Define a solar system.

A solar system is defined as a gravitationally bound system that includes a star at its center and the various celestial bodies that orbit around it. Typically, this central star is a sun, and the orbiting bodies include planets, moons, asteroids, comets, and other objects such as dwarf planets. The critical aspect of this definition emphasizes the gravitational relationship: the star's gravity holds the other celestial bodies in orbit around it, which is fundamental to the structure and dynamics of a solar system.

In contrast, other options do not capture this complete definition. A system containing only planets is missing the essential star that provides the gravitational force needed to bind them together. A cluster of galaxies refers to a much larger structure in the universe, well beyond the scale of a solar system, which includes multiple galaxies rather than planets and smaller bodies orbiting a single star. Lastly, a collection of comets and asteroids does not constitute a solar system as it fails to include the star itself, which is crucial for defining the orbital relationships within such a system.