 By 1851, the number of "planets" had grown to 23 (the eight recognised today, plus fifteen between Mars and Jupiter), and it was clear that hundreds more would eventually be discovered.Astronomers began cataloguing them separately and began calling them "asteroids" instead of "planets".With the discovery of Pluto by Clyde Tombaugh in 1930, astronomers considered the Solar System to have nine planets, along with thousands of smaller bodies such as asteroids and comets.