The
International Astronomical Union (IAU) has spoken on the status of Pluto. The
only thing missing when they announced the decision at their press conference
was the "Mission Accomplished" banner. Yes, I'm afraid this matter is about as
settled as the Iraq war in 2003.
You've
probably heard the basics: Pluto is no longer to be considered a "real" planet,
but will instead be part of a new class of objects called "dwarf planets."
These midgets may number anywhere between a handful and hundreds in our own
solar system, depending on how you count them and how hard we search for them
with more powerful telescopes. But no matter how you cut it, this new
definition takes away any pretense of Pluto being a member of the same elite,
planetary club as Earth and Jupiter.
As a
scientist, I think this was a pretty good outcome, though some of the
justifications used to achieve it are dubious. But as a teacher, textbook
writer, and builder of scale model solar systems, I have some reservations. In
particular, for the model solar systems I've helped develop on the University of Colorado Boulder campus and on the National Mall in Washington, DC, I have to
ask: Should we now bulldoze Pluto?
A little
background: Pluto was discovered in 1930, at a time when astronomers were
searching for an object thought to be causing slight perturbations to the
orbits of other planets around the Sun. Neptune itself had been discovered in
just this way in 1846, after scientists used perturbations in Uranus's orbit to
predict the existence and location of an "eighth planet." Neptune was found as
soon as astronomers pointed telescopes to the calculated position, which is I
like to say that Neptune was discovered with physics and mathematics, and only
confirmed with a telescope.
Over the
ensuing decades, a few scientists claimed to see ongoing orbital discrepancies
and embarked on a search for a "ninth planet" that might be causing them. Pluto
was found during this search, though about 12 full-moon-widths away from the
predicted position. And though hailed as a planet upon discovery, its status
gradually became suspect, as we learned that its orbit is much more tilted and
elongated than that of any of the other planets, and that it has a mass much
less than 1% that of Earth. Worse, reanalysis of past observations suggested
that the claimed orbital discrepancies had simply been errors in measurement,
making Pluto a solution to a nonexistent problem.
In 1951, by
analyzing comet orbits, astronomer Gerard Kuiper predicted the existence of a
"comet belt" beyond Neptune -- now officially named the Kuiper belt --
analogous to the asteroid belt between Mars and Jupiter. Telescope technology
caught up with this prediction in the 1990s, and astronomers soon confirmed the
existence of vast numbers of "Kuiper belt objects" orbiting the Sun in the
general vicinity of Pluto. Scientifically, it became obvious that Pluto was
much more like a large member of this group than like a small version of any of
the other 8 planets. But as long as Pluto was the largest Kuiper belt
object, the IAU felt it acceptable to leave its planetary status intact.
Then, in
July 2005, astronomer Mike Brown announced the discovery of a new Kuiper belt
object -- still nameless today but nicknamed "Xena" or "Planet X" -- that is
slightly larger than Pluto. This discovery was a fatal blow to the status quo
of nine planets; after all, if Pluto is big enough to count as a planet, then
Xena must belong to the club too. But what of the dozens of Kuiper belt objects
only a little smaller than Pluto, and of future discoveries yet to be made? How
would we draw the line between "planet" and "large Kuiper belt object?" Like
all groups facing a tough decision, the IAU appointed a committee.
At first,
the committee proposed roundness as a primary criterion for planethood, which
would have admitted both Pluto and Xena to the club. However, their proposal
would have also admitted Pluto's moon Charon (for technical reasons), perhaps
as many as 40 or more other Kuiper belt objects, and even the asteroid Ceres.
Given that Ceres was discovered in 1801, we would have had to start saying
silly things like "The ninth planet, Neptune, was discovered in 1846, when it
was mistakenly identified as the eight planet."
The IAU
members saw the light, and modified the definition to keep Ceres and Charon out
of the club by adding that a planet must not only be round, but must also have
"cleared the neighborhood around its orbit." Ceres fails this criterion,
because it is just one of many asteroids in the asteroid belt. Charon returns
to being a moon of Pluto. But Pluto loses its status too, because it is just
one of many objects in the Kuiper belt, and in fact crosses over the orbital
path of Neptune.
Some of my colleagues are quite upset, pointing out that no
object has truly cleared it its orbit -- that's why collisions still sometimes
occur, like the comet that smacked into Jupiter in 1994 or the asteroid that
struck Siberia in 1908. Others wonder why we need to add the new term "dwarf
planet," when "large asteroid" or "large Kuiper belt object" can already
describe objects like Ceres and Pluto and Xena quite well. And imagine the
confusion if we someday discover a Mars- or Earth-size ice ball in the Kuiper
belt: officially, such an object would count as a dwarf planet, but it would be
larger than some of the non-dwarfs. Indeed, many astronomers wonder why we need
an official definition of "planet" at all. That's why I doubt this debate is
over, and it brings me back to my point: do we really need to take Pluto out of
songs, place mats, mobiles, and model solar systems? I don't think so.
The important thing to remember is that the new definition
was established by a vote, making it politics, not science. The politics may
yet change again, but Pluto will remain the same. I suspect it will stay a
planet in hearts and minds no matter what the IAU says, much as Europe and Asia remain separate continents to everyone except the geologists. Pluto, after all, is not
just any planet -- it's the only planet with a famous dog named
after it, and some people may not take well to sending the dog home just
because it didn't turn out quite as big as originally hoped. So let's keep the
bulldozers away from the Pluto pedestals on the CU campus and the National
Mall. In fact, if someone will provide the budget, I'd recommend adding a
pedestal for Xena, if astronomers ever vote to give it a real name.
Defining Moments: The Saga's History
Jeffrey
Bennett is the author of more than a dozen books, including Life in the
Universe (with Seth Shostak), Max Goes to the Moon, Max Goes to
Mars, and the forthcoming Beyond UFOs.
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