The most up-to-date list of Sol's nearest neighbors is that published yearly in the Royal Astronomical Society of Canada's Observer's Handbook.

The RASC list for 2014 lists the 50 stars (and their stellar and planetary companions) known to lie within 5 parsecs (16.3 ly) of Earth. Of course, this list includes only objects, (faint stars and brown dwarfs) bright enough to be detectable in visual light--and it makes no claim to being complete. This new object, detectable only by using extraordinary technology due to its extreme cold, is nearer than all but three of the stars on the RASC catalogue: Sol, The Alpha Centauri triple system (4.24 ly) and Barnard's Star (5.98 ly). See the movie I posted of Barnard's in the thread on "proper motions", below.

This dark and cold little visitor may indeed be an escaped gas giant planet from long ago and far away, or it may be a representative of a very common and highly numerous new class of object. We know the Mass Function for stars (the number of objects plotted against their mass) increases explosively for smaller masses, but the shape of the curve is poorly understood because faint brown dwarfs are difficult to detect and study unless they are relatively nearby. Those too cool to emit any visible light, like this one, are cloaked to our view. We can't rule out they may be the most common objects in the Universe, far outnumbering stars or bound planets! They are, in spite of their numbers, collectively such a small fraction of the total mass of the galaxy that they give no gravitational clue to their numbers. For a while, it was thought that the galaxy's "missing mass" could be explained by vast populations of these guys roaming around in space, but this possibility was ruled out for a variety of other reasons.

So what is this object? In appearance, something very similar to Jupiter or Saturn, a cold gas giant, but with no nearby star to illuminate it. At one time it might have glowed feebly with the leftover gravitational energy of its initial contraction from its primordial nebula. But that has long since radiated way.

The question remains, is this guy an outlier, a fluke, or is it just the first discovered of an extremely numerous stellar population we never even suspected existed? The more we learn about the Galaxy, the more complex and wonderful it seems to get.