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Radial velocity measurements showed evidence that the M dwarf GL 581 mighthost a planet, GL 581d, in the so-called "habitable zone" of the star. A studyof Halpha in GL 581 demonstrated that changes in this activity indicatorcorrelated with radial velocity variations attributed to GL 581d. An exopplanetthat was important for studies of planet habitability may be an artifact ofstellar activity. Previous investigations analyzing the same activity data havereached different conclusions regarding the existence of GL 581d. We therforeinvestigated the Halpha variations for GL 581 to assess the nature of theradial velocity variations attributed to the possible planet GL 581d. Weperformed a Fourier analysis of the published Halpha measurements for GL 581dusing a so-called pre-whitening process to isolate the variations at theorbital frequency of GL 581d. The frequency analysis yields five significantfrequencies, one of which is associated with the 66.7 d orbital period of thepresumed planet Gl 581d. The Halpha variations at this period show sine-likevariations that are 180 degrees out-of-phase with the radial velocityvariations of GL 581d. This is seen in thefull data set that spans almost 7years, as well as a subset of the data that had good temporal sampling over 230days. Furthermore, No significant temporal variations are found in the ratio ofthe amplitudes of the Halpha index and radial velocity variations. Thisprovides additional evidence that the radial velocity signal attributed to GL581d is in fact due to stellar activity.

Periodic Halpha variations in GL 581: Further evidence for an activity origin to GL 581d