: In the books, the Prophets knew that Earth was the human homeworld, but they
: didn't know where it was. The presence of Humanity on Earth was a shock to
: them, so they clearly didn't think that our homeworld and their target
: were the same thing. This was made pretty clear at the end of Outskirts...

: It's being spoonfed a little to us now, but the map that led them here is
: something that removes some of the general vagueness surrounding events in
: the plot... Hope there's a terminal that bridges chief's actions between 2
: and 3...

In First Strike, we knew they intended to go to the Sol system (but that they didn't know of Earth as humanity's homeworld), only that a planet in that system contained a significant Forerunner artifact.