Final answer:
Brown dwarfs are objects that fall between planets and stars, with masses insufficient to sustain regular hydrogen fusion but enough to fuse deuterium. Classified by the ability to fuse deuterium, brown dwarfs have a similar radius to Jupiter despite having a much larger mass range.
Step-by-step explanation:
Brown dwarfs are celestial objects that reside in the grey area between planets and stars. They have an approximate mass range from about 1/100 of the mass of the Sun up to the lower mass limit for sustaining nuclear reactions, which is about 0.075 the mass of the Sun. Brown dwarfs are unique in that they can fuse deuterium—a heavier isotope of hydrogen—though they lack the ability to sustain regular hydrogen fusion like main-sequence stars.
The defining feature between a low-mass brown dwarf and a high-mass planet, as designated by the International Astronomical Union, is the object's ability to fuse deuterium. If an object has enough mass, roughly 13 times the mass of Jupiter (MJ) or greater, it is classified as a brown dwarf. Objects with less mass are generally considered planets.
Another interesting characteristic of brown dwarfs is that they all have a similar radius to that of Jupiter, regardless of their mass, which spans from about 13 to 80 times that of Jupiter. This similarity makes it difficult to distinguish between a high-mass planet and a low-mass brown dwarf based solely on size. Mass and temperature variations, along with spectral analysis, are critical factors in distinguishing these types of celestial bodies.