Hydrothermal settings host heated fluids reacting physically and chemically with surrounding rocks. On Earth, those occur deep in the critical zone marked by coupled biological, chemical, geological, thermophysical and atmospheric processes. Decoupled from photosynthesis, microbes can use leachate-rich water via organic-rich sediments, redox gradients, and thermal energy for metabolism, making habitable zones plausible throughout the solar system. Thermal sources tied to planetary evolution, such as magma bodies, meteoritic impacts, radiogenic heating, tide-flexing and metamorphism underpin hydrothermalism. Hydrothermal cooling can also modulate geophysical evolution, by curtailing the viscoelastic relaxation of subsurface topography. However, the extent of thermal sources, signatures of paleo-hydrothermalism, and associated timescales are poorly known. Accordingly, we welcome works on the role of hydrothermal processes in geologic and biologic evolution across the solar system, including ocean worlds like Europa. Laboratory and field findings, such as of amagmatic hydrothermal sites, and connections between Earth and planetary systems are especially welcome.