NAM 2013: Small stars could magnetically bully planets
BY KEITH COOPER
Posted: 1 July 2013
Red dwarf stars may be able to literally bully the magnetic fields of orbiting planets into the ground, according to new research being presented at the Royal Astronomical Society's National Astronomy Meeting taking place at the University of St Andrews this week.
A combination of factors conspire against planets orbiting close to red dwarf stars. First, red dwarf stars have particularly strong magnetic fields (and sport frequent activity such as star-spots and flares) and second, their planetary systems are scaled down compared to our own Solar System, as would be expected for smaller, cooler stars. This means their planets are much close to the star, as is the habitable zone where temperatures are suitable for liquid water. For example, the habitable zone around the red dwarf star Gliese 667C, where three super-earths were recently found, extends from around 15 million kilometres to 37 million kilometres. In contrast, the Earth is 149.6 million kilometres distant from the Sun on average. However, being so close to their star means that the red dwarf's powerful magnetic field could have a dramatic effect on the planets.
Aline Vidotto of the University of St Andrews led a team of researchers who have calculated that the pressure of a red dwarf's magnetic field, carried on a stellar wind of charged particles like the solar wind, could compress the magnetic field of a close orbiting planet right down to the ground, or even below the surface. Without the magnetic protection, the wind would be free to strip away the lighter elements of the atmosphere, depending on the ability of the planet’s gravity to hold onto its atmosphere.
The second get-out is age. A star's magnetic dynamo is linked to its rotation and, as the speed of a star's rotation decrease with age, its magnetic field correspondingly weakens. "The dynamo that operates inside a star is what generates its magnetic field and its magnetosphere," Vidotto tells Astronomy Now. "It is known that the faster a star rotates - in particular for a red dwarf star - the more active it is and therefore the more intense the magnetic fields it can generate."
Vidotto estimates that stars which rotate once every few months won’t have strong enough magnetic fields to impinge on their planet's magnetospheres, meaning older stars could be more hospitable. Perhaps habitable conditions get off to a late start on these worlds - after all, red dwarfs have life expectancies of trillions of years, so there's no rush. They may also have a saving grace - should life somehow manage to arise on one of these magnetically battered worlds, it would be treated to some wonderful auroral lights.
"We have estimated that the size of the auroral oval of the planets orbiting around the red dwarfs could be significantly larger than the size of the auroral oval of the Earth," says Vidotto. "This means that aurora would not be restricted to high latitudes, but could even be seen close to the equatorial region!"