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NAM 2013: Small stars could magnetically bully planets

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.

An artist's impression of a planet losing its atmosphere to a stellar wind, the planet's magnetosphere pounded into the ground by the onslaught of the star’s magnetic field. Image: NASA.
A planet's magnetic field is essential for holding onto a planet's water and the bulk of its atmosphere. Recently it has been shown in our Solar System that Earth is able to retain its water vapour thanks to the protective barrier of its magnetosphere, which extends 70,000 kilometres out into space. Venus and Mars, on the other hand, lack a magnetic field and both have seen their water stripped away by the abrasive solar wind.

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.

An artist's impression of a red dwarf, with an orbiting planet transiting against it. Image: ESO/L Calçada.
There may be two get-out clauses for such unfortunate planets. One is that a planet so close to its star will inevitably be tidally locked, meaning it always shows the same face to its star. Vidotto’s team have only considered the compression of the planetary magnetic field on the star-facing dayside of the planet, whereas the nightside that always faces away, or near the terminator, could potentially be left relatively undisturbed.

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!"