Not so super for lifeforms (Image: SOHO/NASA/ESA)

ROCKY planets a few times heavier than Earth that we thought might be life-friendly may lack one vital feature: a protective magnetic field.

Planets are thought to owe their magnetic fields to an iron core that is at least partly molten. But 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, and his team (arxiv.org/1010.5133).

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. 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 reckons 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.”