Based on answers and comments on some previous questions (especially this one), and as very clearly illustrated by the scatter-plot here, there are some very definite limitations on our ability to detect exo-planets that are below certain sizes or beyond certain distances from their stars.

From what I've been able to determine, 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 we have reasonably good data regarding how many of those kinds of planets are generally present in a system, but we can't 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.

My specific question: Are there reasonable scientific research data, models, estimates, theories, evidence, etc., that describe the likelihood of these "undetectable" (or very hard to detect) planets being present in a planetary system?

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.

Is our system considered to be a "normal" planetary system, by the standards of current known science? What does science predict that we 'would' find, if we 'could' detect all planets around all stars? What does science predict to be the 'average' number of each type of planet in a random planetary system? Are gas giants really more abundant? Or does it only seem that way cause they're easier to see? Is our 4 rocks and 4 giants, a 50-50 split, normal? Or do we expect to find double the rocks for each giant in most systems (or vice versa)? Is 8 planets on the high end of the predicted spectrum, the low end, somewhere in between?

There are similar questions here on SE, but I haven't found any that provide the information I need:

Creating a realistic world(s) map - planetary systems

I've determined how many planets my solar system could plausibly have. How do I figure out what kind of planets they are?

How many planets should I have in my planetary system?

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    $\begingroup$ There is no way to know this. I don't see how this question can be answered with current knowledge. With no way to answer, the only solution for a Worldbuilder is to make it up. Hence, the 'fiction' in science fiction. $\endgroup$
    – kingledion
    Nov 12, 2018 at 22:16
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    $\begingroup$ This makes it hard for me to determine a realistic planetary system for my world to occupy, since I can't find "realistic" data to base it on. Unless you intend astronomers to be your only reader base, your average reader won't know, either. You'd be surprised how much we humans suspend our disbelief - especially when we don't know enough to know otherwise. Thus, I agree completely with @kingledion, fiction is your friend (unless you're writing a documentary/biography, but even that hasn't stopped authors from using fiction). $\endgroup$
    – JBH
    Nov 13, 2018 at 0:58
  • $\begingroup$ Protip: whether you're writing a story or designing a game or just making worlds for your own amusement, it really helps to learn how not to lean to heavily on science and on the extremely limited scope it can provide us for understanding the universe. $\endgroup$
    – elemtilas
    Nov 13, 2018 at 1:55
  • $\begingroup$ @kingledion Yes, I understand that there is no way to "know". That's the reason for, my emphasis on things like "estimates". I'd simply like to have a scientifically based starting point to work with when I do 'make it up', and I haven't been able to locate one. $\endgroup$
    – Harthag
    Nov 14, 2018 at 14:09
  • $\begingroup$ @JBH No, I don't underestimate it at all. Quite the contrary, I intend to use it to its fullest extent allowed by my story. But that doesn't replace the desire to know just how far I'm stretching things, and why. $\endgroup$
    – Harthag
    Nov 14, 2018 at 14:11

1 Answer 1


Normal is not yet defined

There are scientists who claim that various system compositions are "normal" but I haven't accepted it nor do I think the field has 'accepted it'. The simple fact is the technology is just too new and too in descript for a concept of "normal" to really emerge.

There is a pattern for large gas giants to exist and there are theories that stipulate that Saturn like planets are critical in supporting Jupiter like planets forming and surviving the early system. (there are however 'normals' with respect to data sets)

There are theories that suggest that the above process is critical for goldilocks planets to exist because in many systems Jovian sized planets exist in the goldilocks zone.

Also last I checked they haven't been able to verify if a planet is actually habitable. They can assume its habitability based on size and orbit but they cant actually see the planet for confirmation.

  • $\begingroup$ The James Webb Space Telescope will be capable of the spectroscopy of exoplanet atmospheres and being able to image planets close to their suns that could show seasonal variations, say like how Pluto was imaged from Earth. $\endgroup$ Nov 12, 2018 at 21:20
  • $\begingroup$ Thank you for the answer, but I was actually hoping not for a definition of normal, but rather for the details of the claims of the scientists you mentioned, I have been trying to find that type of data, and haven't been successful. I hope to use that type of data to try to work out for myself what I want to use as "normal" for my world building scenario. $\endgroup$
    – Harthag
    Nov 14, 2018 at 14:24
  • $\begingroup$ You aren't going to get that information here Dalila its too hard to get. At the moment it is so decentralized and buried in publications that it would take weeks of dedicated digging effort to compile an impartial model to your requested level of detail. $\endgroup$
    – anon
    Nov 14, 2018 at 14:34

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