A belated welcome to the New Year and a new decade* - it's got to be an improvement on the last one.
Via Winch of Atomic Rockets, a first attempt at a statistical estimate of how common planetary systems like ours are. The answer: probably not rare, but not especially common either, with perhaps about 1 star in 7 having multiple giant planets in the outer system, the researchers' characterization of a system 'like ours.'
This estimate comes from microlensing, a technique aptly described as 'looking for planets you can't see around stars you can't see.' The method relies a flukish event, a star passing in front of a more distant star and, through gravitation lensing, allowing us to briefly register the presence of the distant star and any planets around it. The planets aren't really discovered in the traditional sense, only glimpsed, but with a galaxy full of stars, flukish revealing glimpses happen often enough to be treated statistically.
Which is pretty cool when you think about it.
But my text for the day, so to speak, is the generous interpretation of 'like ours'-ness. I remember back before 1995, when it was pretty much taken for granted that most planetary systems would be variations on a theme - about 5 to 15 major planets, most in nearly circular orbits, arranged as a few rocky inner planets, then a handful of gas giants, then some icy outer bodies.
The first extrasolar planets were found in 1993. They orbit a pulsar, a discovery so plain weird that both astronomy and science fiction mostly blinked and ignored it. We wanted real planets around real stars, not cinders left over from a supernova. Then came 51 Pegasi b, a planet that would have been been laughably unbelievable until it was discovered.
Of the 350 or so planetary systems now known, some 50 with more than one (known) planet, hardly any fit the solar template. This is not statistical evidence, because most were discovered using techniques (radial velocity and transits) that favor discovery of big planets close in to the parent star. But this is still a LOT of planetary systems that don't fit the expected mold.
The most surprising thing to me, besides the hot Jupiters, is the prevalence of eccentric orbits. Very few known extrasolar planets have orbits resembling Solar System planetary orbits. Nearly circular orbits are common enough, but mostly for close-in planets whose orbits have been tidally circularized.
One tantalizing hint, unrelated to the linked story, might suggest a pool of planetary systems more like ours. At the Extrasolar Planets Encyclopedia they have a nifty little graphing tool. Plot semi-major axis (i.e., average distance from parent star, in AU) against orbital eccentricity.
You'll see first of all that most planets discovered so far are within a few AU of the star, with a handful detected at dozens of AU. Their orbits also have a wide range in eccentricity. Now set the distance bounds at a minimum of 1 AU and maximum of 6 AU, excluding the close-in planets and a handful of remote ones. The (few) planets discovered between 4 and 6 AU, where giant planets form, tend to have less eccentric orbits than those inward of them, which got there by migrating.
Is this significant? I don't know, but these worlds are recent discoveries. More beyond!
* Cheerfully ignoring chronology pedants.
The image of the Pleiades, from Astronomy Picture of the Day, is irrelevant but gorgeous.
Related post: Last August I looked at exoplanet destruction.