Re: planets observed in other galaxies
While you’re right that there ars billions of other galaxies and rach of those have billions of stars, I don’t think we are capable of resolving images, visible or radio frequencies, of stars in other galaxies sufficient to discern the tell-tale wobble of even a binary star much less the wobble of a planet. Our extra-solar planetary discoveries are pretty much limited to the local area in our own galaxy and that is a pretty small volume... as astronomic volumes go. We actually are working with a very small dataset.
I agree with the article’s premise that our moon is a rarity. But I think the postulated 5-10% is way too high. That percentage would almost guarantee that every star system with a planetary population of ten or more planets would have a similar dual planet created from a similar hypothetical ancient collision. I find that hard to believe.
Such collisions would require large numbers of planetary bodies having formed by accretion with wildly differing orbital shapes... Something that the stellar accretion ring to planet theory does not support. Any planet formed from such an accretion ring around the same star would orbit in essentially the same plane and in the same direction in orbits mimicking the circular orbits of their accretion ring.
The forces necessary to change the orbit of any planet accreted in the same stellar system from its original near circular orbit to a potential collision prone parabolic orbit are, well, astronomic. Gravity alone cannot account for it... Tidal forces will have already been automatically adjusted during the billions of years of accretion (otherwise the planets could not have formed).
The odds that numerous planet/moon combinations like ours were caused by collisions with cosmic interlopers, planets from outside the stellar system, are also beyond astronomic. First we would have to postulate a free roaming planet that had somehow escaped its own birth star. Then we would have to have sufficient time for this hypothetical impacting planetoid to cross interstellar distances while only moving at non-relatavistic speeds. And then it would have to either pass by the target planet at just the right velocity to be captured or be vectored just exactly right to impact another planet of similar size (too large and no material is blasted off to form a moon; too small and the interloper is not even captured by the star). Space is huge... A miss is far more likely than a hit.
Neither of these scenarios is likely to have occurred often enough for there to be that high of a percentage... I think one in a million would be too high.
Or perhaps planets are not formed as we think and some other mechanism than gravity is at work in their formation.
What was in #24? :’D