In an answer to a previous question of mine, one that asked about the planar orbits of inner planets, I was told the following (emphasis mine):

On the subject of different solar systems, I would expect tidal disturbances from close passes with neighboring stars to be the most dominant effect in determining how closely planets' orbital planes coincide. So... "urban" star areas would have more close passes than "rural" ones, and also more metal pollution. Ergo, if anything I would expect systems with higher metals to be less coplanar.

Now, this last sentence got me thinking whether or not there was any evidence for this, so do stars of higher metallicity have more planets in highly-inclined Pluto-like orbits? (higher metallicity means more dust, I'd presume - but would it significantly increase the gas to dust ratio?) I actually would expect the opposite hypothesis (at first), since there might presumably be more friction with more dust (and the amount of dust wouldn't really affect the amount of gas, since gas still forms the overwhelming majority of particles)

  • $\begingroup$ Wasn't this answered in the earlier question? "Ergo, if anything I would expect systems with higher metals to be less coplanar." $\endgroup$
    – ghoppe
    Jul 28, 2011 at 18:53
  • 2
    $\begingroup$ "I would expect" is not an answer. It's just a hunch that I'd like to see more evidence for (or against) $\endgroup$ Jul 29, 2011 at 1:40
  • $\begingroup$ That was my comment. Indeed, I was speculating, not expounding from authority. $\endgroup$
    – Andrew
    Aug 10, 2011 at 18:40

1 Answer 1


The paper The stability and dynamics of planets in tight binary systems (The Astrophysical Journal, 2009-04-01. PDF, 561 KB) probably has more about this topic than you'd ever want to know. I read the abstract. From that, it seems that I was more or less right in my previous comment (see last sentence of the abstract), except that in binary systems, it is easy to eject a planet entirely.

Therefore you have two, competing effects from one phenomenon- the presence of another star will both decrease coplanarity by perturbation and increase coplanarity through ejection of the most perturbed planets. You'd have to read more of that paper than I'm willing to to find out which wins.


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