Recognizing the ${\mathrm{G}}_2$-horospherical manifold of Picard number 1 by varieties of minimal rational tangents

Pasquier and Perrin discovered that the ${\mathrm{G}}_2$-horospherical manifold $\mathbf{X}$ of Picard number 1 can be realized as a smooth specialization of the rational homogeneous space parameterizing the lines on the 5-dimensional hyperquadric, in other words, it can be deformed nontrivially to the rational homogeneous space. We show that $\mathbf{X}$ is the only smooth projective variety with this property. This is obtained as a consequence of our main result that $\mathbf{X}$ can be recognized by its VMRT, namely, a Fano manifold of Picard number 1 is biregular to $\mathbf{X}$ if and only if its VMRT at a general point is projectively isomorphic to that of $\mathbf{X}$. We employ the method the authors developed to solve the corresponding problem for symplectic Grassmannians, which constructs a flat Cartan connection in a neighborhood of a general minimal rational curve. In adapting this method to $\mathbf{X}$, we need an intricate study of the positivity/negativity of vector bundles with respect to a family of rational curves, which is subtler than the case of symplectic Grassmannians because of the nature of the differential geometric structure on $\mathbf{X}$ arising from VMRT.