Suppose an astronomer discovers a large, spherical-shaped body orbiting the Sun. The body is composed mostly of rock, and there are no other bodies sharing its orbit.

The answer is "Comet".

Explanation:

The comets were interstellar frozen gas of snowballs, stone, and debris orbiting the sun, which are also the size of average towns while frozen. As even the orbit of the comet passes near the sun, it warms up and spreads dust and gas into a big floating head greater than most stars.  

6.4 cm (max) - 25 cm (min)