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."
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