I just read about the planet KELT-9b, and one thing which stuck out to me was the fact that this planet orbits this star parallel to the star's axis of rotation (the article says "perpendicular", but comments below say "parallel" is correct). I'm aware of the concept of "frame dragging", but I'm not advanced at all in my understanding of physics.

Would this planet experience an "uneven" gravitational field as its position aligned with the poles of the star, relative to the rest of its orbit's path? How would this manifest in the planet's orbit?

Would it not be truly elliptical?

  • $\begingroup$ I'd be surprised if the effect of this is even measurable, but calculating it is way beyond me. $\endgroup$
    – zeta-band
    Commented Jun 7, 2017 at 17:13
  • 4
    $\begingroup$ Minor detail: you (and the article) mean parallel to the axis of rotation, that is, perpendicular to the star's equator. Being perpendicular to the axis of rotation (parallel to the equator) is what you usually expect. $\endgroup$
    – Javier
    Commented Jun 7, 2017 at 17:28
  • $\begingroup$ @Javier: Good catch. Updating my question. $\endgroup$
    – loneboat
    Commented Jun 7, 2017 at 17:29
  • $\begingroup$ I assume this means the planet was captured after the star and stellar system formed then? Otherwise we would expect it to orbit in the stellar ecliptic equivalent, correct? $\endgroup$ Commented Jun 7, 2017 at 18:33
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    $\begingroup$ There is nothing special that would happen other than potential interactions with other satellites in different planes. $\endgroup$
    – ProfRob
    Commented Jun 7, 2017 at 20:25

1 Answer 1


The question posits general relativistic effects. There are good old Newtonian effects that would swamp the smallish relativistic effects of such an orbit.

The first is precession. The star about which KELT-9b orbits spins very rapidly, making the star rather oblate. KELT-9b's orbit will undergo nodal precession due to the parent star's large J2. (We use this effect to our advantage. The sun-synchronous artificial satellites that observe the Earth depend on this nodal precession to make those orbits be sun synchronous.)

Another is the Kozai mechanism, aka the Kozai-Lidov mechanism. An object in a highly inclined orbit can oscillate wildly in terms of eccentricity and inclination due to perturbations caused by other orbiting bodies with more staid inclinations. A number of scientific journal papers have speculated that many of the hot Jupiters we see are a result of Kozai oscillations coupled with circularization.


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