With the discovery of Pluto, was the Solar System complete? No, far from it!

WHY SEARCH FOR MORE TRANS-NEPTUNIAN OBJECTS?

In 1930, soon after Pluto's discovery, the American astronomer Frederick C Leonard (1896–1960) wondered whether Pluto was the first of many trans-Neptunian objects awaiting discovery, but he does not seem to have acted on his prescient speculation. In July 1943, in the Journal of the British Astronomical Association, the Anglo-Irish polymath Kenneth Essex Edgeworth (1880–1972) stated that beyond Neptune the solar nebula was too thinly dispersed to have made planets, but instead many smaller bodies were present, some of which become comets. He expressed similar views in 1949 in an edition of the Monthly Notices of the Royal Astronomical Society (MNRAS). This was a year before the Dutch astronomer Jan Hendrick Oort (1900–1992) proposed the existence of a distant spherical shell of small icy bodies that enveloped the Solar System and supplied long period comets to its inner regions – the Oort cloud (Section 1.2). Such a cloud is not the belt of trans-Neptunian objects that Edgeworth had proposed; his belt was not so far away and was largely confined to the planes of the planetary orbits.

Whether there was such a belt of trans-Neptunian objects was also considered by the Dutch-American astronomer Gerard Kuiper (1905–1973) in a 1951 edition of the journal Astrophysics, but he concluded that such a belt no longer existed! This was based on the then belief that Pluto was about the mass of the Earth and would thus have scattered the bodies away.