POISED TO BE A PLANET

Sky at Night Magazine - - DAWN ARRIVES AT CERES -

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

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