Brown Dwarfs are part of our Universe but they are neither planets nor stars.
Well what are they?
They are giant objects that do not shine of their own light;
They have such a mass that is impossible to classify them into the planets category;
They are not born to progressive accretion from a fizzy shell around a solid nucleus but they are born from a gravitational collapse of an interstellar cloud of gas and dust;
They do not produce energy by hydrogen fusion;
They are not stars because their mass it is not enough to activate hydrogen fusion;
They belong to a system that could be low-heavy chemical elements, indeed heavy elements are a hint of planets existence;
They range from 13 to 90 times the Jupiter asses, around a tenth the mass of the Sun (by contrast planets may reach the size of about 10 Jupiter masses
When they were discovered?
Brown Dwarfs were originally called by Shiv Kumar, in 1963, black dwarfs, a classification for dark substellar objects floating freely in space that were not massive enough to sustain hydrogen fusion*, because of their little mass. They named “black” referring to a cold white dwarf. Later, in 1975, they were defined “Brown Dwarfs” by Jill Tarter (an American astronomer and the former director of the Center for SETI Research,) for the first time.
And what about studies about them?
Simply stated, then the INAF science team, in Palermo, have analysed high-resolution spectra of 53 FGK stars known to host a brown dwarf companion, with the aim of determine their chemical abundances and look for differences between those hosting a brown dwarf companion more and less massive than 42.5 Jupiter masses. Therefore, the result is the evidence that the brown dwarfs with a minor mass are those accompanied by stars having slightly larger abundance of heavy elements. On the other hand, stars with massive brown dwarf companion share similar chemical abundances of stars without known planetary companion.
(written by Carlotta Bellisai)