has part has quality composed primarily of iron atom iron molecular entity planetary mantle planetary core terrestrial planet iron planet cannonball planet Mercury is the largest iron planet in the Solar System, the other terrestrial planets being composed of silicate rocks. Some extrasolar planet candidates that may be composed mainly of iron are KOI-1843 b,[5] Kepler-70b and Kepler-10b. A planet which in primarily composed of an iron-rich core with little to no mantle envoAstro decreased size decreased mass