Planetary nebula

From Memory Alpha, the free Star Trek reference

A planetary nebula is formed when an extremely hot star, such as a red giant or supergiant, ejects a shell of gas, exposing the star's inner core. Planetary nebulae have nothing to do with planets - they are only so called because of their resemblance to a planet.

The Enterprise NX-01 surveyed a planetary nebula three weeks before discovering a comet in 2151. (ENT: "Breaking the Ice")