Classification of compact homogeneous spaces with invariant $G_2$-structures

In this note we classify all homogeneous spaces $G\mathrm{/}H$ admitting a $G$-invariant $G_2$-structure, assuming that $G$ is a compact Lie group and $G$ acts effectively on $G\mathrm{/}H$. They include a subclass of all homogeneous spaces $G\mathrm{/}H$ with a $G$-invariant ${\tilde{G}}_2$-structure, where $G$ is a compact Lie group. There are many new examples with nontrivial fundamental group. We study a subclass of homogeneous spaces of high rigidity and low rigidity and show that they admit families of invariant coclosed $G_2$-structures (resp. $\tilde G_2$-structures).