Artist’s illustration of the potentially life-supporting “super-Earth” planet K2-18b (upper right) and its recently discovered neighbor, K2-18c. Both planets orbit in the habitable zone of the red-dwarf star K2-18, which lies 111 light-years from Earth in the constellation Leo. K2-18b, about 2.2 times bigger than Earth, was discovered in 2015 by researchers using NASA’s Kepler space telescope. Credit: Alex Boersma
(Download Image)Squeezing Iron for Clues About ‘Super-Earth’ Cores
Iron-compression experiments on NIF have provided new insights into the possible interior structure and composition of large, rocky exoplanets known as "super-Earths." The findings could help in developing new models of planetary interiors.
The research by a team of scientists from LLNL, Princeton University, Johns Hopkins University, and the University of Rochester was reported in a Nature Astronomy cover article published online on April 16.
"The discovery of large numbers of planets outside our solar system has been one of the most exciting scientific discoveries of this generation," said LLNL physicist Ray Smith, lead author of the paper. "These discoveries raise fundamental questions: What are the different types of extrasolar planets and how do they form and evolve? 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 extrasolar planets, those that are one to four times the radius of Earth are now known to be the most abundant. "Determining the interior structure and composition of these super-Earth planets is challenging," Smith said, "but is crucial to understanding the diversity and evolution of planetary systems within our galaxy."
As core pressures for a planet with five times the mass of Earth can reach as high as 20 million atmospheres, a fundamental requirement for understanding exoplanetary composition and interior structure is an accurate determination of the material properties at these extreme pressures. Iron is an abundant element in the universe and, as the dominant constituent of terrestrial planetary cores, is a key material for studying super-Earth interiors. A detailed understanding of the properties of iron at super-Earth conditions is an essential component of the team’s experiments.
In their paper, the researchers describe a new generation of high-power laser experiments which use ramp, or shockless, compression techniques to provide the first absolute equation-of-state measurements of iron at the extreme pressure and density conditions found within super-Earth cores. Shock-free dynamic compression is uniquely suited for compressing matter with minimal heating to terapascal (TPa) pressures (1 TPa=10 million atmospheres).
Interpreting Observational Data
The experiments produced the first experimentally based mass-radius relationship for a hypothetical pure iron planet at super-Earth core conditions. The discovery can form the basis for future planetary interior models, which in turn can be used to more accurately interpret observational data from the Kepler space telescope and aid in identifying planets suitable for habitability.
NIF, the world’s largest and highest-energy laser, can deliver up to two million joules of laser energy over 30 nanoseconds (billionths of a second) and can provide the necessary laser power and control to ramp-compress materials to TPa pressures. The team’s experiments reached peak pressures of 1.4 TPa, four times higher than previous static results, representing core conditions found in a planet with three to four times the mass of Earth.
"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 core.
"Furthermore," he added, "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. Such information is crucial for advancing our understanding of the structure and dynamics of large rocky exoplanets and their evolution."
Future experiments on NIF will extend the study of planetary materials to several TPa while combining nanosecond x-ray diffraction techniques to determine crystal structure evolution as pressure increases.
Smith was joined on the paper by LLNL colleagues Dayne Fratanduono, David Braun, Peter Celliers, Suzanne Ali, Amalia Fernandez-Pañella, Richard Kraus, Damian Swift and Jon Eggert, along with Thomas Duffy from Princeton University, June Wicks from Johns Hopkins University, and Rip Collins from the Laboratory for Laser Energetics at the University of Rochester.
