Planet sizes

Comparison of sizes of planets with different compositions

An iron planet is a type of planet that consists primarily of an iron-rich core with little or no mantle. Mercury is the largest celestial body of this typeTemplate:Citation needed in the Solar System, but larger iron-rich exoplanets may exist. Also known as a Cannonball.[1]


Iron-rich planets may be the remnants of normal metal/silicate rocky planets whose rocky mantles were stripped away by giant impacts. Some are believed to consist of diamond fields. Current planet formation models predict iron-rich planets will form in close-in orbits or orbiting massive stars where the protoplanetary disk presumably consists of iron-rich material.[2]


Iron-rich planets are smaller and more dense than other types of planets of comparable mass.[3] Such planets would have no plate tectonics or strong magnetic field as they cool rapidly after formation.[2] Since water and iron are unstable over geological timescales, wet iron planets in the goldilocks zone may be covered by lakes of iron carbonyl and other exotic volatiles rather than water.[4]


Some extrasolar planet candidates that may be composed mainly of iron are KOI-1843 b,[5] Kepler-70b and Kepler-10b.

See alsoEdit


  1. Gillett, Stephen L. (1996). Ben Bova World-Building. Cincinnati, Ohio: Writer's Digest Books.
  2. 2.0 2.1 "Characteristics of Terrestrial Planets" by John Chambers, from "The Great Planet Debate: Science as Process", August 14–16, 2008, The Johns Hopkins University Applied Physics Laboratory Kossiakoff Center, Laurel, MD.
  4. "Big Planets: Super-Earths in Science Fiction" by Stephen Baxter, JBIS Vol 67, No 03 (March 2014), p.108