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As the exoplanet 81 Ceti b orbits a star 330 light-years from Earth, the exoplanet’s gravity causes the star to wobble. In 2008, astronomers ______blank this wobble through shifts in the color of the star’s spectral light—blueshifts indicating longer wavelengths and movement toward the observer, redshifts shorter wavelengths and movement away—deduced that the fluctuation was caused by the gravitational force of a previously undetected exoplanet.