Lara Benedetti and colleagues from the University of California at Berkeley, the University of Missouri and the Lawrence Livermore National Laboratory sealed liquid methane between two diamond 'anvils' and squeezed it to 50 gigapascals - 25 million times atmospheric pressure. A laser was then used to heat the sample to 3000 Kelvin - producing the atmospheric conditions that exist 7000 km below the cloud tops of Neptune and Uranus.

Scientists have long suspected that the core of the outer planets could consist of diamond, but this is the first evidence for diamond formation in the middle layers of the planets. Both Neptune and Uranus consist of 10-15% methane and, according to Benedetti and colleagues, large quantities of diamond could effect both the luminosities and magnetic fields of the planets.