It's about death.

The planet that destroys the earth is meant to represent death. I thought it represented depression, but then the scene when Kirsten Dunst it's naked next to a river under the light of Melancholia will make no sense if this were true.

She was laying naked that way because after she beated her horse, that for me represented conventional happiness ideas like getting married, having children, living in the country, having an steady job and such, (notice how she talks to the horse saying cheerfully "i got married"), at that point she wanted a piece of that conventional happiness for her, but then realized that such experiences didn't fit with her personality.

So getting back to what i was saying, she lays naked under Melancholia because she isn't afraid of death anymore, she accepts her vulnerability in the form of being naked, that's the most vulnerable state someone can be in. Justine was struggling to achieve some kind of comfort in her life, but it was very difficult because of her parents, they were both selfish *beep* who didn't care for her very much.

When Justine understands that she's not meant to be happy, looses her fear towards death, that's why she's so calmed when the planets are about to crash.