metal

Within astrophysics, the term metal is used with different meaning than the usual: astrophysics uses the term for all elements other than hydrogen and helium. Thus, the matter of the observable universe can be divided into hydrogen, helium, and metals. Metallicity is a measure of the amount of metals in an astronomical body such as a star. Such metal was nearly non-existent in the early universe, and has since been produced within stars as well as supernovae and other cataclysmic events. Its abundance relative to hydrogen and helium is still low throughout the stars and clouds of the universe, but it is key to the occurrence of rocky planets.

The more common meaning of metal is a particular class of substances that are hard, opaque, transmit heat and electricity, and can be heated to the point of malleability, a usage that definitely has applications within planetary science. Science refines this usage to mean elements in which the electrons in the outer orbit are easily removed, which is the basis of the electrical and heat conductivity. Under extremely high pressure (millions of atm), hydrogen and helium are theorized to have a metal-like phase that conducts electricity, called metallic hydrogen (or conductive hydrogen) and metallic helium, which are believed to be present within Jupiter and Saturn.