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Bow shocks are ubiquitous astrophysical phenomena resulting from thesupersonic passage of an object through a gas. Recently, pre-transit absorptionin UV metal transitions of the hot Jupiter exoplanets HD 189733b and WASP12-bhave been interpreted as being caused by material compressed in a planetary bowshock. Here we present a robust detection of a time-resolved pre-transit, aswell as in-transit, absorption signature around the hot Jupiter exoplanet HD189733b using high spectral resolution observations of several hydrogen Balmerlines. The line shape of the pre-transit feature and the shape of the timeseries absorption provide the strongest constraints on the morphology andphysical characteristics of extended structures around an exoplanet. Thein-transit measurements confirm the previous exospheric H-alpha detectionalthough the absorption depth measured here is ~50% lower. The pre-transitabsorption feature occurs 125 minutes before the predicted optical transit, aprojected linear distance from the planet to the stellar disk of 7.2 planetaryradii. The absorption strength observed in the Balmer lines indicates anoptically thick, but physically small, geometry. We model this signal as theearly ingress of a planetary bow shock. If the bow shock is mediated by aplanetary magnetosphere, the large standoff distance derived from the modelsuggests a large equatorial planetary magnetic field strength of 28 G. Betterknowledge of exoplanet magnetic field strengths is crucial to understanding therole these fields play in planetary evolution and the potential development oflife on planets in the habitable zone.

Optical hydrogen absorption consistent with a thin bow shock leading the hot Jupiter HD 189733b