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Pluto: The Case of the Demoted Planet
by Callie McNew, IfA Outreach Assistant
The panel, from left to right, Richard Crowe (UH Hilo and `Imiloa's
astronomer in residence), Jay Farihi (Gemini Observatory), William Heacox
(UH Hilo), David Jewitt (IfA), Inseok Song (Gemini), Michihiro Takami (Subaru
Telescope), and David Tholen (IfA). Behind them is a chart showing the
relative sizes of trans-Neptunian objects.
Is Pluto a planet? That was the question the International Astronomical
Union (IAU) General Assembly tried to answer at its triennial meeting last
August. The announcement of Pluto's demotion to "dwarf planet" received
international attention both within the ranks of professional astronomers
and among members of the public. To recognize the public's interest
in and concern about the status of Pluto, the `Imiloa Astronomy Center
of Hawaii presented "Bye Bye Pluto: The Case of the Demoted Planet" on
September 29.
The event featured an excerpt from the BBC documentary Bye
Bye Pluto and
a panel discussion by six Hawaii astronomers specializing in small solar
system bodies and extrasolar planets. Bye Bye Pluto, made before the August
IAU meeting, tells the story of how IfA astronomer David Jewitt and his
colleague Jane Luu discovered the first Kuiper Belt object (KBO, also called
trans-Neptunian object) in 1992, and explores the implications of this
discovery for the classification of Pluto. Essentially, finding the first
KBO was the beginning of the end for Pluto as a planet, because it showed
that Pluto is just another member of a vast reservoir of objects beyond
Neptune's orbit. At the `Imiloa event, Jewitt gave an inspiring personal
account of the discovery of the first KBO and the technology necessary
to detect such objects.
IfA's David Tholen offered passionate arguments against Pluto's
demotion. He was particularly disturbed by the IAU's voting method
in which fewer than 500 of the approximate 9,000 IAU members—only
those attending the final day of the August meeting—were eligible
to vote. Tholen also objected to the new nomenclature. It is confusing
because a dwarf star is a type of star, but a dwarf planet is not a planet,
he said. Tholen acknowledges that Pluto is somewhat of an oddball compared
with the other planets, but he does not concur with the IAU's new
criteria for planets.
According to the new criteria, a planet is a celestial body that orbits
the Sun, has sufficient mass to assume a nearly round shape, and "has
cleared out the neighborhood around its orbit." It is this last part
that disqualifies Pluto, since it is located in the Kuiper Belt, a ring
of millions of solar system formation remnants. Tholen would prefer an
easier definition, perhaps based on magnitude. If an object is brighter
than an absolute magnitude of zero, it could be called a planet. If that
were the case, Pluto would still be considered a planet. The other panelists
agreed that the term "dwarf planet" is ambiguous, but they
still considered it sensible to exclude Pluto from the list of planets.
Both Tholen and Jewitt agreed that it makes little difference to scientists
whether Pluto is called a planet or a dwarf planet. They will study Pluto
in either case. The definition is really for the public, since it affects
the way people view the solar system. It was obvious from their questions
and comments that most members of the audience wanted to retain Pluto as
a planet. In any case, a change cannot be made until the next IAU General
Assembly meets in 2009.
Tholen's Objections
"The problem with the IAU definition is that it will not be possible
to apply it to a newly discovered object in the distant reaches of
the solar system. Suppose someone discovers a 24th magnitude (very
faint) object with a nearly circular orbit 500 times the distance
of Earth from the Sun. It may be as large as Earth, but we would
have no information on whether it has cleared its zone, so the question
as to whether it is a planet could not be answered, which I suspect
would be objectionable to the public. My proposal to base the definition
on whether an object has an absolute magnitude of less than zero
solves the problem, but at the expense of scientific rigor. Because
the definition is more for the public than for the scientist, I don't
have a problem with a lack of scientific rigor.
"We've had thousands of minor planets (asteroids) in the solar
system for decades. A minor planet is still a planet in the same
way that a compact car is still a car. Indeed, the original question
was whether Pluto was a major planet or a minor planet. The discovery
of the Kuiper Belt made the situation analogous to the asteroid belt
between Mars and Jupiter, where we now have over 300,000 minor planets.
Somehow the question morphed from whether Pluto is really a minor
planet into whether Pluto is a planet. The IAU didn't just do away
with one planet, but rather hundreds of thousands of planets, though
minor ones." |
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