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Planetesimal is a term used in models of planet formation, including the currently most-accepted ideas: the combining of dust into small (solid) objects, which combine in turn, leading to planets. The term planetesimal is often used for such an object sufficiently large that gravity holds it together. In the solar system, such an object would be minimally on the order of a kilometer in diameter. The motion of such bodies is presumed to be dominated by a gravitational orbit rather than the effects of surrounding gas. Use of the term varies regarding any maximal size or mass, but undoubtedly decidedly smaller than the solar system planets. Many solar system objects of that general size, including many comets and asteroids, are presumed to be left-over such planetesimals.