The brown dwarf is a sub-stellar object. In other words, this is a celestial body, which is a cross between a planet and a star. Scientists were able to find brown dwarfs and begin to study them only in 1995, moreover, many information about these celestial bodies is still being clarified or refined, since it is extremely difficult to study them.
Brown dwarfs used to be classified as either very light stars or very heavy planets. To make it easier to understand why scientists held such opinions, one can compare such celestial bodies with stars and planets. The mass of brown dwarfs varies from 0.012 to 0.0767 solar masses, or 12.57 to 80.35 masses of Jupiter. To better understand the situation, consider the fact that the mass of Jupiter is 2.47 times the mass of all other planets in the solar system combined.
In brown dwarfs, as in stars, thermonuclear reactions occur at the beginning of their life. However, there is a difference between these objects: the fact is that brown dwarfs cool down very quickly, and the temperature in their depths is too low to ensure a continuous reaction of converting hydrogen into helium, accompanied by the release of heat and light. By the way, the very color of these celestial bodies is due to their relatively low temperature, which is less than 2000 degrees Kelvin. In addition, brown dwarfs lack a radiative transfer zone, and heat transfer occurs only due to convection. In particular, lithium, which either burns out in stars at the initial stage of life, or remains in the upper layers, in brown dwarfs gradually passes from the cold upper layers to the hot inner ones, ensuring the mixing of substances and the relative homogeneity of the structure of the celestial body.
Brown dwarfs have long been thought of as planets because their average diameter is about the same as that of Jupiter. In addition, they are not able to sustain thermonuclear reactions for a long enough time. However, there are also significant differences between these celestial bodies. First, brown dwarfs differ from planets in density and mass. As noted above, their mass can be 80 times the mass of the gas giant Jupiter. Secondly, brown dwarfs, unlike planets, are capable of emitting in the infrared and sometimes in the X-ray range, which allowed astronomers to detect many of these celestial bodies far beyond the solar system.