  Bow shocks are ubiquitous astrophysical phenomena resulting from the
supersonic passage of an object through a gas. Recently, pre-transit absorption
in UV metal transitions of the hot Jupiter exoplanets HD 189733b and WASP12-b
have been interpreted as being caused by material compressed in a planetary bow
shock. Here we present a robust detection of a time-resolved pre-transit, as
well as in-transit, absorption signature around the hot Jupiter exoplanet HD
189733b using high spectral resolution observations of several hydrogen Balmer
lines. The line shape of the pre-transit feature and the shape of the time
series absorption provide the strongest constraints on the morphology and
physical characteristics of extended structures around an exoplanet. The
in-transit measurements confirm the previous exospheric H-alpha detection
although the absorption depth measured here is ~50% lower. The pre-transit
absorption feature occurs 125 minutes before the predicted optical transit, a
projected linear distance from the planet to the stellar disk of 7.2 planetary
radii. The absorption strength observed in the Balmer lines indicates an
optically thick, but physically small, geometry. We model this signal as the
early ingress of a planetary bow shock. If the bow shock is mediated by a
planetary magnetosphere, the large standoff distance derived from the model
suggests a large equatorial planetary magnetic field strength of 28 G. Better
knowledge of exoplanet magnetic field strengths is crucial to understanding the
role these fields play in planetary evolution and the potential development of
life on planets in the habitable zone.
