He is best known for his discovery (published in 1866) of the Kirkwood gaps. He also explained how the Cassini and Encke Divisions in Saturn’s rings resulted from orbital resonances with the planet’s larger satellites. He pointed out that certain groups of asteroids shared very similar orbital elements, such as (153) Hilda and other asteroids with periods two-thirds of Jupiter’s (see hirayama family). In 1880 Kirkwood was the first to suggest the existence of a group of sungrazing comets.