Experts meet to decide Pluto fate, Finally we'll know what a 'planet' is... Options

[karolp]

And I fully agree with Alan Stern in this matter - the definition IS premature and stretched. It is not the word "planet" that should have been defined but the Kuiper Belt, after which Pluto should have been included in it. But that was not the case, which resulted in a really obvious stretch. And I fear this may be used as a beachhead in the process of undoing IAU's decision - which, if it really happens, would further undermine IAU's authority. A good way out of this would be keeping Pluto demoted and dropping the "dwarf planet" mist altogether. Even pro-Pluto people can see the "dwarf planet" to be a stretch as Ceres and Xena do not have much in common. But restoring Pluto now would cause much more serious confusion among public and make the attempt at prematurely defining "planet" a few orders of magnitude more laughable than it already is.

[Stephen]

Nope, that Terrestrial Belt of yours is not a crowd of SIMILARLY SIZED objects (Mercury, Venus, Earth and Mars are clearly outstanding in that regard, being an order of magnitude larger and many orders of magnitude more massive than NEOs).

1) That reference to "similarly sized objects", are you suggesting that there are no equivalents of NEOs (let's call them Near Pluto Objects--NBOs) out there in the Kuiper Belt?

2) I ask that because it seems to me you're comparing the KBOs in the Kuiper Belt and the TBOs in "my" Terrestrial Belt using different yardsticks when judging what's dominant and what's not. Out in the Kuiper Belt you seem to be comparing Pluto, Xena, et al with one another. That then allows you to claim them to be a "a crowd of SIMILARLY SIZED objects".

Down sunward here in the Terrestrial Belt, on the other hand, rather than comparing Mercury, Venus, Earth and Mars with one another you're comparing them instead with NEOs! That then allows you claim that "Mercury, Venus, Earth and Mars are clearly outstanding". But then so would Pluto, Xena, Sedna, & other large KBOs if you were comparing them with objects only a few dozen miles wide, rather than with one another. (Out in the Kuiper Belt objects the size of an NEO are doubtless too small to be seen with the telescopes available back on faraway Earth.)

3) You would make a better case for Mercury, Venus, Earth and Mars being "clearly outstanding in that regard"--unlike Pluto--if you'd explained that Pluto's orbit is so highly eccentric it spans--indeed defines--the Kuiper Belt (from 29 AU to 49 AU). In doing so it crosses the orbits of lots other KBOs of similar size, unlike the orbits of Mercury, Venus, Earth and Mars in the Terrestrial Belt, which only intersect with a few NEOs rather than with one another. In that context one might well argue that Earth (say) is dominant in its orbital zone whereas a body like Pluto is not.

Furthermore, it does not end abruptly at 5 AU but much closer as JUPITER is at 5 AU and has CLEARED the space near its orbit.

Conceded. But then I chose 5 AU as a conveniently round figure--much as the 30 AU & 50 AU figures you gave for the inner and outer edge of the Kuiper Belt are doubtless also conveniently round figures

(Although I notice from this Wikipedia diagram that some--including Xena--are way out beyond 50 AU.)

I just happen to have listened to a podcast recording of voting on the IAU assembly (the Jodcast). One of the astronomers put forth an argument similar to yours. He was immediately reminded of the 3:2 resonance - many objects have been locked in that resonance with Neptune, including Pluto. The word "cleared" is not that good but the astronomers were simply forced to define things prematurely without examining a sufficient number of specimens. I guess "influenced" would have come out as second thought instead - after careful consideration - but they went for "let us vote now quick 'cause we need a coffee break" way of doing things instead.

Nobody "forced" them to define anything. They could have always postponed the decision, or better still set up a group to study the issue in detail and present findings to the next IAU meeting. That they chose to *act* prematurely implies they did so without first fully assessing of the issues involved, which in turn suggests that the action they did take (that definition) was probably ill-conceived and not well thought through.

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Stephen

[odave]

FYI:

The November 2006 issue of Sky & Telescope has the Pluto flap as its cover story. The article is written by Owen Gingerich, chair of the IAU's planet definition committee. It gives his inside perspective (and frustration) with the entire process and its outcome.

The issue's editoral, written by Editor in Chief Richard Fienberg (titled Pluto Doesn't Care), lays out S&T's position on the definition of a planet:

...Sky & Telescope won't use it without qualifiers. For example, we'll refer to Pluto as a "Dwarf Planet" (with quotation marks) or as a so-called dwarf planet, but never simply as a dwarf planet, because to sensible people that phrase means "a little planet", which is not what the IAU intends. And we have no plans to remove Pluto from our Sun, Moon and Planets pages.

[David]

As I suggested before either the proposal was presented or the vote took place, the IAU should have made clear that the only function of its "planet" definition was nomenclatorial - to decide whether new largish bodies that were discovered that were bigger than Pluto should be named according to the old Roman-god model for the other planets, or could be named according to the various minor planet models. That question was answered effectively: objects in (at least) the <Mercury, >Pluto range will be named and numbered as minor planets. And whatever you think about the rationale, that is a decision I think most people can live with, and I haven't heard a single voice protesting against the name "Eris" for 2003 UB313, even though it is obviously not Latin nor the name of a major deity.

The problem comes when the decision is blown up into something much bigger than that, some sort of major scientific or philosophical statement about the nature of the universe. And that's just silly. "Planet" in a general sense no more needs or deserves a scientific definition than "world" or, for that matter, "orbis terrarum". Who decides whether something the size of, say, Io or Enceladus is rightfully called a "world"? It's a matter of literary convention. "Planet", I think, is no more scientific than "world", and if this orbit-clearing definition is really coherent and relevant, then they should have come up with some new and non-confusiing term for the objects picked out by it -- because historically, although "planet" has gone through many definitions, nothing like that has ever been held to be part of the definition of a planet before.

As a bookkeeping decision on how to categorize and name solar system objects, I have no problem with the results of the IAU vote. But the word "planet" is far older than the IAU, and ultimately I think it was ultra vires -- beyond the powers they can rightfully claim, for the IAU to presume to present a definition of "planet" for the whole world to follow.

[JRehling]

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