Which of the following is NOT a type of galaxy?

A planetary galaxy is not recognized as a type of galaxy in astronomical classification. The three main types of galaxies are spiral, elliptical, and irregular.

Spiral galaxies are characterized by their distinctive spiral arms that wind around a central bulge, and they often contain significant amounts of dust and gas. Elliptical galaxies, on the other hand, vary in their shape from nearly spherical to elongated and generally display a smooth, featureless light profile with very little star formation. Irregular galaxies lack a distinct shape and often have a chaotic appearance, usually rich in gas and dust, allowing for active star formation.

In contrast, there is no classification for a planetary galaxy in established astrophysical terminology, making it a non-existent type among the recognized galaxy forms. This distinction is crucial for understanding the variety of galaxy types and their specific characteristics in the universe.