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A coreless planet is a theoretical type of terrestrial planet that has undergone planetary differentiation but nevertheless has no metallic core, i.e. the planet is effectively a giant rocky mantle. Despite the name this is not the same as a hollow earth.
In the first, the planet accretes from chondrite-like fully oxidized water-rich material, where all the metallic iron is bound into silicate mineral crystals. Such planets may form in cooler regions farther from the central star.
In the second, the planet accretes from both water-rich and iron metal-rich material. However, the metal iron reacts with water to form iron oxide and release hydrogen before differentiation of a metal core has taken place. Provided the iron droplets are well mixed and small enough (<1 centimeter), the predicted end result is that the iron is oxidized and trapped in the mantle, unable to form a core.
A fully silicate coreless planet will not have a molten core and therefore no magnetic field. The predicted sizes of coreless and cored planets are similar within a few per cent, which makes it difficult to interpret the interior composition of exoplanets based on measured planetary masses and radii.