Why was the planet called Melancholia?

Because it's a metspö or something for depression, actually this film's point is mainly to tell how it is to have a deep depression, it's like a planet sliwly crash ingen on you with no hope or meaning anymore and with no way out.

Here are some other reasons why Melancholia is significant:

Melancholia has been a great subject of interest since the Ancient times.

If you have ever seen Albrecht Dürer’s engravings, his famous Melencolia I comes to mind.

Melancholia is often seen as the seat of creativity but it also suffers from an inability to act (depression does this), while Dürer depicts an infant to accompany her, who represents the ability to act but without thinking.

Here is a cool link that I like that briefly studies the subject:

http://quod.lib.umich.edu/j/jii/4750978.0002.205?rgn=main;view=fulltext

It's interesting that creativity seems to share a link with mental illness, and it looks like even the oldest scholars saw this pattern.

Melancholia is also frequently depicted as a disease of the idle rich (and well, Justine is pretty wealthy). When I studied Samuel Johnston he wrote frequently about hard work as being a cure for depression. There is evidence that he suffered from it himself. So perhaps also the film in a way critiques the ennui that the wealthy fall into because they have nothing to work for.

I'm not sure if Lars Von Trier thought of these things when he made the movie but he seems like a smart guy so it wouldn't surprise me. These were just ideas that came to me while I watched the film.

You can theorize about it all day, and there is no canon for it.

The way it seemed to me was that it was named that because the serious scientists knew it would hit the Earth.

Why can't you be a non-conformist like everyone else?

It's called melancholic becuz Lars von trier, the creator, wanted to make a film to express a horrific bout of depression he had just recovered from..hence, the title and the planet. Think about what the planet does, then think about debilitating depression

Yeah I agree. Why would scientists just simply name a planet "Melancholia"??

It just seems really odd. Like you said, planets are named usually with greek references, or in relation to who discovered it, or in some relation to the solar system. Even if they knew the planet was on a collision course with earth, that still wouldn't justify the name. It sounds like it came straight out of a comic book. They might as well have just called it "Doom".

To me it seems that Lars just decided to name the planet Melancholia as an ode to his highly depressed state. Just doesn't seem scientifically logical to give a planet such a name.

Omega would be a better name.