I do not think Melancholia ever drifted away from the Earth, it was a deception Kiefer's character used to calm down his wife, the planet was getting closer and closer. But you could be right, Justine might have been depressed even before, but she also definitely could tell things beyond what normal people can. Like the other redditor mentioned, she said "I know things" and she knew how many beans were in a jar even before they were counted. So, when I think about it now, she might as well known about Melancholia at that point.

In either way, I think the movie is beautiful, Justine falls apart and finally finds her center, while her sister, the obvious "normal" person, falls apart and cannot face anything. Amazing juxtaposition of characters, probably the best I've seen in a long time.