From Brandon Spektor at LiveScience:
Earth’s Inner Core Shouldn’t Technically Exist
In the paper, the researchers argued that the standard model of how the Earth’s core formed is missing a crucial detail about how metals crystallize: a mandatory, massive drop in temperature that would be extremely difficult to achieve at core pressures.
Weirder still, the researchers said, once you account for this missing detail, the science seems to suggest that Earth’s inner core shouldn’t exist at all. More.
We don’t know as much as many people think about the Moon either.
See also: Textbook theory of moon’s origin is challenged
Space.com: Scientists finally know how old Moon is What’s surprising, really, is how little we know about the moon in general.
Another moon origin theory: Epic crash
How the Moon Formed: 5 Wild Lunar Theories (Mike Wall at Space.com, 2014)
Our moon formed in collision with embryo planet?
Origin of the moon still shrouded in mystery