Bow Shock Leads the Way for a Speeding Hot Jupiter

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As hot Jupiters whip around their host stars, their speeds can exceed the speed of sound in the surrounding material, theoretically causing a shock to form ahead of them. Now, a study has reported the detection of such a shock ahead of transiting exoplanet HD 189733b, providing a potential indicator of the remarkably strong magnetic field of the planet.

Rushing Planets

Due to their proximity to their hosts, hot Jupiters move very quickly through the stellar wind and corona surrounding the star. When this motion is supersonic, the material ahead of the planet can be compressed by a bow shock — and for a transiting hot Jupiter, this shock will cross the face of the host star in advance of the planet’s transit.

In a recent study, a team of researchers by Wilson Cauley of Wesleyan University report evidence of just such a pre-transit. The team’s target is exoplanet HD 189733b, one of the closest hot Jupiters to our solar system. When the authors examined high-resolution transmission spectra of this system, they found that prior to the optical transit of the planet, there was a large dip in the transmission of the first three hydrogen Balmer lines. This could well be the absorption of an optically-thick bow shock as it moves past the face of the star.

Tremendous Magnetism

Operating under this assumption, the authors create a model of the absorption expected from a hot Jupiter transiting with a bow shock ahead of it. Using this model, they show that a shock leading the planet at a distance of 12.75 times the planet’s radius reproduces the key features of the transmission spectrum.

This stand-off distance is surprisingly large. Assuming that the location of the bow shock is set by the point where the planet’s magnetospheric pressure balances the pressure of the stellar wind or corona that it passes through, the planetary magnetic field would have to be at least 28 Gauss. This is seven times the strength of Jupiter’s magnetic field!

Understanding the magnetic fields of exoplanets is important for modeling their interiors, their mass loss rates, and their interactions with their host stars. Current models of exoplanets often assume low-value fields similar to those of planets within our solar system. But if the field strength estimated for HD 189733b’s field is common for hot Jupiters, it may be time to update our models!

Bonus

Check out this video from Cauley’s website, which provides an action view of the transit data for HD 189733b and the possible bow shock leading it. The upper panel shows the transit as viewed from the side, the right panel shows a top-down view of the orbit, and the plot shows the transmission data (points) and model (solid lines) for the three hydrogen lines monitored. All sizes and distances are to scale.

Citation

P. Wilson Cauley et al 2015 ApJ 810 13. doi:10.1088/0004-637X/810/1/13

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