Haret’s major scientific contribution was made in 1878, in his Ph.D. thesis "Sur l’invariabilité des grandes axes des orbites planétaires". At the time it was known that planets disturb each other’s orbits, thus deviating from the elliptic motion described by Johannes Kepler’s First Law. Pierre Laplace (in 1773) and Joseph Louis Lagrange (in 1776) had already studied the problem, both of them showing that the major axes of the orbits are stable, by using a first degree approximation of the perturbing forces. In 1808 Siméon Denis Poisson had proved that the stability also holds when using second degree approximations. In his thesis, Haret proved by using third degree approximations that the axes are not stable as previously believed, but instead feature a time variability, which he called "secular perturbations". This result implies that planetary motion is not absolutely stable. Henri Poincaré considered this result "a great surprise" and continued Haret’s research, which eventually led him to the creation of chaos theory. Félix Tisserand recommended the extension of Haret's method to other astronomic problems and, much later, in 1955, Jean Meffroy restarted Haret’s research using new techniques.