The question has come up many times as to why the planets in our solar system lie in essentially the same plane. The most common answer is that they started out that way due to the accretion disk around the sun. But now we know that such is not the case and that collisions plus planetary migration likely made the orbits chaotic.

Today we observe that, with the exception of Earth, eccentricity from the solar equatorial plane increases with distance.

Frame drag is a tiny effect that decreases with distance. However, because it's an effect of gravity it's influence must be inexorable. So, over the 4 billion years of the system even the most distant planets must be effected; bringing their orbits closer to the solar plane.

It seems to me that we already have an example of the frame-drag effect with the moons of Uranus which orbit the planet's equator and could only have assumed that orbit due to frame-drag.

Let's assume Neptune were to be deflected by some passing object such that it was increasing it's eccentricity by 1 foot per year. And let's assume a frame-drag of one inch per year. The deflection would be a one time event while frame-drag wold be continuous. Though tiny, the Frame-drag influence would have to overcome and reverse the deflection of the planet.

Does that make sense? or am i completely off here?

  • $\begingroup$ 1. "But now we know that such is not the case and that collisions plus planetary migration likely made the orbits chaotic." [citation needed] 2. What do you mean by a "frame-drag of one inch per year" and, more generally, by "frame dragging" here? Frame dragging is a term for several distinct GR effects, e.g. the Lense-Thirring effect. Unless you specify which of these effects you are talking about, it is hard to see such quantifications of the magnitude of the effect as meaningful. $\endgroup$ – ACuriousMind Aug 4 at 14:27
  • $\begingroup$ It's not the case that the only mechanism for the moons of Uranus to end up orbiting its equator is some GR-related effect ('frame-dragging'): Uranus is spinning, so is non-spherical, so has a non-spherical Newtonian gravitational field. The motion of bodies orbiting in such a field is certainly going to be complicated in purely Newtonian terms, and I suspect it is the case that the whole system tends to end up with the moons orbiting around the equator. This answer in the astronomy SE states the same thing, but does not cite clear sources. $\endgroup$ – tfb Aug 4 at 15:56

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