 multiplanet systems are known), it would have been dismissed as implausible. I would have dismissed it as implausible. Jupiter sized planets in sub-Mercury orbits? Get real.
Big oops. At least for now it is our own system whose orderly grand-plan structure seems to be the exception. This itself may be a mere selection effect - if we were observing a twin of the Solar System, we would only just be able to detect Jupiter, and not yet any of the others. The extrasolar systems we now observe may turn out to be in the minority, the ones that happen to be easiest to find.
The obvious lesson here is that the universe is full of surprises. The only slightly less obvious lesson is that it is hard to project from a sample size of one. We will be surprised again, and repeatedly. Meanwhile, though, we have a lot more freedom of invention when it comes to planetary systems than we thought not so long ago.
The image, via Astronomy Picture of the Day, is of a crescent Neptune with Triton. (View the image carefully!) Chaos creeps even into our own orderly system; Triton is the only large moon in a retrograde orbit, and is probably a former Kuiper Belt object, captured during a Solar System past that is now thought to have been fairly turbulent itself.
Related posts: Extrasolar planets don't just orbit backwards. They collide, and take death plunges into their parent stars.
I should add (and just did, on edit) to treat the comments as a handy open thread for any pertinent - or even impertinent - observations.