Well, Raven, a Planet Comet is possible. I did some research on it. The thing is that a Planet Comet can not sustain life. Pluto may be an example of a Planet Comet, but I'm not sure. A Comet is mostly ice, a Planet is a large sphere caught in a gravitational orbit around a star, a Moon is a sphere caught in a gravitational orbit around a Planet.
Nibiru is supposed to be a name for "Planet X" the Comet that enters our part of the Galaxy once every 3600 years, roughly. The thing is that during the time this planet is between Pluto, our farthest planet, and whatever other galaxy it goes through, the surface would ice over and the life on that planet would be silenced. I have already mentioned this to Revan and he apologized for his mistake. I'm waiting for the next message to see whether or not this will lead to the end of his roleplay.