It literally rains diamonds on Neptune and Uranus.

Deep within the atmospheres of Neptune and Uranus, temperatures reach thousands of degrees and pressures are millions of times greater than on Earth. These extreme conditions break apart methane molecules, which consist of one carbon atom and four hydrogen atoms. Once the carbon is isolated, the intense pressure compresses it into a crystalline form, creating solid diamonds.Researchers from the SLAC National Accelerator Laboratory first confirmed this phenomenon in 2017 using the Linac Coherent Light Source X-ray laser. By applying heat and pressure to polystyrene, a material that mimics the hydrocarbon structure of methane, they observed the formation of nanodiamonds in real-time. This experiment proved that carbon can transition into a solid diamond state within the specific environments of ice giant planets.Once formed, these diamonds are thought to be much larger than the microscopic versions created in labs, potentially reaching millions of carats in weight. Because diamonds are denser than the surrounding hydrogen and helium fluids, they slowly sink toward the planetary core. This process, often called diamond rain, may generate additional heat through friction as the stones descend through the mantle.This discovery helps explain why Neptune is much hotter than predicted by standard planetary models. The energy released by the falling diamonds contributes to the planet's internal heat budget. Similar processes may occur on other gas giants and exoplanets throughout the galaxy, making diamond rain a potentially common cosmic occurrence.