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A planetary nebula is an astronomical object consisting of a glowing shell of gas and plasma formed by certain types of stars when they die. These are usually quite colourful and visible via a telescope as glowing objects in the sky.

A planet, on the other hand, is a celestial body that orbits a star, is spherical in shape, and has cleared its orbit of other debris. It may be visible from Earth as a bright point in the night sky but does not emit its own light.

There are multiple ways an astronomer can tell the difference between a planetary nebula and a planet. Firstly by their appearance, as a planet doesn't emit its own light, it will look like a bright point in the sky, while a planetary nebula emits light and gas, so it will have a more diffused appearance with intricate shapes and colors. Secondly, through spectral observation. Planets generally reflect the light of their star, so their spectroscopic signature is similar to that of their star. A nebula has a completely different substance and produces a distinct emission spectrum with bright emission lines. Thirdly by their movement, Planets move relative to the stars in the sky since they are orbiting a star. In contrast, a nebula is much further away and appears fixed in relation to the stars.