Recreating the Interior of Exoplanets with Laser-driven Dynamic Compression to Terapascal Pressures

Deep inside (exo)planets and in the aftermath of giant impacts, extreme density, pressure and temperature strongly modify the properties of the constituent materials. In conjunction with numerical simulations, experimental constraints on phase transformations and how they affect thermodynamic and transport properties at the extreme conditions are crucial to determine a planet’s internal structure and evolution.

Laser-driven dynamic compression can easily reach the TeraPascal range typical of the pressure existing deep inside large planets and exoplanets, or generated during hypervelocity impacts with large entropy creation during single-shock compression resulting in large shock-heating. We find that combined compression and heating can transform typical rocky minerals into dense, shiny fluid able to conduct electrical current, thus blurring the distinction between metals and rocks to accurately model planetary collisions [1].

In addition, the versatility of large lasers can also be exploited to design advanced shock compression schemes that allow us to probe thermodynamic states other than the pressure/temperature conditions obtained in a single-shock experiment (Hugoniot), therefore opening to the possibility of tackling fundamental questions on the behavior of planetary relevant materials at lower temperatures near 1000-10000 K.

To illustrate the interest of developing such advanced dynamic compression schemes to explore new states of matter relevant for exoplanetary science, I will discuss recent experimental results including the discovery of superionic water ice [2,3], the insulator-to-metal transition in dense fluid hydrogen [4] and the demixing of H and He at Jovian planet conditions [5].

Prepared by LLNL under Contract DE-AC52-07NA27344.

[1] Millot, M. Shock compression of stishovite and melting of silica at planetary interior conditions. Science, 347, 418-420 (2015). [2] Millot, M. et al. Experimental evidence for superionic water ice using shock compression. Nat. Phys. 14, 297–302 (2018) [3] Millot, M. et al. Nanosecond X-ray diffraction of shock-compressed superionic water ice. Nature 569, 251–255 (2019). [4] Celliers, P. M, Millot, M. et al. Insulator-metal transition in dense fluid deuterium. Science 361, 677–682 (2018) [5] Brygoo, S., Loubeyre, P., M, Millot, et al. Evidence of hydrogen–helium immiscibility at Jupiter-interior conditions. Nature 593, 517–521 (2021).