The study describes a new generation of high-power laser experiments that provide the first absolute equation of state for iron under extreme pressure and density conditions.
A group of researchers from the Livermore National Laboratory. Lawrence (LLNL), Princeton University, Johns Hopkins University and the University of Rochester (USA) for the first time experimentally determined the mass-radius dependence of a hypothetical metal planet with the properties of a super-Earth core. The work of scientists is presented in the journal Nature Astronomy.
“The discovery of a large number of planets outside the solar system was one of the most exciting scientific discoveries of this generation. These studies raise fundamental questions. What are the different types of extrasolar planets, and how do they form and evolve? Which of these objects can maintain acceptable living conditions on the surface? To address these issues, you need to understand the composition and internal structure of these objects,”says Ray Smith, physicist at LLNL and lead author of the study.
The results can be used to estimate the composition of large rocky exoplanets, forming the basis for future models of planetary depths, which, in turn, can be used to more accurately interpret observational data from the Kepler space mission and help determine habitable planets.
It is known that of more than 4,000 exoplanets and candidates for this role, the most common are those that exceed the Earth's radius by 1-4 times. Such extrasolar worlds are not represented in our system. This indicates that planets are forming in a wider range of physical conditions than previously thought. Determining the internal structure and composition of super-Earths is challenging, but critical to understanding the diversity and evolution of planetary systems in our galaxy.
Since the pressure in the core of an exoplanet 5 times the mass of Earth can reach two million atmospheres, a fundamental requirement for limiting the composition of an exoplanet and its internal structure is to accurately determine the properties of the material under extreme pressure. Iron is the dominant component of the planetary cores of earthlike planets. A detailed understanding of the properties of iron in super-earth conditions became a major challenge in the research of Ray Smith's team.
Scientists have described a new generation of powerful laser experiments that provide the first absolute equation of state for iron under extreme pressure and density conditions in the super-Earth's core. The method is suitable for compressing matter with minimal heating up to a pressure of 1 terapascal (1 TPa = 10 million atmospheres).
Recreation of the core of the super-earth in the NIF camera as seen by the artist. Credit: Mark Meamber (NIF).
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The experiments were carried out at the LLNL National Ignition Complex (NIF). The NIF, the world's largest and most powerful laser, can deliver up to 2 megajoules of laser energy in 30 nanoseconds and deliver the required laser power and control of material compression up to TPa pressures. The team's experiments reached a peak pressure of 1.4 TPa, four times the pressure of the previous static results, which described the basic conditions of a super-earth 3-4 times the mass of Earth.
“Internal planetary device models based on the description of composite materials at extreme pressures typically extrapolate low pressure data and create a wide range of possible material states. Our experimental data provide a solid foundation for determining the properties of a super-earth and a hypothetical metal planet. In addition, the study demonstrates the ability to determine equations of state and other key thermodynamic properties of planetary core materials at pressures well above conventional static methods. Such information is critical to gaining an understanding of the structure of large rocky exoplanets and their evolution,”says Ray Smith.
Future NIF experiments will expand the study of materials under several TPa pressures by combining nanosecond X-ray diffraction techniques to determine the evolution of crystal structure as a function of pressure.
Arina Vasilieva