@futurebird The Moon, are we technically a binary planet / dwarf planet system; would intelligent life even be possible without the moon's influence; why is the moon exactly the right size for a full solar eclipse; is the moon really an artificial construction (probably not but you can't completely rule it out).
@futurebird runner up is Planet X, gravitational influences indicate it should be there, lurking in the Kuiper Belt. They been doing serious science to try and find it pretty much my entire life, it's never where they look for it. One semi-fringe but plausible explanation is it's a primordial black hole.
@lienrag@futurebird I remember a collection of dragon based Sci Fi short stories I read as a preteen and my favorite was this giant momma dragon shows up, looks for an egg where Vulcan should be, then turns to Mercury which hatches into a baby dragon. I think the story ended there, but the implications were clear. I've never been able to figure out the name of the collection or story though.