POISED TO BE A PLANET
When Ceres, Pallas, Juno and Vesta were discovered in the first years of the 19th Century they were listed as the eighth, ninth, tenth and eleventh planets of the Solar System. William Herschel, who discovered Uranus in 1781, was the only one who suggested these small bodies belonged to a new class, for which he coined the term ‘asteroids’ – meaning ‘star-like’, a reference to their point-like appearance in a telescope.
Herschel may have simply disliked the idea of sharing his status of planet discoverer with others. But when more and more small bodies were found between the orbits of Mars and Jupiter in the mid-1800s, astronomers indeed reclassified them as ‘minor planets’ or asteroids. Neptune, discovered in 1846, became the eighth ‘regular’ planet. The demotion of Ceres, Pallas, Juno and Vesta was not unlike what happened to Pluto in 2006. When Pluto was discovered in 1930, it was regarded as the ninth planet, albeit a very small one in an unusual orbit. Only since the 1990s, when more and more ice dwarfs were found beyond the orbit of Neptune, did it become clear that Pluto was just one of the largest Kuiper Belt Objects. In August 2006, the International Astronomical Union (IAU) decided to strip Pluto of its undeserved planethood.
Together with a small number of other Kuiper Belt Objects, both Pluto and Ceres have officially been classified by the IAU as dwarf planets: they are large enough to be spherical under the influence of their self-gravity, but not massive enough to have cleared their orbital regions of smaller objects.
Pallas Ceres Mars The major planets of the inner Solar System, as envisioned at the dawn of the 19th Century Sun Vesta Earth Juno SUN, PLANETS AND ASTEROIDS NOT SHOWN TO SCALE