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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 type in the Solar System (as the other terrestrial planets are silicate planets), but larger iron-rich exoplanets may exist.
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Origin
Iron-rich planets may be the remnants of normal metal/silicate rocky planets whose rocky mantles were stripped away by giant impacts. Some are thought 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.
Characteristics
Iron-rich planets are smaller and more dense than other types of planets of comparable mass. Such planets would have no plate tectonics or strong magnetic field as they cool rapidly after formation. These planets are not like Earth. 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.
In science fiction, such a planet has been called a "Cannonball".
Candidates
Some extrasolar planet candidates that may be composed mainly of iron are KOI-1843 b, Kepler-70b and Kepler-10b.
The closest object in our Solar System to an iron planet is Mercury, which is a mixed iron-silicate planet with a high density of 5.4 g/cm3, compared to 7.9 g/cm3 for pure iron at standard temperature and pressure. The Earth is also a mixed iron-silicate planet, although with a lower percentage of iron than Mercury. Notably, both planets have magnetic fields, unlike the silicate planets in the inner Solar System (Venus, Mars, and the Moon).