Why are planetary nebulae called planetary even though they are not planets?

Planetary nebulae are not related to planets; they are actually expanding shells of luminous gas expelled by dying stars.1

The term "planetary" is a historical misnomer. When these objects were first discovered in the late 1700s, they appeared as relatively round, compact, and disk-like shapes when viewed through the limited astronomical instruments of that era. Because they resembled the appearance of planets, they were labeled "planetary nebulae," a name that has persisted despite our modern understanding that they are stellar phenomena rather than planetary bodies.1