美国有线新闻 CNN 科学争论再起 将行星身份还给冥王星（在线收听）

Which of these is not a requirement for a celestial body to be considered a planet? Must orbit the sun, must clear its path of orbit, must have a moon or must be round. A planet does not need a moon to meet the International Astronomical Union’s definition of a planet.

But as option B indicated, it does have to clear the neighborhood in its path of orbit. Meaning knock other space rocks out of the way if necessary and that's why the International Astronomical Union announced in 2006 that Pluto was no longer a planet. The Union said it was just too small to clear its path. But that decision caused a lot of controversy and not just among those who were taught there were nine planets in our solar system.

New research published in the scientific journal Icarus argues that Pluto should be reinstated to planetary status. A University of Central Florida scientist says that, nobody actually uses the path clearing requirement in their research.

He says his team looked at two centuries worth of materials and found only one study from the early 1800's that used the path clearing requirement.

He adds that the way a celestial body is formed and whether it's big enough to have a spherical shape are more important factors for its classification. The IAU says it hasn't received any formal requests to reevaluate Pluto but that it's good to debate these topics.