The Earth's moon formed in a similar way when Earth crashed into an object the size of Mars.
Do we know this as fact?
What object was it?
It's considered pretty certain based on analysis of rock and dust samples brought back from the Apollo missions.
We are apt to imagine that we could discover these effects by the mere operation of our reason, without experience. We fancy, that were we brought on a sudden into this world, we could at first have inferred that one billiard ball would communicate motion to another upon impulse, and that we needed not to have waited for the event, in order to pronounce with certainty concerning it. Such is the influence of custom, that, where it is strongest, it not only covers our natural ignorance but even conceals itself, and seems not to take place, merely because it is found in the highest degree. But to convince us that all the laws of nature, and all the operations of bodies without exception, are known only by experience, the following reflections may, perhaps, suffice. Were any object presented to us, and were we required to pronounce concerning the effect, which will result from it, without consulting past observation, after what manner, I beseech you, must the mind proceed in this operation? It must invent or imagine some event, which it ascribes to the object as its effect, and it is plain that this invention must be entirely arbitrary. The mind can never possibly find the effect in the supposed cause, by the most accurate scrutiny and examination. For the effect is totally different from the cause, and consequently can never be discovered in it. Motion in the second billiard ball is a quite distinct event from the motion in the first. nor is there anything in the one to suggest the smallest hint of the other. --David Hume
regarding supposed impact origin of the Moon:
http://www.freerepublic.com/focus/f-chat/1234919/posts?page=10#10
there is a huge ring of debri between earth and mars- they think that may have once been a planet
Today we call the resulting post crash debris by the unimaginative names, "Earth" and "Moon".
Facts are elusive and science does often produce absolutes, but the crash hypothesis explains why the Earth's spin axis is titled so profoundly with respect to its orbital plane and that huge gash west of California called the Pacific Ocean, and why the Earth-Moon system is the closest thing to a double planet in the solar system, inter alia.