The official definition of an asteroid is ambiguous, and boilds down to asteroids being rocky, dusty objects orbiting the Sun that aren’t planets. They come in all sizes with the largest (named Ceres) being over 900 kilometers across and the smallest being mere specks of dust. Some have been found to have small amounts of ice on them, so that nobody’s quite sure whether to call them asteroids or comets, and others cross over into the definition of Minor Planet. Wherever you decide to draw the line, though, asteroids are made up of the solid left-overs from the birth of the Sun and its Solar System.
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