More about Pluto

Pluto and Charon and the two little moons

The Case for Pluto by Alan Boyle

After reading The Pluto Files a couple weeks ago and posting about it, I got a comment from a ubiquitous blogger who’s on the “pluto IS a planet” side of things.  Her blog recommends The Case for Pluto as an explanation of why Pluto should be called a planet, so in the interest of due dilligence, I read that too.

Boyle gives a more thorough discussion of the two conflicting viewpoints about the word planet, and gives a more balanced discussion of why the IAU decision was so thoroughly disputed.  I’ll discuss that bit below, but first a couple tidbits from Boyle’s book that I enjoyed:

• The guy who named Pluto’s moon wanted to name it after his wife, Charlene, so he decided to take the first syllable of her name and add the suffix -on to be similar to proton or neutron.  When the committee on naming things told him he had to name it after a god, he was bummed.  But then he discovered that the ferryman across the river Styx was named Charon.  In Greek, it’s pronounced “Care – on” but apparently astronomers use the pronunciation “Share – on” to reflect the original intent.
• One of the first people to raise the “should Pluto be a planet” question did so at the 50th anniversary of Pluto’s discovery, in front of Tombaugh and his family.  Gaff!
• In the next few years, we expect to find many pluto-sized bodies (or bigger) in the Kuiper belt.  There will probably be some as big as Mars.

The whole things turns out to be a narrow window of semantics that gets spliced onto a question of common concern for regular people.  Here’s how it breaks down:

Planetary scientists generally think anything big enough to be rounded by its own gravity and small enough not to have fusion at its core should be a “planet.”  Then, they suggest that we should have a series of adjectives that describe what kind of planet we’re talking about: rocky dwarf planet (Ceres), ice dwarf planet (Pluto, Eris, et al), rocky planet (Mercury-Mars), Gas giant planet (Jupiter, Saturn) and Ice giant planet (Uranus and Neptune), with more to be added as needed.  The distinction here is that all rounded objects would be planets.

Dynamicists are scientists more interested in the way bodies influence one another through gravity and so on.  For example, Neptune was discovered because the gravitational mathematics said there should be a big planet somewhere in that neighborhood.  To this layperson, such calculations seem unreliable, as other predictions have failed to turn up similar discoveries.  (At one point, there was thought to be a planet called vulcan inside Mercury’s orbit.)  Dynamicists prefer the IAU designation because it defines planet as something that has “cleared its neighborhood” of bodies, like the classic 8, while something that hasn’t cleared the neighborhood would be a dwarf planet.  “But not a REAL planet.”

As I said, to the layperson this seems like a pretty minor difference, as both are using multiple sets of criteria, while the IAU version puts “planet” above “dwarf planet.”  Where the extra tension comes from, for me, is the public complaint about Pluto’s being ‘demoted’ to dwarf planet.  I think the ruckus in the public sphere has to do with the change from the “original 9″ to a different definition.  When people learn that having Pluto be a planet means that we will also have four more right now, plus dozens to come in future years, I think they’ll be equally irritated with the change.  In other words, there are two groups complaining about Pluto not being a planet anymore.  One group of scientists have a legitimate beef and want all the rounded objects to be recongized as planets.  But the citizenry, who appear to be on the side of these scientists, are really in favor of keeping the old order intact.  They don’t just want Pluto to be a planet, they want there only to be nine planets.

So I guess I’m convinced that any rounded object should be called a planet.  But I’m also convinced that given our current information, it’s not appropriate to put Pluto in the special category it’s been in for all these years (as one of nine).  Thus, the decision Tyson made in The Pluto Files was right on — Pluto is better represented as part of the Kuiper Belt objects.  A “planet,” yes, but one of a group separate from the other eight (who are, in fact, two or three different groups themselves).