Researchers at MIT's Kavli Institute for Astrophysics and Space Research are searching for exoplanets by detecting periodic dips in the star's brightness as the planet transits across the face of the star. If the star happens to be rotating in the same direction as the planet's orbit, the beginning of the transit will show light of a higher wavelength than the end of the transit, due to Doppler shifts in the rotating photosphere of the star. What is the name of this effect?