Brown dwarfs are neither planets nor stars. They’re “substellar objects,” too low in mass to sustain hydrogen fusion reactions in their cores, unlike normal stars. They range in size between the heaviest gas giants (think Jupiter) and the lightest stars, with an upper limit of around 75 to 80 Jupiter masses (MJ).
Brown dwarfs heavier than about 13 MJ are thought to fuse deuterium. Those above ~65 MJ, fuse lithium as well.
Despite their name, brown dwarfs are rarely brown and come in an array of colors. Many brown dwarfs would likely appear magenta and orange/red to the naked eye.
There is a debate as to whether brown dwarfs have experienced fusion at some point in their history. Some planets are known to orbit brown dwarfs.
The nearest known brown dwarf was discovered by NASA’s Wide-field Infrared Survey Explorer (WISE survey), named Luhman 16 and part of a binary system of brown dwarfs, at a distance of about 6.5 light years.
If WISE found the brown dwarf in our Solar System, which many serious scientists have calculated exists in a binary orbit with our own Sun, they’re not telling. This theoretical object is often referred to these days as “Nibiru,” based on the writings of Zechariah Sitchin. It is also popularly referred to as Planet X and Hercolubus and has been called Nemesis in scientific literature.
The strong belief by many that this object exists and that it’s moving towards our outer Solar System and is predicted to disrupt – if not entirely extinguish all life on our planet is the subject of many YouTube videos and if nothing else a symptom of how completely distrustful of any authority that many people have become during our post-9/11 era.