Lava Worlds
Some planets can hold onto residual heat for billions of years sustained by radioactive material and tidal forces.
Planets can be molten during formation and subsequent bombardment.
A close, hot star can liquify the inner side of a tidally locked planet.
For a research project studying the color and observational signature of molten rock on other planets. Some of the planets we have discovered like COROT-7b, Kepler-10b, and Kepler-78b are expected to have high surface temperatures that could partially or completely melt their surfaces. Contributing factors to the creation of a lava planet could be a large abundance of radiogenic material, high tidal forces, direct heating from the host star, bombardment, or the release of gravitational potential energy from initial collapse.