  The use of statistical methods to model gravitational systems is crucial to
physics practice, but the extent to which thermodynamics and statistical
mechanics genuinely apply to these systems is a contentious issue. This paper
provides new conceptual foundations for gravitational thermodynamics by
reconsidering the nature of key concepts like equilibrium and advancing a novel
way of understanding thermodynamics. The challenges arise from the peculiar
characteristics of the gravitational potential, leading to non-extensive energy
and entropy, negative heat capacity, and a lack of standard equilibrium. Hence
it has been claimed that only non-equilibrium statistical mechanics is
warranted in this domain, whereas thermodynamics is inapplicable. We argue
instead that equilibrium statistical mechanics applies to self-gravitating
systems at the relevant scale, as they display equilibrium in the form of
metastable quasi-equilibrium states. We then develop a minimal framework for
thermodynamics that can be applied to these systems and beyond. Thermodynamics
applies in the sense that we can devise macroscopic descriptions and
explanations of the behaviour of these systems in terms of coarse-grained
quantities within equilibrium statistical mechanics.
