Well this particular planet (with a theoretical mass of 10x that of Earth) would be in our own solar system, which makes it somewhat more significant to us than just some random planet somewhere on the other side of the galaxy.
Right now astronomers are only seeing hints of its existence from the gravitational perturbations exerted on other, smaller bodies in the outer solar system.
What's kind of funny is that astronomers have been able to find dozens, if not hundreds, of extra-solar planets far beyond our own solar system, yet this mysterious, massive Planet X, right in our own backyard, has thus far proven too elusive for them to find any conclusive proof of its existence.