I did a quick google on Gravity Terutz and did not find anything. As for the influence of stuff outside the solar system on the orbit of the Earth it is pretty close to 0. While the sun is orbiting the center of the Milky Way in more or less circular orbit there is very little influence on the planets due to the fact that everything outside the solar system is so very far away. Also mass is to a first order approximation distributed around us evenly*, so the gravity cancels out. (The gravitational attraction inside a uniform spherical shell is 0, its comes down to a quirk of the geometry of an inverse square force).
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