Detecting the Magnetic Fields of Exoplanets May Help Determine Habitability

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.

The bow shock leads the planet – causing the light to drop a little earlier than expected.  The amount of light blocked by the bow shock, however, will change as the planet moves through a variable stellar wind. If the stellar wind is particularly strong, the resulting bow shock will be strong, and the transit depth will be greater. If the stellar wind is weak, the resulting bow shock will be weak, and the transit depth will be less.

The video below shows the light curve of a bow shock and exoplanet.

“We found that the shockwave between the stellar and planetary magnetic fields will change drastically as activity on the star varies,” Llama told Universe Today. “As the planet passes through very dense regions of the stellar wind, so the shock will become denser, the material in it will block more light and therefore cause a larger dip in the transit making it more detectable.”

While there were no transit observations for this study, this theoretical outlook demonstrates that it will be possible to detect the bow shock, and therefore the magnetic field, of a distant exoplanet. Dr. Llama comments: “This will help us to better identify potentially habitable worlds.”

The paper has been accepted for publication in Monthly Notices of  The Royal Astronomical Society and is available for download here.


Shannon Hall

Shannon Hall is a freelance science journalist. She holds two B.A.'s from Whitman College in physics-astronomy and philosophy, and an M.S. in astronomy from the University of Wyoming. Currently, she is working toward a second M.S. from NYU's Science, Health and Environmental Reporting program. You can follow her on Twitter @ShannonWHall.

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