We define a notion of modular forms of half-integral weight on the quaternionic exceptional groups. We prove that they have a well-behaved notion of Fourier coefficients, which are complex numbers defined up to multiplication by
${\pm }1$
. We analyze the minimal modular form
$\Theta _{F_4}$
on the double cover of
$F_4$
, following Loke–Savin and Ginzburg. Using
$\Theta _{F_4}$
, we define a modular form of weight
$\tfrac {1}{2}$
on (the double cover of)
$G_2$
. We prove that the Fourier coefficients of this modular form on
$G_2$
see the
$2$
-torsion in the narrow class groups of totally real cubic fields.