Scientists blast iron with lasers to study the cores of rocky exoplanets

By blasting a small iron sample with high-powered lasers at the Lawrence Livermore National Laboratory, scientists can replicate the extreme pressure and density conditions found inside the cores of large, rocky exoplanets.

The experiments have offered scientists unique insights into the core conditions found inside faraway super-Earths.

"The discovery of large numbers of planets outside our solar system has been one of the most exciting scientific discoveries of this generation," Ray Smith, a physicist at LLNL, said in a news release. "These discoveries raise fundamental questions."

"What are the different types of extrasolar planets and how do they form and evolve?" Smith said. "Which of these objects can potentially sustain surface conditions suitable for life? To address such questions, it is necessary to understand the composition and interior structure of these objects."

Of the more than 4,000 confirmed and candidate exoplanets discovered by Kepler and other planet-hunters, the largest percentage are so-called super-Earths, rocky planets with a radius between and one and four times that of Earth.

"Determining the interior structure and composition of these super-Earth planets is challenging but is crucial to understanding the diversity and evolution of planetary systems within our galaxy," Smith said.

The larger the rocky exoplanet, the more intense the pressure found inside its core. Because iron is the most abundant compositional element inside super-Earths, scientists set out to study its properties under extreme pressure.

Scientists used high-powered lasers and ramp compression techniques to replicate the extreme conditions. The laser at LLNL's National Ignition Facility can deliver 2 megajoules of laser energy over 30 nanoseconds, enough to compress the iron sample to 1.4 TPa, with a single TPa equaling 10 million atmospheres. That is four times the pressure achieved during previous experiments and the equivalent of the pressure found inside a rocky exoplanet with three to four times the mass of Earth.

Researchers described the experiments this week in the journal Nature Astronomy.

"Planetary interior models, which rely on a description of constituent materials under extreme pressures, are commonly based on extrapolations of low-pressure data and produce a wide range of predicated material states," Smith said. "Our experimental data provides a firmer basis for establishing the properties of a super-Earth planet with a pure iron planet."

"Furthermore, our study demonstrates the capability for determination of equations of state and other key thermodynamic properties of planetary core materials at pressures well beyond those of conventional static techniques," he said. "Such information is crucial for advancing our understanding of the structure and dynamics of large rocky exoplanets and their evolution."

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