In a Planet-or-Not Debate, Some Astronomers Say "Long Live Planet Pluto"

Sunday, September 27, 2014 | 12:35am

By Nadia Drake | PUBLISHED SEPTEMBER 26, 2014

PHOTOGRAPH BY NASA, ESA, H. WEAVER (JHUAPL), A. STERN (SWRI), AND THE HST PLUTO COMPANION SEARCH TEAM

Pluto, seen with its large moon Charon and smaller satellites Nix and Hydra, is the poster child for a contentious debate over how to define the term "planet."

Pluto is a planet.

No, its not!

It depends what you mean by "planet".

Last week, the Harvard-Smithsonian Center for Astrophysics revisited the perennial debate over how to define a planet—and whether icy little Pluto qualifies. Three accomplished experts weighed in at the event, two pro-planet, one anti-planet.

After those arguments, the audience in Cambridge, Massachusetts—a mix of scientists, teachers, and the public—voted on their favorite definition of a planet, and whether Pluto is in or out.

Planet Pluto won.

It seems to always wins the popular vote, except for that one time in 2006 when the International Astronomical Union redefined "planet" and stripped Pluto of its planethood.

The basis of that decision: A number of other worlds have been discovered at the margins of the observable solar system, and Pluto might not even be the largest of these frosted runts. Astronomers suspect there are hundreds more of these worlds waiting to be discovered.

How, then, could Pluto alone be called a planet? The IAU needed to figure out how to classify Pluto and its friends, and describe what made them different from the classical eight planets.

So, the assembly voted to call them "dwarf planets," and Pluto became one of the first entries in the new official category. Joining it are Ceres, in the asteroid belt, and Eris, Haumea, and Makemake, which like Pluto live in the icy Kuiper Belt beyond Neptune's orbit.

Watercolor editorial artwork showing Pluto's demotion from planet status.

ILLUSTRATION BY MARC JOHNS

Now, "dwarf planet" may not sound so objectionable. But it's a classification that, despite how it reads, is not synonymous with "small planet"—and therein lies much of the trouble.

In the same confusing way that king cobras are not actually cobras, dwarf planets are not, in fact, planets. They meet two of the three IAU criteria for a planet: They're round, and they orbit the sun. But unlike every world from Mercury through Neptune, the dwarfs haven't grown massive enough to dominate their orbits and clear those paths of other solar system debris, by either knocking it away or reeling it in with their gravity.

"Jupiter has cleared its neighborhood. Earth has cleared its neighborhood. Ceres, which is in the main asteroid belt, hasn't. Pluto hasn't," said Gareth Williams, associate director of the IAU's Minor Planet Center, who presented the IAU definition at the Harvard debate. "In my world, Pluto is not a planet."

A dwarf planet, according to the IAU definition, is not a type of planet. It is, one could say, just its own thing.

Source: http://news.nationalgeographic.com/news/2014/09/140926-pluto-planet-definition-science-debate/?