Detecing Exoplanet's Magnetic Fields

Astronomers may soon be able to observe the shockwaves between the magnetic fields of exoplanets and the flow of particles from the stars they orbit.

Magnetic fields are crucial to a planet’s (and as it turns out a moon’s) habitability. They act as protective bubbles, preventing harmful space radiation from stripping away the object’s atmosphere entirely and even reaching the surface.

An extended magnetic field – known as a planetary magnetosphere – is created by the shock between the stellar wind and the intrinsic magnetic field of the planet. It has the potential to be huge. Within our own Solar System, Jupiter’s magnetosphere extends to distances up to 50 times the size of the planet itself, nearly reaching Saturn’s orbit.

When the wind of high-energy particles from the star hits the planetary magnetosphere, it interacts in a bow shock that diverts the wind and compresses the magnetosphere.

Recently a team of astronomers, led by PhD student Joe Llama of the University of St. Andrews, Scotland, have worked out how we might observe planetary magnetospheres and stellar winds via their bow shocks.

Llama took a careful look at the planet HD 189733b, located 63 light years away toward the constellation Vulpecula. From the Earth, the planet is seen to transit its host star every 2.2 days, causing a dip in the overall light from the system.

As a bright star, HD 189733b has been studied extensively by astronomers. Data collected in July 2008 by the Canada-France-Hawaii telescope mapped the star’s magnetic field. While the magnetic field varied, it was on average 30 times greater than that of our Sun – meaning that the stellar wind is much higher than the solar wind.

This allowed the team to carry out extensive simulations of the stellar wind around HD 189733b – characterizing the bow shock created as the planet’s magnetosphere passes through the stellar wind. With this information they were able to simulate the light curves that would result from the planet and the bow shock orbiting the star.

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