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New research on carbon cracks open secrets deep inside exoplanets

An artist’s rendering of 55 Cancri e, an exoplanet rich in carbon. For the first time in a laboratory setting, researchers—including the University of Rochester's Gilbert (Rip) Collins and Ryan Rygg—have achieved extreme pressures that help them to understand the structure of carbon that sits in the interior of carbon-rich exoplanets like 55 Cancri e. (NASA/JPL-CalTech)
Measuring carbon at the highest pressures ever achieved in a laboratory, researchers report first model of carbon structures that may make up planets outside the solar system.

Carbon is one of the most prevalent elements in existence. As the fourth most abundant element in the universe, it’s a building block for all known life and forms the interior of carbon-rich exoplanets.

Decades of research has shown that carbon’s crystal structure has a significant impact on a material’s properties. In addition to graphite and diamond—the most common carbon structures found at ambient pressures—scientists have predicted that there are several new structures of carbon that could be found at pressures above 1,000 gigapascals (GPa). The pressures, which are approximately 2.5 times the pressure in Earth’s core, are important for studying and modeling the interiors of exoplanets. However, it has historically been difficult to achieve such pressures in a laboratory setting and impossible to determine the structure of matter under those pressures. 

That is, until now. 

An international team of researchers, including researchers at the University of Rochester’s Laboratory for Laser Energetics (LLE), has successfully measured carbon at pressures reaching 2,000 GPa (five times the pressure in Earth’s core), nearly doubling the maximum pressure at which carbon’s crystal structure has ever been directly probed. Their results were published in the journal Nature.

“This is the highest pressure any atomic structure has been measured, placing key constraints on the equation of state, material strength, melting, and chemical bonding of carbon,” says Gilbert (Rip) Collins, the Tracy Hyde Harris Professor of Mechanical Engineering and associate director of science, technology, and academics at the LLE. “In our studies of the many recently discovered and yet-to-be discovered massive, carbon-rich planets, we will have to consider the diamond structure of carbon at pressures well beyond its predicted stability range.”

The research team, which was led by scientists from Lawrence Livermore National Laboratory (LLNL) and the University of Oxford, compressed solid carbon to 2,000 GPa using ramp-shaped laser pulses, simultaneously measuring the crystal structure using an X-ray diffraction platform to capture a nanosecond-duration snapshot of the atomic lattice. The experiments nearly double the record high pressure at which X-ray diffraction has been recorded on any material.

The researchers found that even when subjected to the intense conditions, solid carbon retains its diamond structure, far beyond its range of predicted stability. The findings indicate that the strength of the molecular bonds in diamond persists even under enormous pressure, resulting in large energy barriers that hinder carbon’s conversion to other possible structures. 

“The diamond phase of carbon appears to be the most stubborn structure ever explored,” says Ryan Rygg, an assistant professor of mechanical engineering and of physics and a senior scientist at the LLE. “This could have implications for carbon in the deep interiors of planets, where the precipitation of diamond is expected. Now we anticipate the diamond structure of carbon will persist over a much greater range of planetary conditions than we previously thought.”

The collaboration and the suite of capabilities available at Rochester’s Laser Lab, the largest US Department of Energy university-based research program in the nation, and at Lawrence Livermore’s National Ignition Facility has in part led to the recently awarded Center for Matter at Atomic Pressures, hosted by the University of Rochester. The center, which is the first major initiative from the National Science Foundation in the field of high-energy-density science, focuses on understanding the physics and astrophysical implications of matter under pressures so high that the structure of individual atoms is disrupted.

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