Rocky Planets as Heat Engines.

Rocky planets act as heat engines, using internal heat to drive mantle convection, tectonic activity, crustal evolution and magnetic field generation. This ultimately shapes the surface environment and long-term habitability. This article reviews the physical and chemical processes governing the interior dynamics of rocky planets in the Solar System and beyond, emphasizing how variations in heat sources, material properties, internal structure, and boundary conditions (e.g., surface temperature and stellar flux) lead to diverse evolutionary pathways. Planetary heat arises from primordial accretion, core formation, radiogenic decay, tidal dissipation, and impact heating, and these energy sources regulate convective vigor and heat loss over geological timescales. Depending on different mechanical, thermal, and compositional controls, rocky planets can exhibit a spectrum of tectonic regimes, including plate tectonics, episodic lid, stagnant lid, and plutonic-squishy lid. Particular attention is given to the role of crustal buoyancy, volatile content, rheology, surface temperature, and magmatic intrusion-extrusion balance in governing lithospheric mobility and recycling efficiency. We further review models of coupled thermal and chemical evolution, including magma ocean solidification, mantle differentiation, crust formation, and volatile cycling between the interior and atmosphere. The implications of interior evolution for planetary magnetic fields are discussed, highlighting how mantle cooling histories influence dynamo generation and longevity. Finally, we review the links between tectonic regime, atmospheric evolution, and planetary habitability, with Earth serving as a reference case within a broader comparative planetary framework. By integrating insights from geodynamics, mineral physics, planetary science, and exoplanet observations, this article aims to provide a unifying framework for interpreting the interior state, thermal evolution, and tectonic diversity of rocky exoplanets, and to support future efforts to understand their dynamics, evolution, and potential for habitability.