Harrington worked at the USNO. Another astronomer there, James W. Christy, consulted with him after discovering bulges in the images of Pluto, which turned out to be Pluto's satelliteCharon.[1] For this reason, some consider Harrington to be a co-discoverer of Charon,[2] although Christy usually gets sole credit. By the laws of physics, it is easy to determine the mass of a binary system based on its orbital period, so Harrington was the first to calculate the mass of the Pluto-Charon system, which was lower than even the lowest previous estimates of Pluto's mass.
For much of his career, he proposed the existence of a Planet X beyond Pluto and supported searches for it, collaborating initially with T. C. (Tom) Van Flandern.[1]
Six months before Harrington's death, E. Myles Standish had used data from Voyager 2's 1989 flyby of Neptune, which had revised the planet's total mass downward by 0.5%—an amount comparable to the mass of Mars[3]—to recalculate its gravitational effect on Uranus.[4] When Neptune's newly determined mass was used in the Jet Propulsion Laboratory Developmental Ephemeris (JPL DE), the supposed discrepancies in the Uranian orbit, and with them the need for a Planet X, vanished.[5] There are no discrepancies in the trajectories of any space probes such as Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2 that can be attributed to the gravitational pull of a large undiscovered object in the outer Solar System.[6] Although most astronomers agree that Planet X, as Lowell defined it, does not exist,[7] as of January 2016 there is speculation concerning Planet Nine.
^Myles Standish (1992-07-16). "Planet X – No dynamical evidence in the optical observations". Astronomical Journal. 105 (5): 200–2006. Bibcode:1993AJ....105.2000S. doi:10.1086/116575.