Rotational speed of a planet Is there a way to calculate the rotational speed (or day length) of a planet?
If not then what are the factors? I mean what affects the speed? Is it just the distance from Sun (like period of a planet) or mass, density and other things are also important?
And of course by planet, I meant solid planets.
 A: Rotational speeds of planets cannot be calculated/predicted because planet formation seems to be highly chaotic.  The spin of planets (both rocky and gas) is determined by many factors, including:  


*

*the angular momentum of the material which was accreted on the planet,  

*gravitational interactions with other planets,  

*the history of collisions as the planet formed  

*tidal interactions with the host star (if the planet is close in) and the gaseous and debris disks while the planet was forming.


In the solar system, for example, Mercury is in a 3:2 spin-orbit resonance --- so it completes 3 rotations every 2 orbits.  The spin periods of Earth and Mars, however are almost identical despite different masses and semi-major axes.  Finally, Uranus has a shorter rotational period than earth --- but is tilted almost 90degrees relative to the orbital plane.
A: About the only relationship worth considering is whether the planet orbits close enough to its parent star so that tidal forces lock the rotation period to the orbital period. Even this is fraught with problems because we currently don't know the exact "tidal friction" coefficients for exoplanets. This will depend greatly on the structure of these planets (e.g. rocky cores, average density, size of envelope) and how long they have spent close to their parent star (since planets migrate) and on the mass and size of the parent star.
It is generally expected that most hot Jupiter's with orbital periods less than a few days will probably be tidally locked.
