How Earth got its Moon

I think part of the reason for the theory is due to the very low density of the Moon. Although the moon is much larger than Pluto, it has half the mass. The theory explains that a large object collided with Earth ejecting enough matter into orbit to form the moon. The matter ejected would have naturally been lighter elements and thus the moon isn't very dense.

It's a good theory, and its probably true, but I'm not willing to commit to the theory yet. It seems to create a ton of questions, some of which Jorrie is asking here.

It amazes me that this is hailed a a recent theory.In 1958, while still in grade school, my science teacher taught us that that was how the moon formed.(Except he said a "Large Asteroid" instead of a 'Mars-sized planet'") It seems to have been forgotten about for nearly 50 years, and is now appearing as a new and profound theory.Wish I still had my science text book from the fifth grade.......

I didn't realize it was hailed as a recent theory.

I just have a hard time visualizing the event and the moon turning out all nice and round and in such a nice orbit. There does seem to be a lot of evidence to support it.

I too, have a hard time visualizing the event. For the theory to hold up, it seems that BOTH the earth and its moon would have to have been blown to smithereens and re-coalesced. Otherwise, there would be a huge crater on earth the size of the moon.

So, if the earth blew up into two clouds (split in a ratio of 1:6 by the trajectory of this missing planet) then I'd expect the densities and constiuents of both earth and the moon to be more similar than they are.

I seems, to me, more reasonable to suspect that the two bodies formed independently and at roughly the same time. Proximity in the original cloud would dictate a high probablity that the bodies would share constituents, but that they could also be disimilar in some of the ways in which all the planets are disimilar.

The Theory, IIRC, (it was a long time ago), involved
a grazing collision, with surface matter ejected,
some of it recaptured, some lost, some forming
a ring, and eventually, the moon.
There is evidence of another high energy event, (the one that
tipped neptune on its side), which argues for another planetary
collision.
Such events were probably common in the early solar system
and possibly we find evidence only of the last ones.

It's not a recent theory - it's one of many fairly old theories. It's just that this is new evidence in support of the collision scenario.