Who you calling a dwarf? Pluto debate heats up

Pluto lost its title as our ninth planet nearly a decade ago, not long after Nasa launched a 3billion-mile mission to the celestial body that reached its destination this week, and not nearly enough time for passions to cool over its demotion.

Nasa’s historic New Horizons mission to Pluto has revived those passions, if not the official debate, between scientists who can’t agree about the definition of a planet. In 2006 the International Astronomical Union took up the question, and a handful of voters decided the issue. Pluto was reclassified from one of the nine planets to a new category, the dwarf planet, based on its size and the discovery of similar bodies in its neighborhood that Pluto’s gravity does not dominate.

Scientists dissented. Citizens protested. Somebody sold “save Pluto” merchandise. But the question persists: what is a planet?

Luminaries including astrophysicist Neil deGrasse Tyson have sided against Plutonian planethood, with Tyson telling Stephen Colbert that to call Pluto a planet “would insult all the other planets”.

On the other side of the ring is New Horizons lead scientist Alan Stern, who said in 2011 the IAU had “embarrassed” itself and that under the definition used Earth would sometimes not be a planet.

“Any definition that produces a result where Earth is not a planet under any circumstance is immediately indicted as ridiculous, because one thing we all agree is a planet is planet Earth!”

Some of his colleagues agree, though they doubt the IAU will reconsider its position.

“I don’t expect the IAU to reverse what they’ve done, but I do expect it to be widely ignored,” said David Grinspoon, a Nasa planetary scientist on the New Horizons mission.

“A lot of planetary scientists don’t take the supposed demotion of Pluto that seriously,” he said. “I’m in that camp. I’ve always considered it a planet.”

Grinspoon conceded that Pluto “needed to be reclassified” and that he’s “happy with pointing out that it’s a different kind of planet”, but said that planetary specialists often don’t see eye to eye with astronomers about the title.

“Planetary scientists think about planets as bodies with processes and systems, and a lot of astronomers mostly think of them as objects orbiting around other objects. But people will keep referring to Pluto as a planet because it is one.

“To me its intrinsic, geophysical properties are more significant - for instance if it’s large enough to be round by self-gravity, and orbiting the sun or a star, then we might consider it a planet.”

But such definitions could add anywhere from three to a dozen more planets to the solar system, said Harvard astronomer Gareth Williams. “If you look at the outer solar system there are a lot of Pluto-size bodies in orbit near Pluto,” Williams said.

“We could’ve immediately looked at it and come up with 12 or 13 objects that could be treated as planets, and within a decade we could have 50 or 100 objects.”

Those objects include Eris, whose 2005 discovery helped provoke the IAU debate. Larger than Pluto, Eris orbits in the solar system’s outer fringe called the Kuiper belt , where ancient fragments of ice and rock drift through the dark.

The man who discovered Eris, astronomer Mike Brown, told National Geographic last year that the question came down to orbits: “Why has the solar system sorted itself into a small number of dominant bodies and a huge number of tiny ones flitting between them? This is the sort of question scientists are actively trying to answer.”

Brown, who calls himself “the Pluto Killer”, tweeted on Wednesday: “Sure, it’s OK to grandfather Pluto in as a planet. Also you can grandfather the Earth as flat, too, if that makes you feel better.”

“Pluto should never have been a planet in the first place,” Williams said, saying that errors in its discovery had mischaracterized it from the start. “You’re simply moving it from the smallest of planets to the king of the Kuiper belt.”

UCLA professor Jean-Luc Margot agreed. “The New Horizons discoveries are very exciting but they will not change the definition of a planet. It’s a beautiful definition and it makes sense.”

Margot said the definition worked because it relied on dynamics and dominance: “A planet has to be able to clear its orbital zone” - be the dominant player in its neighborhood - “and if you look at that aspect in the solar system there are eight bodies that can very clearly do that. There’s a very, very sharp distinction between the eight bodies.”

Definitions about roundness or composition would require close inspection of everything in space, he said, while the current definition could be adjusted to include planets outside the solar system.

Other scientists feel the IAU took on something of a fool’s errand. Astronomer Owen Gingerich told a Harvard symposium last year that ultimately “it’s a culturally defined word” whose meaning has shifted over centuries, as all language does.

“I thought it was really dumb that the IAU took as a category ‘dwarf planet’ and then said: ‘But they’re not planets,’” he said. “I think this is an abuse of the language.”

Grinspoon agreed: “The strange linguistic trick of saying it is ‘a dwarf planet and a dwarf planet is not a planet’, that just seemed vindictive.”

He also observed that almost everyone, astronomers and planetary scientists alike, realized that their “strong feelings” paled next to their excitement about the mission.

“There’s a lot of people who don’t care,” he said. “They’re probably the wisest among us.”

Guardian service