When astronomers talk about metals, they’re using the term in a specific sense. A metal in stellar terms is any element heavier than helium. Thus iron, silicon, magnesium and carbon qualify, all elements that are components of small, rocky planets. It was iron that John Michael Brewer (Yale University), Debra Fischer and colleagues singled out as a proxy in their recent work on the metal content of exoplanet systems. The work focuses specifically on compact, multi-planet systems as one of several system architectures found in close orbit of a host star.