July 5, 2013
How Stellar Magnetic Fields Affect The Chances Of Life Beyond Earth
John P. Millis, PhD for redOrbit.com - Your Universe Online
The presence of an atmosphere - among many other factors - is vital for the evolution of life as we know it on a planet. However, this seemingly simple requirement is bathed in a multitude of variables that can affect its creation and existence. Thus, establishing an atmosphere is by no means a trivial detail.
Scientists believe mechanisms such as volcanic eruptions, other tectonic activity and even comet impacts could all play a role in creating an atmosphere on a planet. But once in place, maintaining it is another problem all together.
First of all, the planet must have sufficient gravity to prevent the lighter elements from escaping into outer space. There must also be mechanisms for replenishment and recycling in place. And, finally, the delicate atmosphere will usually require protection from cosmic radiation, which can easily destroy it.
In the case of this last point, the presence and geometry of a planetary magnetic field is important. As charged particles - primarily originating from the stellar wind of its parent star - stream toward the planet, the shell of a magnetic field known as the magnetosphere that surrounds it will deflect, trap and otherwise divert them from the atmosphere. Of course, some charges will still penetrate this shield of sorts, but overall the catastrophic effects will be mitigated.
At least this is how it works on Earth. In other systems where the planet is too close to the star, the planetary magnetic field is too weak, or the stellar wind too strong, the charged particles streaming from the star can completely erode, leaving no chance for life to exist on the surface.
Earlier this week, new research from St. Andrew's University revealed red dwarf stars in particular could be potential threats to life because of their close habitable zones and high-strength magnetic fields. In general, however, it has been difficult to tell how exactly the interaction between stellar and planetary magnetic fields evolves.
Following up on this work, the St. Andrew's team on Thursday released new results indicating scientists might soon be able to observe the shockwaves created between planet and star as the stellar wind compresses the planetary magnetic field. Doctoral candidate Joe Llama demonstrates that the shock waves created from the magnetic field interactions can affect the dips in light that astronomers observe when a planet passes in front of a star.
Known as the transit method, this observational tool has been used to study many of the worlds found outside of our solar system. Now, using simulations of stellar wind interactions of the nearby system HD 189733, located 63 light-years away in the direction of the constellation of Vulpecula, Llama and his colleagues have found the magnitude of the observed dip and brightness will vary significantly as the planet passes through different densities of stellar wind, which themselves can have a significant gradient over a single orbital period.
The work reveals that if any Earth-like planets existed around the star in the habitable zone - where the energy from the star is just right for liquid water to persist on the surface - that the atmosphere would be completely demolished by the stellar wind.
"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," says Llama. "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."
Llama presented the new model this week at the Royal Astronomical Society's National Astronomy Meeting in St Andrews, Scotland.