Probing giant planets’ dark hydrogen

Hydrogen is the most-abundant element in the universe, but there is still so much we have to learn about it. One of the biggest unknowns is its transformation under the extreme pressures and temperatures found in the interiors of giant planets, where it is squeezed until it becomes liquid metal, capable of conducting electricity. New work measures the conditions under which hydrogen undergoes this transition in the lab and finds an intermediate ‘dark hydrogen’ state.