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The term planetoid is generally used to mean a minor planet, i.e., a solar system body that is not a moon or (major) planet, or comet, and is larger than a meteoroid. It serves as an alternative to the term asteroid, specifically implying a generalization to include bodies outside the asteroid belt.
The term planetoid is also common in discussions of planet formation, for planet-like bodies smaller than what we know as planets. Some theories of solar system planet formation presume an early population of planetoids that each either grew, combined with others, was ejected, was broken up, or remained as an asteroid. It is presumed other stars could be undergoing such theorized processes, but their planetoids would be hard to detect for being small.