Estimates for "undetectable" planets in extra-solar systems
As far as I've been able to determine, it appears there are some significant limitations on our ability to detect exo-planets that are below certain sizes or beyond certain distances from their stars.
It seems that Uranus and Neptune are about as small as we can detect unless the planet is relatively close to its star, and Earth is near the lower size limit we can detect, regardless of proximity to the star.
In other words, our bias toward detecting large planets, and planets close to their stars, means I have been able to find reasonably good data regarding how many of those kinds of planets are generally present in a system. But since we can't seem to tell (by direct observation) how many small planets are generally present in a system, or how many planets are present that are just too far from their star for us to detect, I'm struggling to find data regarding how many plants of those types are common in planetary systems.
For example: our system has 8 planets, 4 rocky, 2 gas giants, 2 ice giants. But chances are that if we were observing (with current technology) our own Solar system from some other solar system, we'd probably only see 2 to 6 of them. We'd almost certainly see Jupiter and Saturn, and we'd almost certainly NOT see Mercury or Mars (they're just too small), Venus and Earth "might be" close enough to the sun for us to see them even though they are on the small end of what we can detect, and Uranus and Neptune also "might be" detectable, even though they are small-ish for their distance from the sun.
My specific question: How many planets are in a 'normal' or 'average' planetary system, according to humanity's best scientific theories available so far?(beyond what we can simply detect.) In other words, where can I find reasonable scientific research data sets, models, estimates, theories, evidence, etc., that describe the likelihood of "undetectable" (or very hard to detect) planets being present in a planetary system?
This is my first question on Astronomy stackexchange, so please be gentle, but don't hesitate to provide constructive criticism if I'm doing anything wrong. 710