The Dumbbell Nebula, a planetary nebula in Vulpecula
An expanding shell of gas - actually the surface layers - ejected from a medium-mass star during its last stages of evolution at a rate of 20 to 30 km/sec, resulting in a circumstellar gas cloud that has spheroidal shape and often appears as a faint disk in telescopes or long-range scanners.
The name is misleading, this kind of nebula has nothing to do with planets except for the rough resemblance to a planet's shape when seen at long range. With the result of the loss of surface layers, the white-hot interior of the star is exposed, which emits intense ultra-violet radiation. This ionizes the gas in the expanding shell, making it glow like a huge ring of fog a light year or more in diameter.
Planetary nebulae are often dense enough to be exploited for raw materials by specialised ramscoop equipment, and planets that remain in the primary system can gather thick surface layers of dust and gases.