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From now on Pluto won鈥檛 just be any dwarf planet, it will be a 鈥榩lutoid.鈥�

The International Astronomical Union (IAU) today after two years of deliberation. The new classification will be used to refer to bright dwarf planets that spend the bulk of their time outside Neptune鈥檚 orbit.

So far, Pluto and its larger neighbour, Eris, are the only named objects that qualify as plutoids, but more dwarf planets are expected to follow.

In 2006, IAU members voted to demote Pluto from planet to 鈥榙warf planet鈥�, defined as an object large enough for gravity to make round, but not big enough to clear out its orbit.

At the same time, they also voted to come up with an alternate name for similar, Pluto-like objects. The name 鈥榩lutoid鈥� was chosen partly in deference to Pluto鈥檚 fallen glory.

鈥淚n the end, we ended up with something Pluto-like, and I don鈥檛 think we could have done better,鈥� IAU general secretary Karel A van der Hucht, told New 杏吧原创.

But the name is not universally loved. 鈥淚t sounds like 鈥榟emorrhoid鈥� and it sounds like 鈥榓steroid鈥�, and of course these objects are planets and not asteroids,鈥� says planetary scientist Alan Stern of the University Space Research Association in Maryland, US.

Minimum brightness

Stern, who has criticised the IAU鈥檚 definition of a planet, says the naming decision was not made openly and is unlikely to carry much weight.

鈥淚 think this pronouncement is one more step in the IAU鈥檚 increasing irrelevance,鈥� he told New 杏吧原创. 鈥淭he IAU could announce tomorrow that the sky is green, but that doesn鈥檛 make it so.鈥�

But van der Hucht says the classification is not binding. 鈥淚t鈥檚 a free world,鈥� he says. 鈥淲e do not issue laws at the IAU. This is a working recommendation that will make life easier for people in the field.鈥�

The term 鈥榩lutoid鈥� will not apply to all dwarf planets. Ceres, for example, which sits in the asteroid belt between Mars and Jupiter, will not be a part of this category.

鈥楶lutoids鈥� must also be a minimum brightness, a requirement that will exclude dozens of dwarf planet candidates that have been spotted in the Kuiper belt, a ring of icy objects beyond Neptune, says Michael Brown of Caltech in Pasadena, US.

鈥淭hat鈥檚 an odd definition,鈥� Brown told New 杏吧原创. 鈥淚t makes objects of exactly the same size 鈥榩lutoids鈥� or 鈥榥ot plutoids鈥� depending on what鈥檚 covering their surface.鈥� If Pluto were covered with dust, Brown notes, it might not be considered a plutoid.