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The new planet definition that relegates Pluto to 鈥渄warf planet鈥� status is drawing intense criticism from astronomers. It appears likely that the definition will not be widely adopted by astronomers for everyday use, even though it is the International Astronomical Union鈥檚 (IAU) official position.

On Thursday, astronomers at the IAU meeting in Prague approved a resolution that says the solar system has only eight planets, with Pluto excluded. Pluto is considered a 鈥渄warf planet鈥� instead.

But the new definition has provoked a backlash. Alan Stern, who heads NASA鈥檚 New Horizons mission to Pluto and works at the Southwest Research Institute in Boulder, Colorado, US, says the new definition is 鈥渁wful鈥�.

鈥淭he definition introduced is fundamentally flawed,鈥� he told 51动漫. 鈥淎s a scientist, I鈥檓 embarrassed.鈥�

Four planets

He says only four of the eight objects mentioned in the IAU definition actually meet the definition鈥檚 criteria 鈥� and Earth, Mars, Jupiter and Neptune do not.

That is because the stipulates that to be a planet, an object must have cleared the neighbourhood around its orbit. But Earth鈥檚 orbital neighbourhood is filled with thousands of near-Earth asteroids, Stern says.

And Mars, Jupiter and Neptune have so-called 鈥淭rojan鈥� asteroids sharing their orbits. 鈥淭his is a half-baked criterion for planethood,鈥� he says.

He says the new definition was pushed by people who are unhappy with having large numbers of planets (an earlier proposal, which was abandoned after heavy criticism at the meeting, would have potentially allowed hundreds of new planets into the fold).

鈥淚t鈥檚 just people that say things like, 鈥楽chool kids will have to memorise too many names.鈥� Do we limit the number of stars because children have to think of too many names? Or rivers on the Earth? It鈥檚 just crazy,鈥� Stern told 51动漫.

Minority report

Stern is also critical of the fact that only astronomers present for the vote, which occurred at the end of the two-week meeting, were allowed to have their say on the matter. No email voting was allowed for the decision 鈥� it was made by a show of hands 鈥� and that meant that less than 5% of the nearly 9000 IAU members actually voted.

鈥淵ou鈥檙e going to see a real backlash in the coming weeks,鈥� he says. 鈥淚 know there is a petition among planetary scientists that鈥檚 getting a lot of support.鈥�

In any case, he says, astronomers are not obligated to use the new definition, since the IAU does not have the power to enforce it. 鈥淚 don鈥檛 think it鈥檚 going to be very widely followed,鈥� he says.

David Weintraub, author of the upcoming book Is Pluto a Planet? and a researcher at Vanderbilt University in Nashville, Tennessee, US, says he thinks Pluto is still a planet 鈥� even under the new definition.

Grammar issue

鈥淎s best I can tell, 鈥榙warf鈥� is an adjective and 鈥榩lanet鈥� is a noun,鈥� he told 51动漫. 鈥淚 think the IAU thinks they defined Pluto to not be a planet. But they in fact have defined Pluto to be a planet 鈥� a particular kind of planet.鈥�

Astronomers who proposed the new definition respond that the term 鈥渄warf planet鈥� is meant to be thought of as a single concept. And others point out that 鈥渕inor planets鈥� 鈥� asteroids and other small bodies 鈥� are not considered planets.

But he agrees with Stern that the stipulation that a planet clears out its neighbourhood is flawed. A better definition would say a planet is an object that orbits a star and is large enough to be spherical, but is not large enough to be a brown dwarf 鈥� a 鈥渇ailed鈥� star with between about 13 and 75 times the mass of Jupiter 鈥� or a star, he says.

鈥楾his is crazy鈥�

鈥淓veryone agrees on those criteria,鈥� he says. 鈥淭he question is, can we agree on additional criteria to refine the definition further? I think the answer is no.鈥�

鈥淓verybody who has communicated with me is saying, 鈥楾his is crazy and we don鈥檛 agree with it,'鈥� he adds. 鈥淚鈥檓 not convinced that the folks who were at the meeting represented well the larger community.鈥�

But not everyone is unhappy with the decision. Richard Conn Henry of Johns Hopkins University in Baltimore, Maryland, US, says he is pleased with the outcome. 鈥淎s far as I鈥檓 concerned, the right decision was made,鈥� he told 51动漫. 鈥淚 know a planet when I see it and there are eight of them.鈥�

He says it makes no sense to call Pluto a planet because it is just one of huge numbers of objects in the Kuiper Belt beyond Neptune.