  The immiscibility of hydrogen-helium mixture under the temperature and
pressure conditions of planetary interiors is crucial for understanding the
structures of gas giant planets (e.g., Jupiter and Saturn). While the
experimental probe at such extreme conditions is challenging, theoretical
simulation is heavily relied in an effort to unravel the mixing behavior of
hydrogen and helium. Here we develop a method via a machine learning
accelerated molecular dynamics simulation to quantify the physical separation
of hydrogen and helium under the conditions of planetary interiors. The
immiscibility line achieved with the developed method yields substantially
higher demixing temperatures at pressure above 1.5 Mbar than earlier
theoretical data, but matches better to the experimental estimate. Our results
suggest a possibility that H-He demixing takes place in a large fraction of the
interior radii of Jupiter and Saturn, i.e., 27.5% in Jupiter and 48.3% in
Saturn. This indication of an H-He immiscible layer hints at the formation of
helium rain and offers a potential explanation for the decrease of helium in
the atmospheres of Jupiter and Saturn.
