Deadly Stellar Winds Could Put a Stop to Life

Illustration of how material ejected from the Sun (on the left) interacts with the magnetic field of the Earth (on the right). The white lines represent the solar wind; the purple line is the bow shock line and the blue lines surrounding the Earth represent its protective magnetosphere. Credit: NASA

Scientists could soon be able to observe the shockwaves between the magnetic fields of planets and the flow of charged particles from the stars they orbit, according to an astronomer at the University of St. Andrews. The fields (magnetospheres) shield the atmospheres of planets and are thought to be crucial to the development of life.

Now PhD student Joe Llama has worked out how the shock waves affect the dip in light seen from Earth as the planet passes in front of its parent star. He presented the new model on 5 July at the National Astronomy Meeting in St. Andrews, Scotland.

Since the early 1990s astronomers and space scientists have found almost 900 planets in orbit around other stars, most of which are far larger than the Earth and far closer to their stars than the Earth is to the Sun. These giant planets are known as ‘hot Jupiters’ because of their sizes and high atmospheric temperatures, in some cases several thousand degrees.

Hot Jupiters have magnetospheres that help protect them to some extent from the high-energy particles that flow from their stars (so-called stellar winds). When the wind hits the planetary magnetospheres they interact in a ‘bow shock’ that diverts the wind and compresses the magnetic field.

Joe Llama looked at earlier work on the planet orbiting the orange dwarf star HD 189733, located 63 light-years away in the direction of the stars of the constellation Vulpecula. From Earth the planet, HD 189733b, blocks the light of the star every two days, an event known as a transit, causing a dip in its brightness. Measuring the way the light drops over time gives scientists a lot of information on the planet and star, including their sizes and the makeup of their atmospheres.

As a relatively bright star, HD 189733 has been studied extensively by astronomers. Magnetic imaging of the star has revealed that the star has a magnetic field approximately thirty times stronger than that of our Sun. The presence of such a strong magnetic field means that the stellar wind is likely to be much more powerful than the solar wind. The scientists at the University of St. Andrews have also carried out simulations of the stellar wind around HD 189733 using the magnetic maps to examine how dangerous this wind is likely to be for the planet.

An artist’s impression of the hot Jupiter planet in orbit around the star HD 189733. Credit: ESA – C. Carreau

Looking at the predicted transits in more detail, Joe and his colleagues have found that the shockwave between the stellar and planetary magnetic fields will change drastically as activity on the star varies. 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.

Their simulations show that the conditions the planet sees as it orbits around its star are going to change dramatically during a single orbit around the star. They find that the magnetosphere, which protects the planet from the radiation of the stellar wind, will experience huge changes in size and orientation which could have devastating consequences for the atmosphere of the planet.

In the case of HD 189733b, this is not a huge problem as it and other hot Jupiters are already far too hostile for life to survive. But strong stellar winds could also strip away the atmospheres of potentially habitable planets further out, something that would have dire consequences for their habitability. Joe comments: “Imagine what the Earth would be like with its air stripped away, placed in a radiation bath. There could be numerous planets like this that in many ways resemble our world, but where life never stood a chance.”

If astronomers look at the way in which the light curves of transits change, they should be able to observe the varying stellar winds and their effects, helping to better identify potentially habitable worlds. Joe concludes: “For more than two decades we have been stepping up the search for other planets like the Earth. Our new work will help refine this quest, enabling us to rule out the sites where dangerous activity on stars would kill off life from the start.”


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