Any nebula which shines via excitation of its atoms is an emission nebula, including planetaries. It's unfortunate that this term got to be associated with the non-planetaries. The prime determinant on nebula visibility is surface brightness (SB), which for emission objects tends to revolve about the presence or lack of oxygen emission; where ...

When a star like our Sun dies, it doesn’t explode into a supernova or collapse into a black hole. Instead, it gently sheds its outer layers, which form a beautiful cloud called a “planetary nebula”, while the dying star’s core becomes a white dwarf.

The difference between planetary nebula and emission nebula is outlined below. Origin and Formation: Planetary nebulae originate from dying low-mass stars during the asymptotic giant branch phase. Emission nebulae form from ionized gas clouds triggered by radiation from nearby hot stars.

The Orion Nebula (M42): a vibrant emission nebula easily visible with small telescopes; The Ring Nebula (M57): a planetary nebula representing a star’s demise, recognizable for its distinctive ring shape