Final answer:
An astronomer observing a very small object made mostly of iron and nickel orbiting the sun has most likely found an asteroid, which is a small, rocky body within our solar system.
Step-by-step explanation:
The astronomer has most likely found an asteroid. Asteroids are defined as stony or metallic objects orbiting the Sun that are smaller than a planet and show no evidence of an atmosphere or other types of activity that are characteristic of comets. Given that the object in question is very small in size and made predominantly of iron and nickel, it fits the key characteristics of an asteroid, which are commonly found within the asteroid belt between Mars and Jupiter. These rocky bodies have survived the heat from the sun, unlike comets, which are composed mostly of ice and would not be made mostly of iron and nickel. Comets typically originate from cooler, more distant regions of the solar system and have different compositional attributes, such as being made of frozen gases like water, carbon dioxide, and carbon monoxide.