As Science Evolves, So Does Pluto
Artist’s impression of how the surface of Pluto might look. The image shows patches of pure methane on the surface.
Credit: ESO/L. Calçada

NEW YORK — Pluto's status nowadays as a so-called plutoid and former planet may be official in the latest textbooks, but someone forgot to tell the astronomers.

A panel of six of them gathered here last Tuesday to debate the former ninth planet's status at the American Museum of Natural History, along with moderator Neil deGrasse Tyson.

Public interest in poor Pluto has peaked ever since the International Astronomical Union demoted Pluto from planet status in 2006. And it became clear at the museum event that fierce disagreement still exists among top scientists at the leading edge of the debate.

The panelists said they remained dissatisfied with the IAU decision, which if anything has only intensified the debate and confused the public with politics amidst uncertainty.

"The IAU vote was really a political action to the Pluto huggers and Pluto haters," said Mark Sykes, director of the Planetary Science Institute in Tucson, Ariz., who has led criticism of the IAU's decision.

The face-off reaches back at least several years further. As director of the American Museum of Natural History's (AMNH) Hayden Planetarium, Tyson found himself facing an outcry from astronomers, science educators and students when the museum opened a new solar system exhibit in 2000 that intentionally omitted Pluto as one of the planets.

Tuesday evening's conversation revealed how uncertainty can breed politics in science — but also how scientists deal with an evolving understanding of the universe.

You say Planet, I say Plutoid

Many people involved in the Pluto debate fall roughly into two camps. One side argues that Pluto, which sits far out beyond the eight current planets, belongs with the icy Kuiper Belt Objects that it hangs out with beyond Neptune. The other side says that Pluto's characteristics distinguish it from lesser bodies in the solar system, and so it deserves the planet moniker.

The International Astronomical Union (IAU) first demoted Pluto to a "dwarf planet" in 2006 after the discovery of Eris, a Kuiper Belt Object larger than Pluto. Last year the IAU went further by reclassifying Pluto as a plutoid, essentially ruling out Pluto's planetary status because it sits beyond Neptune and fails to clear out asteroids and other smaller debris from its orbital neighborhood.

Many of the astronomers and astrophysicists at the recent debate agreed that the IAU's use of the word "clear" was confusing, because not even Jupiter completely clears its neighborhood. Instead, they suggested that Pluto does not "gravitationally dominate" its neighborhood like planets such as Earth, Jupiter or even Mercury.

"Classification is meant to facilitate communication between scientists," said Steven Soter, a planetary scientist at AMNH who co-hosted the "Cosmos" television series with Carl Sagan. "Now about clearing, that was an unfortunate term, because planets never fully clear their orbits."

But the museum panel strongly disagreed on whether or not to include Pluto, Eris and other round Kuiper Belt Objects as planets. Soter said that he agreed with the idea of partly defining planets by where they hang out in the solar system, which brought an immediate response from other astronomers.

"By the IAU's definition, when a cowboy herds his cattle he becomes a cow by association," said Alan Stern, a planetary scientist and former NASA science director who was deemed "Mr. Pluto" by Tyson. "This is why I like characteristics and not association."

Stern wondered at the absurdity of a definition that would exclude an Earth-like object with "oceans, continents, blue sky, people and Broadway lights" if it sat beyond a certain distance in the solar system.

The planet hunters

The idea of what counts as a planet becomes even less certain beyond Earth's solar system, where space telescopes have uncovered more than 300 exoplanets orbiting stars other than the sun.

"No one is writing a law or rule that you have to call them this or that," said Sara Seager, an astrophysicist at MIT.

She pointed out that while most attention has focused on the lower limit of the planet debate with Pluto, scientists also fiercely debate the upper limit where a planet becomes a star. A star is typically defined by being able to carry out fusion, but some super-massive brown dwarfs and other objects blur current planetary definitions.

"People have some cutoff they like, but lo and behold, astronomers discovered three planets orbiting a central star," Seager noted. "According to the IAU, they can't be planets because they're too massive."

And if defining a planet by its characteristics can run into some difficulties, defining planets by location gets even trickier. Tyson wondered about objects known as "planemos," or "rogue planets" that float through space without a star to orbit.

"Why not have a classification scheme starting with planemos ... I don't really like that word," Tyson mused.

Gibor Basri, an astrophysicist at the University of California-Berkeley, chimed in. "It didn't really catch on," said Basri, who coined the term.

Uncertainty aside, all of the panel members spoke eagerly of NASA's Kepler space telescope, which launched on March 7. That mission is designed to search for signs of smaller, rocky planets like Earth among over 100,000 stars.

"Let me be bold and say that perhaps planetary science is still in its infancy, and has no business classifying anything at all yet," Tyson said, noting that new data from Kepler's survey could change the debate down the road.

Pluto politics

The whole debacle has painted a new picture of how planetary scientists operate.

"I think this has been one of the more disappointing episodes for science with regard to the IAU," Stern said. "Now school kids see science as voting, and that's not the best way to do science."

"I like to call it the Irrelevant Astronomical Union," Stern added. He summed up the messiness of the scientific process as being "like cats herding themselves."

Basri agreed that "whenever you get a [scientific] issue decided by a vote, I think you can infer that people don't know what they're talking about." He observed that scientists did not vote on the existence of gravity.

All the arguments can also hide the fact that scientists continue to do research and pursue new knowledge, Seager suggested. "I'm not calling Alan [Stern] and saying, 'What do you think today, is it a planet?'" she said.

A 'teaching moment'

The science may remain far from settled on Pluto and other planets, but the panelists also saw a bright side in the mess.

"The best thing about this debate is that it got people interested and became a teaching moment," said Jack Lissauer, a theoretical physicist at NASA's Ames Research Center in California.

Much of the original controversy over Pluto's demotion may stem from how science has been taught in schools, Tyson said. He criticized the idea of branding facts into the brains of young schoolchildren, given that science remains a dynamic and ever-changing process.

"I see this as maybe a rare moment to see the birth of a new way of thinking about the solar system and the richness of objects that orbit it," Tyson said. He recalled recent letters from third graders born in 2000 who are now "kind of OK with Pluto being something other than the ninth planet," because they grew up with Pluto's planetary status already in question.

Seager mentioned that she had kids just starting school at a time when the "whole toy industry has already dropped Pluto," but also expressed confidence that students would be okay with a changing understanding of the solar system.

After all, the panelists suggested, it's perhaps more important to love the scientific process rather than the scientific facts.

"I'm one of the people who don't characterize Pluto as a planet," Lissauer said, drawing applause as he brought out a Disney "Pluto" stuffed toy. "But I care about solar system objects, and I am a Pluto hugger."