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Tricky star plays sleight-of-light
Two signatures in starlight thought to be written by extrasolar planets may turn out to be forgeries. Astronomers often attribute periodic shifts in a star's apparent motion toward and away from us to the tug of orbiting planets. Robertson et al. studied archival spectra of the star Gliese 581 to assess another potential cause: magnetic activity on the star's surface. The signals once attributed to two planet candidates instead resulted from the combined effect of starspots and stellar rotation. Planet hunters be warned—correct for stellar activity in your data analyses.
Science, this issue p. 440
Abstract
The M dwarf star Gliese 581 is believed to host four planets, including one (GJ 581d) near the habitable zone that could possibly support liquid water on its surface if it is a rocky planet. The detection of another habitable-zone planet—GJ 581g—is disputed, as its significance depends on the eccentricity assumed for d. Analyzing stellar activity using the Hα line, we measure a stellar rotation period of 130 ± 2 days and a correlation for Hα modulation with radial velocity. Correcting for activity greatly diminishes the signal of GJ 581d (to 1.5 standard deviations) while significantly boosting the signals of the other known super-Earth planets. GJ 581d does not exist, but is an artifact of stellar activity which, when incompletely corrected, causes the false detection of planet g.
