An object that is too massive to be a planet but not massive enough to be a star is called a:

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An object that is too massive to be a planet but not massive enough to be a star is called a brown dwarf. Brown dwarfs are also known as failed stars because they are not massive enough to initiate sustained nuclear fusion in their core like a true star, but are still much larger than planets. A red dwarf is a small and cool star, while a white dwarf is the remnant core of a once-massive star that has exhausted its fuel. A T Tauri star is a young, pre-main sequence star.

An Object That Is Too Massive to Be a Planet