Coreless Planet

Coreless Planets are terrestrial planets that are composed entirely of a silicate mantle and lack a metallic core. These planets could conceivably form in the outer regions of a star system, where it could accrete material where all the iron is bound into silicate mineral crystals, thus keeping the planet’s composition largely homogenous. These planets would differ very little in mass and radius from their "cored" cousins, as their composition is no different from "cored" planets, except that it is homogenous rather than layered.

Although a coreless planet cannot achieve a magnetic field in the same way as a “cored” planet, it is conceivable that Magnesium Oxide could exist in a molten state deep inside a sufficiently large coreless planet, where heat and pressure are immense, and thus could generate a magnetic field in the mantles of these very large coreless planets.

Worldbuilding in Practice
"Orbiting the red dwarf Eitri at a distance of 9.26 AU is the unassuming rocky terrestrial planet Smar. With a mass of 0.3 M⊕ and a radius of 0.71 R⊕, it is an unremarkable planet in the backwaters of the Eitri system. However, new research from orbital probes suggest that, based on complex scans of the planet, the planet lacks an iron core. Smar is the first coreless planet discovered in Earth Federation space."