Reading about Uranus almost 90° tilt, I was wondering if some rocky planet with mass concentration at one pole could possibly spin around its own axis, while still being locked to its star? Which means the more massive pole always points toward the star?

  • $\begingroup$ Almost? The axial tilt of Uranus is more than 90°, it's 97.77° relative to its orbital plane. $\endgroup$ – PM 2Ring Jan 16 '19 at 9:26
  • $\begingroup$ Maybe it's spinning retrograde with 82.23° axial tilt? $\endgroup$ – qq jkztd Jan 16 '19 at 9:41
  • $\begingroup$ The most improbable is to have mass concentrated at the pole, Unless you have a more generic system in mind rather than planetary $\endgroup$ – Alchimista Jan 16 '19 at 13:02
  • $\begingroup$ Would be the case if the axe of rotation were right 90°, tough tidally locked might not be the right term (just always facing to. .. instead of locked)? $\endgroup$ – Alchimista Jan 16 '19 at 13:16

I think what you are envisioning is having one pole always pointed towards the primary star. So the planet would rotate about that pole, but then the direction of the pole would change over the course of a year to keep the pole pointing towards a star.

This phenomenon of the direction of the pole changing is called "precession". The Earth does it over a period of 26,000 years, but the pole only traces out a fairly small circle in the sky, rather than twisting about a perpendicular line like you envision.

I think your scenario is improbable at best. Planets are like gyroscopes and resist having their axes of rotation be twisted about a perpendicular axis like that. Even a planet with mass concentrations at both poles (maybe a long cigar shape) to maximize tidal force would probably end up losing the perpendicular rotation to tidal friction, and just end up rotating slowly about an axis nearly parallel to the axis of the orbit.


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