Like the planets in our own solar system, they produce no light of their own and are lost in the bright light of their mother stars. Indirect means reveals their presence, however. So far, so good. We realize they have engines, brakes, perpetually empty gas tanks, windshield wipers and the like, and we exclude things like vans and pickup trucks from our car category. Astronomers thought they had a grip on the definition of a planet, and then brown dwarfs came along.
Brown dwarfs are star wannabes.
They're smaller and less massive than stars and didn't have enough material to produce the intense pressure and heat in their cores for thermonuclear nuclear fusion to begin. The nuclear powerhouses in the centers of stars turns hydrogen into helium and releases the energy we call heat and light. It's what makes a star a star. Brown dwarfs don't emit light and thus they're impossible to see in visible wavelengths. They are hot, however, and show up the best in the infrared.