The distinction between gas giant planets and brown dwarfs (“failed stars”) is a vague one. To separate these objects into different groups, astronomers focus on their temperatures and masses. Astronomy: Roen Kelly
Maybe. It depends on what gas giant you’re looking at and how you distinguish a “planet” from a “failed star.” A gas giant found in orbit around a star might be considered a planet like Jupiter around the Sun. An isolated gas giant is probably a failed star. A failed star has such a low mass that its core never gets hot enough to fuse hydrogen, although it might fuse other elements like lithium and deuterium. A planet has an even lower mass and thus never fuses anything.
