There is a difference between 'we did not detect any planets there' and 'there are no planets there'. 500 km diameter Enceladus class planets (ice moon planets on the Meghar Scale of Planetary Mass Classification) are undetectable by any known technology at this time. One thing most scientists are very careful about is unvetted and indiscriminate use of words like 'no', 'not' 'none' and 'never'.

It's quite possible that some stars may not have any planets as most people define them. After all, it's a big universe, and it's a fuzzy, ill defined problem. Once we get to the point where we can detect most 500 km objects around nearby stars, then we can maybe state something a little more specific, but right now it looks like most stars have 'stuff' around them. All you have to do is find a star which has recently formed, and then look at that stuff. What we do know is that the lower limit for planetary size is something like 500 km, less for water planets that have melted in their past. That is the resolution we are aiming for.

In lieu of that the next level on the Meghar scale is 1000 km for asteroid planets. At that level we can begin to really define the structures of extrasolar systems.