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Published online by Cambridge University Press: 26 May 2016
The Doppler residuals to the Keplerian fits for extrasolar planets reveal important properties of the planets and host stars. Stellar magnetic fields modify the photospheric velocity fields, causing Doppler fluctuations with unknown time scales. This Doppler “jitter”, seen prominently in the magnetic stars Epsilon Eridani and ξ Boo A, compromises the detectability of planets. The Doppler residuals during the transit of HD209458 reveal that the planet orbits in the same direction as the star spins. Moreover, the transit path across the star is nearly parallel to the stellar equator. Most interestingly, the Doppler residuals of known planets often reveal additional coherent variations, probably caused by additional companions. Both 55 Cancri and HD168443 reveal such coherent Doppler residuals. Another five planet–bearing stars observed at Lick show trends in the Doppler residuals indicating the presence of additional companions. Remarkably, about half of the known extrasolar planets reveal such coherent variations. This suggests that stars with planets have a high occurrence rate of harboring more distant companions, planetary or otherwise.