Astronomers have discovered a lot of exoplanets—nearly 5000, according to NASA. Now that so many distant worlds have been identified, researchers are studying how those planets may have formed and whether they could harbor life. (For more on the search for life on exoplanets, see the article by Mario Livio and Joe Silk, Physics Today, March 2017, page 50.) A critical step toward those research goals is determining the exoplanets’ interior structure (see Physics Today, March 2019, page 24).
Earth’s core is mostly iron, and that’s expected to be true for other terrestrial planets too. The swirling liquid-iron outer core of Earth generates the planet’s magnetosphere, which protects us and every other organism from genetic damage due to cosmic radiation. A better understanding of the melting curve of iron at the temperatures and pressures of planetary interiors may help identify the kinds of planets that have magnetospheres.
Of course, finding that an exoplanet has a life-preserving magnetosphere doesn’t necessarily mean there’s life there. And potential life beyond Earth may not even need a planetary magnetosphere to survive. Still, understanding whether certain planets have magnetospheres would be an important data point for evaluating those worlds’ evolution and potential habitability.
The search for planetary magnetospheres, however, has been hampered by a lack of experimental data on the pressure–temperature melting curve of iron. Until recently the record-high-pressure measurement, accomplished by using diamond anvil cells heated with a laser, was taken at 290 GPa, lower than the 330 GPa found at the boundary between Earth’s solid inner core and the liquid-iron outer core. That mismatch means that to study the melt curve and phase space of iron, researchers have had to extrapolate by about an order of magnitude to the pressures expected in some terrestrial planet interiors.
That pressure limit has now been overcome. Using the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory in California, Richard Kraus and his colleagues have determined the melting curve for iron at up to 1000 GPa. From their results, the researchers predict that planets with masses four to six times that of Earth can sustain magnetospheres for the longest time, approximately eight billion years.
Sébastien Merkel, a geophysicist at the University of Lille in France, is enthusiastic about the experimental advance. “The melting curve of iron has been debated for decades,” he says. “This paper and the data it contains anchor the melting curve of iron at pressures well above previous work.”
Competing models
With only low-pressure iron data to work with, conclusions of researchers studying the interiors of exoplanets “have varied dramatically,” says Kraus. One model predicts that the outer core of a planet smaller than or similar to Earth in size would be composed of liquid iron. Larger planets would have a completely solid core. But another model predicts the exact opposite. And yet another model—which focuses on super-Earths whose masses are several times that of Earth—finds that iron crystallization begins at the outer core and precipitates toward the center as iron snow.
To break the impasse among the various competing models, Kraus and some of his collaborators emulated the pressure conditions at the center of a super-Earth core using light sources at NIF, the largest laser facility in the world. They combined 16 laser beams to create a coherent shock wave that passed through a 2 mm2 iron sample. The ensuing compression and decompression of the sample melted it to a liquid phase. Then the laser intensity was dialed up in precise increments to increase the pressure applied to the sample, up to 1000 GPa in about 10 ns.
Kraus and colleagues compressed the sample incrementally so that the thermodynamic path on which the sample traveled was nearly isentropic—that is, the increase in pressure added negligible entropy to the sample. That detail matters because no appreciable heat flow or work done by viscous forces modified the pressure–entropy phase measurements of the experiment.
During the experiments, an additional 24 laser beams energized a germanium or zirconium foil, which produced a hot plasma. The plasma emitted x rays that were then directed to the iron sample. The resulting x-ray diffraction pattern was used to determine whether the iron solidified at various pressure states and, if so, to identify its atomic structure.
Bottom-up solidification
The P–S (pressure–entropy) experimental results indicated that the solidification of iron is pressure driven, which implies that exoplanet cores will solidify from the bottom up. “I was surprised we actually saw solidification,” Kraus says. “Before this experiment, many experts thought you could not dynamically solidify any material on a nanosecond time scale.” He argues that the small amount of impurities in their high-purity iron sample and the 10 000 °C temperature maximum helped to accelerate the kinetics of solidification.
To better compare the measurements with other results, the researchers transformed their P–S data into a P–T (pressure–temperature) phase diagram. They found that the melting temperature of iron at Earth’s inner-core boundary is 6230 K, which is about the same, within the uncertainties, as temperature estimates obtained from extrapolations of previous lower-pressure experiments.
The results, plotted in the figure below with previous lower-pressure research, support the idea that large exoplanets with a compositional structure similar to Earth’s may have bottom-up core solidification. That mechanism would allow those planets to have liquid-iron outer cores and potentially magnetospheres.
From the new data, the researchers infer that core solidification in super-Earths would take 30% longer than for Earth because of the additional heat that must dissipate. (The model estimates that Earth’s core would take 6 billion years to cool.) That means that the liquid core would stick around longer and thus provide a prolonged period of protection from cosmic radiation. To reach that inference, the researchers used their measurement of the entropy change along the melt curve and assumed that the heat flow out of the core scales as a function of planet size.
Most of the work on core solidification has focused on iron. But Earth’s core also has nickel. Other planetary cores may have nickel, too, and perhaps other metals. “We’re starting with pure iron to provide a baseline of fundamental information,” says Kraus. “People can take this information and say, based on the composition of a super-Earth, how do melting curves change with alloying effects?”
