@ProfRob's answer to Is it coincidence that the earth's rotation and revolution are in the same direction? explains several important aspects of models for planetary formation, angular momentum and vorticity.

I love our planets and don't mean to disrespect them, but it's a small and quirky dataset.

So I'd like to ask:

Question: What will it take to start measuring the rotation rate and direction of exoplanets? Which instrument(s) current or future are most likely to do it?

Are there current or future instruments that mention this capability as part of their goal? How might it be done?

Feel free to answer for "rate" and "direction" seperately of those measurements can be approached differently.


1 Answer 1


The first exoplanet to have its rotation period measured was Beta Pictoris b in 2014. This was done using the Cryogenic highResolution InfraRed Echelle Spectrograph CRIRES located at the Very Large Telescope (VLT) of the European Southern Observatory.

The rotation of three directly imaged exoplanets was measured with the near-infrared spectrograph NIRSPEC at the Keck II 10m telescope.

More info on rotation rate of exoplanets is at https://en.wikipedia.org/wiki/Exoplanet_orbital_and_physical_parameters#Rotation_and_axial_tilt

For the direction of rotation or axial tilt see the question Axial tilt of exoplanets

  • $\begingroup$ If the rotation rate of Beta Pictoris b was measured by observing differences in Doppler returns, then surely we know in which direction it rotates? And if it has been directly imaged, surely we know which way around its star it orbits, so it should be possible to say if they rotate in the same direction. Or did I miss something? $\endgroup$
    – jcaron
    Commented Feb 25 at 13:07
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    $\begingroup$ @jcaron The question astronomy.stackexchange.com/questions/35234/… explains how rotation speed can be measured. It doesn't involve knowing which direction it is rotating. $\endgroup$
    – sno
    Commented Feb 25 at 14:42
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    $\begingroup$ ah, the two sides of the planet are not resolved independently, it’s the width of the band (possibly on a single pixel/dot) which gives the difference and thus the speed. Thanks. $\endgroup$
    – jcaron
    Commented Feb 25 at 14:48

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