Kuiper Belt and Oort Cloud objects can exert gravitational effects on the planets, but the real question is whether such perturbations are detectable. Based on the observation that long-period comets can appear in any part of the sky, we generally think that the Oort Cloud of comets is a near-spherical structure surrounding the solar system at a distance of about 20,000 to 100,000 astronomical units (AU, which is the average Earth-Sun distance) from our star. The nearly spherical nature of the cloud means that there would be no net gravitational effect on planetary orbits — all of the small gravitational pulls would cancel one another out.
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