Exactly how many planets orbit any given star is still a major unknown in exoplanetary science. The two primary techniques for detecting planets and quantifying their characteristics have significant limitations that blinker us to the full contents of other solar systems. Radial velocity measurements pick up the tell-tale motion of a star around a system's common center-of-mass, or balance point, due to the gravitational pull of any planets. But the smaller the planets and the further they are from the star the weaker the signal. Multiple planets and longer orbital periods confound the situation by producing complex patterns that may also be incompletely sampled in data that spans only a few years. Transit observations, such as those undertaken by the Kepler mission, are biased towards the detection of large planets in small orbits around small stars where it is most likely for a planet to block the light from the star more frequently.
All of this means that in essentially all currently known systems we may have only incomplete information about the true number of orbiting planets. Nonetheless, stars with multiple planet detections certainly crop up. Of the over 550 confirmed exoplanetary systems there are over 90 with more than one planet (a total of more than 760 worlds). Now a new study of radial velocity data from the HARPS instrument suggests that one of these systems, HD 10180, may harbor nine major planets - usurping our own solar system from the top of the pile of planetary richness.
In a recent paper Mikko Tuomi applies a sophisticated Bayesian (probabilistic) analysis technique to measurements taken over a period from 2003 to 2009. The star HD 10180 is a close solar-analog, about 6% more massive than the Sun and of similar composition it lies some 127 light years from us. Previous investigations of this data had suggested that there could be six or perhaps 7 planets in the system, Tuomi claims the presence of two more objects, albeit at relatively low statistical significance. The masses of these worlds ranges from more than about 1.3 times the mass of the Earth to more than 65 times the mass, and the orbits place all of them within the equivalent of Jupiter's orbit in our solar system.
If further observations bear out this claim, it will make the HD 10180 system an extraordinarily busy place. Nine planets, ranging from possibly Earth-sized to Neptune-sized and larger, all crammed into this star's inner realm. Previous studies of other systems have already suggested that planets are often 'maximally packed' - if they can form they will and as close together as allowed by gravitational dynamics. In such cases, any reduction in orbital spacing between worlds or increase in their masses would make the system unstable. In the case of HD 10180, the computer simulations that will tell us if these 9 planet candidates can actually form a stable system over billions of years have yet to be run, but it looks promising.
All of which raises an even more interesting question. There is no obvious reason for these to be the only planets in the HD 10180 system. The fact that the outermost candidate appears to have a 6 year orbital period is a strong function of the limitations of the current set of measurements, and the packed nature of this system doesn't exclude even more objects on larger orbits. In other words, there is no telling how many planets actually orbit further out around HD 10180. The total mass of the 9 candidates is at least 170 Earth masses, but this is just over half the mass of Jupiter, and even if it were double this, it suggests that the original proto-planetary disk that made these planets could have had plenty of additional material to make more (assuming some similarity to our own solar system).
Just as we've seen that the galaxy is rife with planets, making our own circumstances seem a little less special, it may be that our own planetary richness is also quite mediocre.