Life on Earth is made possible by our planet’s magnetosphere – an invisible radiation shield that protects the surface – now it turns out that super-Earths could have magnetospheres too
The melting point of iron has been measured in conditions similar to those found in the cores of super-Earths, planets with masses several times that of our world.
A molten iron core is a feature of many planets, including our own. On Earth, the molten core is responsible for generating a magnetosphere: a spherical magnetic field that shields our planet from radiation and allows life on the surface to survive. Understanding the conditions under which iron melts, or stays solid, can tell us how likely it is that other types of planet will be similarly protected with a magnetosphere, and for how long.
Richard Kraus at the Lawrence Livermore National Laboratory, California, and his colleagues used one of the world’s most energetic lasers, at the lab’s National Ignition Facility, to recreate the pressures found at the centre of super-Earths. They then used diffracted X-rays to work out whether iron would be solid or liquid under these conditions.
“We mapped out the melting curve… to nearly four times greater pressure than anyone had studied before,” says Kraus. “Then we were able to address the question of how much heat a liquid iron core needs to lose in order for it to fully solidify.”
The melting temperatures that Kraus and his team measured suggest that planets four to six times the mass of Earth retain liquid metal cores for the longest – longer than Earth will. This means these super-Earths should have very long-lasting magnetospheres.
While the measurement of pure iron under such extreme conditions is useful for understanding exoplanets – and Earth’s own core – core impurities and confounding effects caused by a planet’s mantle will also have an impact on the strength and duration of an exoplanet’s magnetosphere, according to Guillaume Morard at the University of Grenoble Alpes, France.
“I think it’s a first step,” says Morard. “But to know exactly how the magnetic fields of these large planets work, there will need to be more modelling on what is going on inside the mantle of the planets.”
Journal reference: Science, DOI: 10.1126/science.abm1472
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