The phase behaviour of warm dense hydrogen−helium (H−He) mixtures affects our understanding of the evolution of Jupiter and Saturn and their interior structures^1,2. For example, precipitation of He from a H−He atmosphere at about 1−10 megabar and a few thousand kelvin has been invoked to explain both the excess luminosity of Saturn^1,3, and the depletion of He and neon (Ne) in Jupiter’s atmosphere as observed by the Galileo probe^4,5. But despite its importance, H−He phase behaviour under relevant planetary conditions remains poorly constrained because it is challenging to determine computationally and because the extremes of temperature and pressure are difficult to reach experimentally. Here we report that appropriate temperatures and pressures can be reached through laser-driven shock compression of H₂−He samples that have been pre-compressed in diamond-anvil cells. This allows us to probe the properties of H−He mixtures under Jovian interior conditions, revealing a region of immiscibility along the Hugoniot. A clear discontinuous change in sample reflectivity indicates that this region ends above 150 gigapascals at 10,200 kelvin and that a more subtle reflectivity change occurs above 93 gigapascals at 4,700 kelvin. Considering pressure–temperature profiles for Jupiter, these experimental immiscibility constraints for a near-protosolar mixture suggest that H−He phase separation affects a large fraction—we estimate about 15 per cent of the radius—of Jupiter’s interior. This finding provides microphysical support for Jupiter models that invoke a layered interior to explain Juno and Galileo spacecraft observations^1,4,6,7,8.

暖致密氢氦 (H-He) 混合物的相行为影响我们对木星和土星演化及其内部结构^1,2的理解。例如，在大约 1-10 兆巴和几千开尔文的 H−He 大气中沉淀出的 He 已被用来解释土星^1,3的过度光度，以及木星中 He 和氖 (Ne) 的耗尽。伽利略探测器观测到的大气^4,5. 但是，尽管它很重要，但在相关行星条件下的 H-He 相行为仍然受到很差的约束，因为计算确定具有挑战性，而且温度和压力的极端值很难通过实验达到。_{在这里，我们报告了通过激光驱动的 H 2}冲击压缩可以达到适当的温度和压力− 已在金刚石砧单元中预压缩的样品。这使我们能够在木星内部条件下探测 H-He 混合物的性质，揭示沿着 Hugoniot 的不混溶区域。样品反射率的明显不连续变化表明该区域在 10,200 开尔文时结束于 150 吉帕斯卡以上，并且在 4,700 开尔文时在 93 吉帕斯卡以上发生更细微的反射率变化。考虑到木星的压力-温度曲线，这些对近原太阳混合物的实验不混溶性限制表明，H-He 相分离会影响木星内部的很大一部分——我们估计约为半径的 15%。这一发现为木星模型提供了微观物理支持，这些模型调用分层内部来解释朱诺和伽利略航天器的观测^1,4,6,7,8.