Planet X is an idea that's been around a long, long, very long time. It once referred to the idea that a planet existed somewhere beyond Neptune, and then after Pluto was discovered, some people decided that Pluto wasn't Planet X and it had to be out past that too. Though really, it basically refers to any sufficiently mysterious planet we haven't found yet. Another historical bugaboo came from before the beyond-Neptune theory, when 'Vulcan' was thought to be circling around somewhere between Mercury and the sun. Those of you who've watched enough Looney Tunes will recognize the term used in a certain cartoon from 1953:
For current purposes, Planet X, which I actually had thought we'd more or less abolished as a concept by now, refers to the outside-Pluto definition, and more specifically refers to a theory that once in a while, a rogue planet, also sometimes called Nemesis, comes along on an orbit that causes a mass-extinction event on Earth.
Well, long story short, you can relax about the hypothetical star that's going to doom us all if it actually exists, because it doesn't. NASA conducted a sweep of pretty much the entire cosmos that they're currently capable of sweeping, and there's no Planet X anywhere on their radar. Although they did find 3,525 new stars and brown dwarfs sitting within 500 light years of the sun, so it's far from a waste of time.
Seriously, though. People were still doing the Planet X thing? And taking it seriously? Wow.