that these four worlds be given their own separate classification, asteroids (meaning "starlike" since they were too small for their disks to resolve and thus resembled stars), though most astronomers preferred to refer to them as planets. This conception was entrenched by the fact that, due to the difficulty of distinguishing asteroids from yet-uncharted stars, those four remained the only asteroids known until 1845. Science textbooks in 1828, after Herschel's death, still numbered the asteroids among the planets. With the arrival of more refined star charts, the search for asteroids resumed, and a fifth and sixth were discovered by Karl Ludwig Hencke in 1845 and 1847. By 1851 the number of asteroids had increased to 15, and a new method of classifying them, by affixing a number before their names in order of discovery, was adopted, inadvertently placing them in their own distinct category. Ceres became "(1) Ceres", Pallas became "(2) Pallas", and so on. By the 1860s, the number of known asteroids had increased to over a hundred, and observatories in Europe and the United States began referring to them collectively as "minor planets", or "small planets", though it took the first four asteroids longer to be grouped as such. To this day, "minor planet" remains the official designation for all small bodies in orbit around the Sun, and each new discovery is numbered accordingly in the IAU's Minor Planet Catalogue. Pluto The long road from planethood to reconsideration undergone by Ceres is mirrored in the story of Pluto, which was named a planet soon after its discovery by Clyde Tombaugh in 1930. Uranus and Neptune had been declared planets based on their circular orbits, large masses and proximity to the ecliptic plane. None of these applied to Pluto, a tiny and icy world in a region of gas giants with an orbit that carried it high above the ecliptic and even inside that of Neptune. In 1978, astronomers discovered Pluto's largest moon, Charon, which allowed them to determine its mass. Pluto was found to be much tinier than anyone had expected: only one-sixth the mass of Earth's Moon. However, as far as anyone could yet tell, it was unique. Then, beginning in 1992, astronomers began to detect large numbers of icy bodies beyond the orbit of Neptune that were similar to Pluto in composition, size, and orbital characteristics. They concluded that they had discovered the long-hypothesised Kuiper belt