How did planetary nebulae get their name?
Planetary nebulae were mistaken for planets and hence their name.
The word nebula comes from the latin word for cloud. William Herschel saw these in the 1780s and thought that they were planets. Hence their name.
Actually planetary nebulae are the remnants of stars which are too small to undergo supernova explosions. Material is expelled from the star to form a planetary nebula. These are relatively short lived lasting only 10s of thousands of years.