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 ninemajor planets – usurping our own solar system from the top of the pile of planetary richness.