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In general, when an object falls from the far Solar System and swings around the Sun, does that usually make the orbit more or less eccentric? Or, assuming it doesn't get ejected from the Solar System entirely, does the eccentricity not change

I'm trying to figure out a reason for the circularity of planetary orbits. This mechanism might tend to make the orbits more circular over time.

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    $\begingroup$ In a 2 body system, the eccentricity is constant. So you need perturbations from other bodies to change the eccentricity. For tiny particles, radiation pressure also affects the orbital parameters, but that's negligible for larger bodies. $\endgroup$
    – PM 2Ring
    Commented Nov 17 at 4:58
  • $\begingroup$ good question, good answer, voting to leave open. $\endgroup$
    – uhoh
    Commented Nov 20 at 2:04

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If we are dealing with a two body system and point particles, the eccentricity is a constant of the orbit and doesn't change.

In practice, there are other bodies in the Solar System exerting forces and bodies are of finite size leading to tidal dissipation. Both interactions with other bodies, and tidal forces, increase towards perihelion. On average, this extracts more energy from the orbit nearer perihelion, which means the object does not reach as large an aphelion and thus reduces the eccentricity.

This is not the reason that planetary orbits are close to circular. See https://astronomy.stackexchange.com/a/58936/2531

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