Plate tectonics governs the topography and motions of the surface of Earth, and the loss of heat from Earth's interior, but appears to be found uniquely on Earth in the Solar System. Why does plate tectonics occur only on Earth? This is one of the major questions in earth and planetary sciences research, and raises a wide range of related questions: has plate tectonics ever occurred on other planets in the past? How did plate tectonics start on Earth? Will it ever end? In the absence of plate tectonics, how do planets lose their heat? This article provides a brief introduction to the ways in which planets lose their heat and discusses our current understanding of plate tectonics and the challenges that lie ahead.