According to a 2008 paper by Sara Seager and Linda Elkins-Tanton,[1] there are probably two ways in which a coreless planet may form.
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.
Earth's magnetic field results from its flowing liquid metallic core, according to the dynamo theory, but in super-Earths the mass can produce high pressures with large viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles.
[2][3] The predicted sizes of coreless and cored planets are similar within a few percent, which makes it difficult to interpret the interior composition of exoplanets based on measured planetary masses and radii.