After graduation, Haret won a scholarship competition organized by Titu Maiorescu and went to Paris in order to study mathematics at the Sorbonne. There he earned a mathematics diploma in 1875 and a physics diploma in 1876. Two years later (on 18 January 1878) he earned his Ph.D. by defending his thesis, "Sur l’invariabilité des grandes axes des orbites planétaires" ("On the invariability of the major axis of planetary orbits"), in front of examiners led by Victor Puiseux. In this work he proved a result fundamental for the "n"-body problem in astronomy, the thesis being published in Vol. XVIII of the "Annales de l'Observatoire de Paris". Haret was the first Romanian to obtain a Ph.D. degree in Paris.