Planet spin and it's effect on the planetary RV

The Spin Effect on Planetary Radial Velocimetry of Exoplanets

Recent detections of the Doppler shift of the planetary spectral lines at HD 209458 and τ Boö have allowed for the planet's radial velocity to be measured, permitting an independent measurement of the mass of the planet by resolving the inclination degeneracy.

A planet's rotation rate will, however, leave an imprint on that Doppler shift that can contribute to the measured planetary radial velocity, especially if the source of the observed spectral line is biased toward one hemisphere or the other. While this may permit constraints to the rotation rate of the planet, it may also complicate estimates of the orbit inclination from direct planet RV measurements.