These are normally young worlds within the first 100 million years of formation. Sometimes though a small dwarf world that is too close to a star to be able to cool down exhibit similar characteristics, and will also be classed as Proto Lithian.
They may have a fully or partially molten surface, and are geologically unstable.
These worlds will often have an atmosphere, though it is a temporary one created by outgassing from the molten surface. Eventually the rate of loss to space will exceed creation, and the atmosphere will finally dissipate.