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Some “dwarf planets” are now “plutoids”

June 12, 2008
Courtesy IAU
and World Science staff

The in­terna­t­ional body in charge of nam­ing ce­les­tial ob­jects has cho­sen “plu­toid” as a name for dwarf plan­ets like Plu­to.

The In­terna­t­ional As­tro­nom­i­cal Un­ion an­nounced June 11 that its Ex­ec­u­tive Com­mit­tee ap­proved the no­men­cla­ture at a meet­ing in Os­lo.

The move comes al­most two years af­ter the or­gan­iz­a­tion in­tro­duced the term dwarf plan­et for ob­jects that are round and plan­et-like, but haven’t “cleared the neigh­bor­hood” around their or­bit through their gravita­t­ional ef­fects on oth­er bod­ies.

The new de­ci­sion means some dwarf plan­ets in our So­lar Sys­tem are re­clas­si­fied as plu­toids. They’re de­fined as such if they or­bit the Sun at a dis­tance great­er than that of Nep­tune. The two rec­og­nized plu­toids are Plu­to and Er­is. The dwarf plan­et Ce­res is not a plu­toid as it is lo­cat­ed in the as­ter­oid belt be­tween Mars and Ju­pi­ter.

The new def­i­ni­tions fol­low long de­bates in re­cent years over ex­actly what is a “plan­et.” Re­cent disco­veries have show that the very con­cept of plan­et can overlap, at its bound­aries, with those of very large as­ter­oids or moons or even ti­ny stars.


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The international body in charge of naming celestial objects has chosen “plutoid” as a name for dwarf planets like Pluto. The International Astronomical Union’s Executive Committee announced June 11 that it approved the nomenclature at its recent meeting in Oslo. The move comes almost two years after the organization introduced the name dwarf planets for objects that are round and planet-like, but haven’t “cleared the neighborhood” around their orbit through their gravitational effects on other bodies. The new decision means some dwarf planets in our Solar System are reclassified as plutoids. They’re defined as such if they orbit around the Sun at a distance greater than that of Neptune. The two recognized plutoids are Pluto and Eris. The dwarf planet Ceres is not a plutoid as it is located in the asteroid belt between Mars and Jupiter. The new definitions follow long debates in recent years over exactly what is a “planet.” Recent discoveries have show that the very concept of planet can overlap, at its boundaries, with those of very large asteroids or moons or even tiny stars.