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

TL;DR

This study detects and models a pre-transit hydrogen absorption feature around exoplanet HD 189733b, consistent with a bow shock caused by the planet's magnetic field, providing insights into exoplanet magnetospheres.

Contribution

First robust detection of a pre-transit hydrogen absorption signature around HD 189733b, modeled as a planetary bow shock indicating a strong magnetic field.

Findings

Pre-transit absorption occurs 125 minutes before optical transit.

Absorption depth is ~50% lower than previous measurements.

Model suggests a planetary magnetic field strength of 28 G.

Abstract

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.