Iron Planet

An Iron Planet is a terrestrial planet composed almost entirely of an iron core with little or no silicate mantle. Mercury is considered an iron planet, with roughly 70% of its composition being metallic and 42% of its volume being its iron core (as opposed to Earth’s 17%). Some planets may be even more metallic. Iron planets would have a density of between roughly 5 and 8 g/cm3 (found around the Fe band on the Mass-Radius graph).

Iron planets are thought to form very close orbits around their stars (larger orbits are permitted if the star is more massive), where the protoplanetary disk in more iron-rich. Iron planets may also form due to collisions stripping away the silicate mantle, or drag from solar nebulae causing lighter materials not to acrete (see theories on Mercury’s formation).

Since these planets are much smaller than silicate planets of a similar mass, they would likely be geologically dead and would not have a magnetic field, due to rapid cooling.

An iron planet may be covered with lakes of iron carbonyl if it exists within a region where it can interact with liquid water (i.e. the Habitable Zone). Some iron planets may also have diamond fields.

Examples might include the KOI 1843.03

Worldbuilding in Practice
"Hephaestus is an iron planet orbiting the the F class star Agamemnon every 30 days. Orbiting at a mere 0.21 AU from its star, it is tidally locked and is heated to about 750 K on its day side. Composed almost entirely of iron, it has a density of just over 8 g/cm3. With a mass of almost 3 M⊕ and a radius of about 1.25 M⊕, it has a comfortable surface gravity of 1.82 Gs, allowing for very profitable mining on its night side, where temperatures are far more manageable."