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WASP-12b: A Magnetic Bow Shock

A number of interesting things are coming out of the Royal Astronomical Society’s now convening meeting in Llandudno, Wales, many of them still embargoed, though we’ll be able to discuss them later in the week. But among the papers now open for discussion, I was drawn to work by Aline Vidotto and colleagues at the University of St. Andrews. Vidotto has been working with the exoplanet WASP-12b, a ‘hot Jupiter’ discovered in transit by the wide-field cameras of the SuperWASP project (WASP stands for Wide Angle Search for Planets). The work focuses on how a planetary ‘bow shock’ can protect an exoplanet’s atmosphere from emissions from its host star.

For the new evidence Vidotto and team are discussing at Llandudno shows that there are signs of a magnetosphere around WASP-12b. Discovered in 2008, this ‘hot Jupiter’ is one of the largest exoplanets yet found, more than 250,000 kilometers in diameter. It’s also an extremely hot planet, orbiting the star designated WASP-12 every 26 hours, at a distance of no more than 3.4 million kilometers (0.0229 AU). Violent interactions between star and planet are inescapable. Vidotto’s team used data from the Hubble Space Telescope to analyze variations in ultraviolet and visible light that can be explained through the effects of a planetary magnetosphere.

WASP-12b bow shock simulation from Joe Llama on Vimeo.

Image: Simulation of a planet and bow shock transiting a limb-darkened star. The parameters have been tuned to be representative of the WASP-12 system. Credit: Joe Llama/University of St. Andrews.

What astronomers had thought to be a flow of material from the planet onto the star turns out to be better explained by bow shock. Close study of the planet has shown that the dip in the star’s light during a transit begins earlier in ultraviolet light than in visible light. Simulations of the planet and its bow shock can reproduce the ultraviolet dip, a window into the interactions between a planetary magnetic field and the magnetic field of the host star. The fact that we’re seeing the interactions in a magnetic field here means that WASP-12b has a conducting, rotating interior.

Bow shocks are helpful things indeed. The Earth’s magnetic bow shock protects us from the solar wind, and in WASP-12b’s case, we can assume that the bow shock offers some protection for the planet’s atmosphere from the charged, energized particles of its parent star, shielding it from erosion. And scientists wind up with a new and useful tool, one that lets us measure the strength of the magnetic field of a planet we can ‘see’ only through transit dips. These observations are anything but static. From the most recent paper on this work:

Observational follow-up suggests that the near-UV light curve presents temporal variations, which may indicate that the stand-off distance between the shock and the planet is varying. This implies that the size of the planet’s magnetosphere is adjusting itself in response to variations in the surrounding ambient medium.

Joe Llama (University of St. Andrews), the PhD student who ran the simulations of the bow shock, notes the significance of the finding:

“Our models are able to reproduce the data from the Hubble Space telescope for a range of wind speeds implying that bow shocks could be far more commonplace than had been thought… Although our model predicts a bow shock similar to that of the Earth, we are not expecting any messages from WASP-12b as it is too hot to support life. But the first hints that extrasolar planets have magnetosphere is a big step forward in understanding and identifying the habitable zones where we ultimately hope to find signs of life.”

The paper is Vidotto, “Transit Variability in Bow Shock-Hosting Planets,” Monthly Notices of the Royal Astronomical Society, published online 6 April 2011 (abstract / preprint). See also Vidotto et al., “Early UV Ingress in WASP-12b: Measuring Planetary Magnetic Fields,” Astrophysical Journal Letters Vol. 722, No. 2 (2010), L168 (abstract / preprint). A news release from the University of St. Andrews is also available.


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  • Adam April 18, 2011, 18:29

    Bow shocks can be formed around space-vehicles via artificial magnetospheres, to make drag, so they’re handy when one wants to slow down without propellant. Of course the caveat is that the magnetosphere has to be BIG to get significant drag.