  In this paper, we introduce the first and third cohomology groups on Leibniz
triple systems, which can be applied to extension theory and $1$-parameter
formal deformation theory. Specifically, we investigate the central extension
theory for Leibniz triple systems and show that there is a one-to-one
correspondence between equivalent classes of central extensions of Leibniz
triple systems and the third cohomology group. We study the $T^*$-extension of
a Leibniz triple system and we determined that every even-dimensional quadratic
Leibniz triple system $(\mathfrak{L},B)$ is isomorphic to a $T^*$-extension of
a Leibniz triple system under a suitable condition. We also give a necessary
and sufficient condition for a quadratic Leibniz triple system to admit a
symplectic form. At last, we develop the $1$-parameter formal deformation
theory of Leibniz triple systems and prove that it is governed by the
cohomology groups.
