I don’t follow. If my “ancient aliens” theory says that Stonehenge should occur on 50% of planets, would you automatically favor it over any theory that sets steeper odds?
If I had confidence that you'd formed that theory without prior knowledge of the existence of Stonehenge, then the fact that you'd predicted Stonehenge would indeed be making your theory look pretty good!
In fact that is an essential part to how theories are evaluated. Not just by what they explain, but by what they predict.
Most theories on physics are evaluated this way. "We theorize that light is a particle, which would explain these properties of light, and means we should expect to see these other properties of light if we test them." If further observation of light matches those properties you predicted then that's pretty good evidence that your theory is on the right track.
So the UFO people are right? They predicted this sort of thing all along, without any prior knowledge of Oumuamua. I don’t recall hearing about fractal comets before?
We're going to need to think carefully about what is covered by "this sort of thing".
If your ancient aliens theory didn't actually predict "Stonehenge", but predicted that the aliens would have left "all sorts of cool weird stuff", and then you point at Stonehenge and say "see, that's the sort of cool stuff I mean", then I'm no longer impressed by the predictive power of your ancient aliens theory.
Predictions have to be precise for a theory to be supported by them: they can't be vague generalities. cf. astrologers, who "predict" the future by predicting nothing.
Well, "my" theory is also pretty wrong if it doesn't allow for the human civilisation that built Stonehenge.
A theory that is wrong will sometimes predict correct things by accident. That's unavoidable, but we a) have statistical methods for modelling how often things go right by chance, and b) have a philosophy of science in which theories are forever tentatively held: your aliens theory might beat any theory which says Stonehenge is impossible, but will soon be replaced by a theory which explains Stonehenge more economically.
