It is known that when a planet orbits very close around its star, the tidal effects will soon (on an astronomical scale) force the day and the year of the planet to have the same length, making the planet show the same face to the star (let's ignore the possibility of orbital resonance). So the body will be tidally locked to its star.

So, if the planet was born from the same gas cloud as the star, it will rotate along an axis that has (almost) no tilt with respect to the orbit.
But what if, for instance, a star captures a rogue planet in a close orbit or an impact with a massive body reorients the rotation axis of a close planet? This planet now has an axis of rotation that is almost 90° tilted with respect to the orbit plane (in other words, the axis is not perpendicular, but parallel to the orbit plane).

So, my question is: what will likely happen in a situation where a planet is tilted of 90 degrees with respect to the plane of its orbit, and is very close (like Mercury, or less) to its star?

  • Nothing? (the planet in spite of the proximity won't experience any change in its rotational dynamics, experiencing particularly extreme seasonal variations, as Uranus)
  • the tidal effects will slowly force the rotational axis to align to the perpendicular of the orbit plane (and eventually tidal-locking it to the star)?
  • the tidal effects will slowly reduce the rotation of the planet, gradually stopping its rotation?
  • something else?
  • $\begingroup$ How close do you have in mind? Mercury is pretty close to the Sun, and is tidally locked, but not in a 1:1 resonance, but rather a 3:2 spin-orbit resonance (as per Wikipedia). $\endgroup$ – user Mar 12 '19 at 18:06
  • $\begingroup$ I've seen recent headlines that suggest the planet may get pushed outward when it has a high axial tilt. Didn't read the articles, so can't answer on this basis, but perhaps someone else did. $\endgroup$ – Zeiss Ikon Mar 12 '19 at 18:06
  • $\begingroup$ @Agrajag That's impossible. I think he means something like Uranus, with a near 90 degree axial tilt. Each pole points through the star once per orbit. $\endgroup$ – Zeiss Ikon Mar 12 '19 at 18:07
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    $\begingroup$ @ZeissIkon That's Uranus, which has a 97.77° axial tilt. Agrajag, I think you'd need to violate a handful of basic laws of physics to keep a highly-inclined planet's axis of rotation pointed toward the star throughout its orbit. Compare en.wikipedia.org/wiki/Uranus#Axial_tilt $\endgroup$ – user Mar 12 '19 at 18:09
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    $\begingroup$ Notice that the planet cannot be captured by the star without the presence of a much more massive planet, which makes it unlikely thst the ex-rogue will have a very close orbit. $\endgroup$ – The Square-Cube Law Mar 13 '19 at 0:23

the tidal effects will slowly force the rotational axis to align to the perpendicular of the orbit plane (and eventually tidal-locking it to the star)

This will, however, take a very long time--hundreds of millions of years, up to possibly billions, depending on precise details of the planets composition, initial spin rate, and exactly how large its orbit is.

The most obvious short-term effect will be slow precession of the planet's axis as tidal effects apply off-axis torques. Depending on which way it spins, this will result in each pole pointing towards the sun either slightly less or slightly more than once per sidereal year.

Over the very long term, the planet will end up tidally locked, with some sort of spin-orbit synchronization and a spin axis close to its orbital axis--but, its initial spin angular momentum can't just disappear. Rather, it will have been transferred into orbital momentum along the way (just as, for example, the Earth's spin angular momentum is slowly being dumped into the Moon's orbital angular momentum by tidal interaction), resulting in a final orbit that is inclined with respect to its initial orbit.


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