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?