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D. W. E. Green (Harvard-Smithsonian Center for Astrophysics)
Trans-Neptunian Objects (TNOs) are generally defined as (a) those objects orbiting the sun with perihelion distances > 30 AU (that of Neptune) and (b) those objects with semi-major axes > 30 AU but having short-order resonances with respect to (and exterior to) Neptune. TNOs with the 2:3 resonance (called "plutinos" after the prototype, Pluto) and 1:2 resonance are now known, and some such objects have parts of their orbits pass just inside that of Neptune. Pluto was discovered some 62 years before the second TNO was found, and hundreds are now known. I shed some light upon who really made predictions about large numbers of TNOs and when (beginning with F. C. Leonard 1930, A.S.P. Leaflet 30); the results differ from what many astronomer now believe them to be (for example, G. Kuiper did not actually predict a large number of objects to be now located beyond Neptune). In this context, I also briefly delineate changing perceptions of planethood vs. minor bodies in our solar system over the last two millennia, including the waxing and waning perception of Pluto since 1930 as a "major planet".