Paul Butler designed and built the iodine absorption cell system at Lick Observatory, which resulted in the discovery of 5 of the first 6 known extrasolar planets. This instrument has become the de facto standard for precision Doppler studies, having been adopted by teams at the University of Texas, Harvard, Europe, and Japan. In addition, Butler built the iodine systems at the Keck, Anglo-Australian, and Magellan Telescopes. The original iodine cell (still in use at Lick) has been requested by the Smithsonian Institution upon its retirement.
Along with his collaborators at Lick, Keck, AAT, and Magellan, Butler has discovered hundreds of extrasolar planets, including the first planet to transit its host star, the first sub-Saturn mass planets, the first Neptune-mass planet, and the first terrestrial mass planet. This work has been featured on several front-page articles in the New York Times and Washington Post, as well as a TIME magazine cover story.
Over the past 25 years, Butler's work has focused on improving the measurement precision of stellar Doppler velocities, from 300 meters per second in the 1980s to 1 meter a second in the 2010s to detect planets around other stars. The ultimate goal is to find planets that resemble the Earth.