Stern, in charge of the robotic probe on its way to Pluto, said the language of the resolution is flawed. It requires that a planet "has cleared the neighborhood around its orbit." But Earth, Mars, Jupiter and Neptune all have asteroids as neighbors.
"It's patently clear that Earth's zone is not cleared," Stern told SPACE.com. "Jupiter has 50,000 trojan asteroids," which orbit in lockstep with the planet.
Stern called it "absurd" that only 424 astronomers were allowed to vote, out of some 10,000 professional astronomers around the globe.
"It won't stand," he said. "It's a farce."
Stern said astronomers are already circulating a petition that would try to overturn the IAU decision.
Owen Gingerich, historian and astronomer emeritus at Harvard who led the committee that proposed the initial definition, called the new definition "confusing and unfortunate" and said he was "not at all pleased" with the language about clearing the neighborhood.
Gingerich also did not like the term "dwarf" planet.
"I thought that it made a curious linguistic contradiction," Gingerich said in a telephone interview from Boston (where he could not vote). "A dwarf planet is not a planet. I thought that was very awkward."
Gingerich added: "In the future one would hope the IAU could do electronic balloting."