Maybe the idea of a large planet is too far fetched for what I have in mind. I'll tone it down to a small moon, either artificial like Publius suggested or one which was ripped out of its orbit as C5 mentioned.
I like the idea of an artificial planet, one which could resemble the death star. I've toyed with the idea of making maybe a world which is artificial but with a holographic surface.
Now I've got lots of good competing ideas; a world ripped out of its orbit and dying slowly in space, one artificial planet which "flies" through space, and one which is a ocean based which was purposefully torn out of its orbit. All great ideas, I'm trying to figure a way to merge them all together.
Some help would be appreciated in trying to put them together.