What characterizes a comet?

A comet is characterized as a small icy body that orbits the Sun. Comets are composed mainly of ice, dust, and rocky materials. As they approach the Sun, the heat causes the ice to vaporize, creating a glowing coma and often a tail that points away from the Sun. This distinctive feature is a result of the interaction between the comet and solar radiation.

The presence of ice and volatile compounds is crucial in defining a comet. Unlike asteroids, which are primarily rocky or metallic, comets contain a significant amount of ices such as water, carbon dioxide, ammonia, and methane. Their orbits can be highly elliptical, which brings them close to the Sun and then sends them back out toward the outer solar system.

In contrast, the other options describe celestial bodies that do not match the characteristics of a comet. For example, the description of a large ball of gas burning in the cosmos pertains to stars. Rocky planets refer to substantial planetary bodies like Earth or Mars with solid surfaces. A cluster of stars would be a distinct astronomical structure known as a star cluster but does not correlate with the icy nature or behavior of comets.