I think an important point not mentioned is the matter that constitutes stars and planets needs time to coalesce.
The first few billion years stars were made of hydrogen turning into helium. Hardly no heavier elements existed. Heavier elements like carbon, oxygen, silicon, iron, et al are made when an exceptionally dense star goes super nova, turning their hydrogen-to-helium generators into producers of even denser elements like carbon and oxygen. You know, the stuff life as we know it is made out of. And even if you postulate life based on other elements rather than carbon, say silicon, it still takes several births, dramatic deaths and rebirths of stars to make enough.
So planet X, 8 billion years old, may not have existed. Where would all the silicon, carbon, iron and oxygen come from, to make up such a body?
Perhaps rocky worlds took a lot longer to evolve themselves. Up until maybe 2/3rds of the life of the universe, planets were mostly gas giants because only hydrogen and helium was abundant enough elements to coalesce around a new star.
Which makes the Earth fairly new in the abundant-enough-elements-to-make-a-rocky-world planet. And then it took a few billion years to make life.
And here we are. We got here as soon as we could, which, to paraphrase Gandalf, is when we arrived precisely when we meant to.
I've been searching high and low for an article I read a few weeks ago that said astronomers are coming around to the realization that there were a lot more heavy elements in the early universe than they originally believed. That's because supernovae were much more common then (same amount of matter more densely distributed in a smaller space, creating lots of monster stars) and those supernovae were spewing out heavy elements like crazy. So rocky planets were common. No luck finding it, though.