Original: To early observers with low-resolution telescopes , M27 and subsequently discovered planetary nebulae somewhat resembled the giant planets like Uranus , and William Herschel , discoverer of this planet , eventually coined the term ` planetary nebula ' for them , although , as we now know , they are very different from planets .
Simplified: This is why people called them ` planetary nebula ' . We still call them this even though we know they are not planets .