PlanetPol

PlanetPol was a ground-based, high sensitivity polarimeter based at the William Herschel Telescope^[1] on the island of La Palma in the Canary Islands, Spain that has now been decommissioned. It was the most sensitive astronomical visual polarimeter ever built in fractional polarisation, a mantle that since its decommissioning now belongs to HIPPI. Although the device could be used for a wide range of astronomy, its primary use was the detection of extrasolar planets.^[1]

PlanetPol did not discover any extrasolar planets, however it was used to provide upper limits to planetary albedos in the known 55 Cnc and τ Boo planetary systems.^[2] Observations with the polarimeter in the Canary Islands, which are affected by dust from the Sahara, also identified airborne dust as a source of polarization within our atmosphere.^[3] Additionally, PlanetPol provided measurements of the polarization of a few dozen nearby stars,^[4] which were later combined with southern hemisphere measurements from PlanetPol's successor, HIPPI,^[5] to provide information about the nature of those stars and the distribution of the interstellar medium.^[6]