Phys. Rev. Lett. 108, 091102 (2012)

Ice giants is the term commonly used to describe planets like Neptune, Uranus and several extrasolar planets discovered in the last few years. Water is abundant in the cores of these celestial bodies, but at those extreme pressures and temperatures it little resembles its liquid and solid forms we see on Earth. Its equation of state has been modelled in different ways, which naturally lead to conflicting estimates of the properties of the planets. Marcus Knudson and colleagues have now used a high-precision shock-wave experiment to study the compressibility of water at several hundred gigapascals of pressure, which resembles the conditions expected in the ice giants. Their results are in agreement with the equation of state predicted by a recent first-principle model, and suggest that the temperature in the core of Neptune and Uranus can be up to 20% lower than previously estimated. Validation of this equation of state will be instrumental in understanding the internal composition of planets of this type.