Tuesday, June 21, 2016

Bow Shock of a Speeding Hot Jupiter

When a planet moves supersonically through the stellar wind or coronal plasma of its host star, a bow shock can form where the pressure between the plasma and the planet's magnetosphere balances out. This is generally ahead of the planet, in the direction of the planet's motion around its host star. If the planet has a strong magnetic field, the bow shock can be displaced several planetary radii ahead of the planet. Material from the stellar wind of the planet's host star can pile up at the bow shock. If the material is compressed sufficiently, the line-of-sight column density of the material in the bow shock can become high enough to generate a visible absorption signature. Furthermore, if the planet transits its host star, the visible absorption signature of the bow shock can be detected prior to the main transit event.

Figure 1: Artist’s impression of a hot Jupiter.

HD 189733b is a transiting hot Jupiter in a close-in orbit around its host star. It is 13 percent more massive than Jupiter and it orbits its host star once every 2.2 days. HD 189733b whizzes around its host star at 152.5 km/s. At this enormous speed, HD 189733b is moving supersonically through the stellar wind if its host star. Transit observations of the host star of HD 189733b revealed a pre-transit absorption feature occurring 125 minutes before the main transit event. This pre-transit absorption feature is consistent with the presence of a planetary bow shock that has a large standoff distance of 12.75 planetary radii from the planet itself. The large standoff distance is a clear indication of a very powerful magnetic field around the planet. In fact, the equatorial planetary magnetic field strength of HD 189733b is estimated to be 28 G. For comparison, the magnetic field strength on Earth is less than 1 G. The strong magnetic field around HD 189733b serves as the planet's first line of defence against the energetic stellar wind from its host star which can potentially erode the planet's atmosphere.

Figure 2: To-scale projections of the planet and bow shock in the orbital plane (top panel) and the view from Earth (bottom panel). Cauley et al. (2015)

Cauley et al. (2015), "Optical hydrogen absorption consistent with a thin bow shock leading the hot Jupiter HD 189733b", arXiv:1507.05916 [astro-ph.EP]