It is chemistry's poster child. From copper's conductivity to mercury's mercurial liquidity, the periodic table assigns the chemical elements to neat columns and rows and so reveals their properties. It is chemists' first reference point in all their endeavours, whether building better catalytic converters, making concrete set faster or looking for the best materials for medical implants. Yet this iconic picture of science is hopelessly parochial. Most of the known matter in the universe doesn't exist under the cool, calm conditions of Earth's surface that the periodic table assumes. By mass, more than 99.9 per cent of normal matter resides within planets and stars - environments of high temperatures, but above all unimaginable pressures. Here, the elements' familiar identities start to blur. "We essentially have a new periodic table at high pressures," says materials scientist Paul Loubeyre of the Alternative Energies and Atomic Energy Commission (CEA) in Bruyeres-le-Chatel, France. As yet, this high-pressure realm is one we know little about - proportionally as if, in thousands of years of Earth exploration, geographers had mapped out a region no larger than Spain.
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