For each simple Lie algebra $\mathfrak{g}$ (excluding, for trivial reasons,
type ${\sf C}$) we find the lowest possible degree of an invariant second-order
PDE over the adjoint variety in $\mathbb{P}\mathfrak{g}$, a homogeneous contact
manifold. Here a PDE $F(x^i,u,u_i,u_{ij})=0$ has degree $\le d$ if $F$ is a
polynomial of degree $\le d$ in the minors of $(u_{ij})$, with coefficients
functions of the contact coordinates $x^i$, $u$, $u_i$ (e.g., Monge-Amp\`ere
equations have degree 1). For $\mathfrak{g}$ of type ${\sf A}$ or ${\sf G}$ we
show that this gives all invariant second-order PDEs. For $\mathfrak{g}$ of
type ${\sf B}$ and ${\sf D}$ we provide an explicit formula for the
lowest-degree invariant second-order PDEs. For $\mathfrak{g}$ of type ${\sf E}$
and ${\sf F}$ we prove uniqueness of the lowest-degree invariant second-order
PDE; we also conjecture that uniqueness holds in type ${\sf D}$.