From the June 2011 issue

Could gas giants be considered stars that failed to ignite?

Joseph Lettieri, Wakefield, Massachusetts
By | Published: June 27, 2011 | Last updated on May 18, 2023
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.