Rocky planets a few times heavier than Earth that were thought might be life-friendly may lack a protective magnetic field that originates from an iron core that is at least partly molten.
A simulation of super-Earths between a few times and 10 times Earth's mass suggests that high pressures will keep the core solid, according to Guillaume Morard of the Institute of Mineralogy and Physics of Condensed Matter in Paris, France. Without a magnetic field, the planets would be bathed in harmful radiation, and their atmospheres would be eroded away by particles streaming from their stars.
The present-day Mars is a prefect example of a planet that lost its magnetic field. Planetary magnetic fields are created by massive molten metal currents within the planet's core. A flowing current creates a magnetic field, even when the current is massive volumes of charged liquid metal moving under the influence of temperature gradients (convection). But magnetic analysis of Martian sites by Berkeley researchers show that the red planet's protective field was switched off half a billion years ago.
Without the magnetic field Mars and perhaps Super Earths in the Milky Way are defenseless against the radiation that constantly pours in from space. Earth is thought to have survived the same space-bombing because of our superior size, with our dynamo maybe stuttering a little but - very importantly - not stopping.
So life would have trouble getting started on super-Earths, even if they lie in the habitable zone around their stars.
However, Vlada Stamankovic of the German Aerospace Center in Berlin reckoned it is too soon to rule out molten iron cores - and magnetic fields - for super-Earths. Their interiors might get hot enough to melt iron, he says. "Actual temperatures could be much larger than assumed - we simply do not know."
About 2,000 exoplanets are now known, but most of these have been hot giants circling too close to their stars, because they are easiest to find. As techniques have been refined, smaller planets have come into view, and a recent survey by University of California astronomers, Andrew Howard and Geoffrey Marcy, concluded that about a quarter of all Sun-like stars should have Earth-size planets.
Analysis of the Kepler Mission "400," may reveal Earth-size planets as well as the existence of magnetic fields and the possibility of life-bearing habitats.
A super-Earth like the one in the artist's conception at the top of the page can grow twice as large as Earth with up to 10 times the mass.
Image credit: David A. Aguilar (Harvard-Smithsonian CfA)