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« Excommunication | Main | This Is Not An Elephant! »

November 22, 2006


Sifu Jones

To solve the moon-as-a-sister-planet "problem", could not the definition also include that an official planet cannot be dominantly effected (in other words, a satellite of) by the gravitational pull of another round, non-fusioning body?

That makes a planet distinct from a moon in that it orbits nothing but a star, assuming it orbits anything at all.

Then again, that poses the problem of a planet being defined by its location, which is accidental. I suppose planets could be divided into more or less equal classes of primary and secondary orbitals. That would allow for things like Jupiter's satellites being bigger than earth, and still being planets.

Jimmy? What do you think about sub-classes of planets in general? I'm not talking about the "lesser planet" theory that's being ascribed to Pluto and others now, just positing another type of categorization.


Planet= a large enough round mass of some substance directly in orbit around a star or stars.

Moon=a large enough round mass of some substance directly in orbit around a planet.


I like the idea of astronomers "alienating" people.


kevin: "in orbit around" isn't necessarily as simple as it seems -- for instance, the center of mass of the Earth/Moon system is only juuuust barely below the Earth's surface. It's effectively a binary planet.


"in orbit around" isn't necessarily as simple as it seems -- for instance, the center of mass of the Earth/Moon system is only juuuust barely below the Earth's surface.

In that case, the Moon is only juuuuust barely a satellite of the Earth, rather than a sister planet. :-) That's the nice thing about having a non-subjective cutoff -- you don't have to sweat the close cases.


I like your definition of planet as a scientific term, but the real question is what does the average person need to know to be culturally literate?

Everybody is "supposed" to know the nine planets. Even when Pluto was "demoted" it was going to remain a "planet with an asterisk" as my uncle described. That is, in all those charts in grammar school text books, Pluto would always be mentioned as a planet, at least as a footnote. Now, if the pendulum swings in the opposite direction, and there are upwards of 6,000 planets swarming around our sun, which planets does the average person need to know? There will inevitably be some division of "major planets" which are divided into their traditional gas giant and rocky groupings, and the thousands of small, rocky "minor planets." In terms of sizes, Pluto would likely be officially grouped with the latter species, but taught as one of the former.

There would/could also be a division among planets that are solar satellites (traditional planets), planetary satellites (where the center of mass of a given system is within the crust of one of the bodies, as mentioned above with the Earth/Moon system or any of the gas giant systems), and outright binary planets (where the center of mass resides outside the physical structure of any of the bodies, like the Pluto/Charon system).

Francis DS

+1 for kevin's definition.

I'd hate to start calling the Moon, "Planet".


"I'd hate to start calling the Moon, 'Planet'." Just think of the problem it would cause poets and lyricists, having to rhyme "planet"!


I did think that Pluto got a raw deal :o)



The Janet will get a lot more poetic notice.

David B.

"The Great Pluto War "


Randolph Carter

What greatly aggravates me about this whole matter is not that our dear friends in the scientific community have chosen to define Pluto as a "dwarf planet", but that we who speak the vernacular English are automatically expected to change the term that we, speaking in the vernacular, use to refer to Pluto, in order to conform with the current scientific lingo.

The definition of what does and doesn't constitute a "planet" is entirely arbitrary. In times long past, as you, Mr. Akin, point out, the term "planet" has been assigned to a variety of different celestial bodies, some of which we no longer consider to be "planets". In terms of our cultural conscious in the English-speaking world, we have been thinking of Pluto as a "planet" since its discovery in the early part of the twentieth century.

Scientists may or may not choose to rework the definition of what is and what isn't a planet however they want, to the inclusion or exclusion of any and all heavenly bodies, because scientists generally assign terms to things to make them easier to classify. We should not, however, be willing to ditch our vernacular definition of what a planet is just to conform to the ever-changing, re-definable categories which are beloved scientists have created for the classification of objects in our solar system.

The vernacular and scientific-lingo are two different things, and I cannot stress this enough. According the narrow scientific definition of what does and doesn't constitute a "bug", we should not be referring to spiders, centipedes, flies, cockroaches and silverfish as "bugs", because there are, in the scientific lingo, only two kings of "bugs": stink bugs and shield bugs. Likewise, because scientists have used the term "fish" to refer to those vertebrates who live underwater and who are possessing of gills, and ONLY to those vertebrates who live underwater and who are possessing of gills; and I have been told by scientists that we should not call starfish "starfish", but rather "seastars" because starfish do not fit the scientific classification of what does and doesn't constitute a fish. I think it goes without saying that whatever fanatic who came up with the detestable moniker of "seastar" should be hung from the ceiling by his thumbs until they fall off, but sadly this sort of invasiveness of the scientific terminology into the vernacular continues to persist. What is next? Can we no-longer speak of "devilfish", "cuttlefish" or "jellyfish"? Herman Melville, while acknowledging the lungs and hair of the whale, still chose to call them "fish", because their phenotypological appearance resembled other little critters that swim in the pelagic deeps. Alas, I suppose Melville was but an ignorant sod, and now we must go back and amend Moby Dick

In closing, the vernacular is quite a different tongue from the sciencespeak, and to those of us who speak the vernacular Pluto has always been a Planet, it is still a planet, and it shall always be a planet, experts be damned!

In case you can't tell, I have quite strong beliefs on this topic :)


I suspect we could do as well as the experts on nomenclature.

OBANGKM stars - which fuse hydrogen, (even though all bodies with atmospheres show similar weather patterns)

Gas giants if < than hydrogen fusion (not this "if the universe is 15 billion years old, and if we ignore the rocky and metallic elements, it might have fused deuterium once upon a time" criteria)

rocky planets

Anything too small to be spherical if made of a normal iron-silicon mix, a _planetismal_

Some name such as KBO for Pluto and kindred, which are round due to being ice and other volitiles, not round due to silicates and metals. Including ice-body spherical moons such as Enceladus, Mimas, Rhea, Dione, Miranda, Ariel, Umbriel and company.

Then, it would be nice to have a category for 'planetoid' for the spherical rocky bodies such as Ceres, Vesta, Pallas and Juneo, the Moon, the Galilean moons, but how to define the term?

Or we could use Star Trek classifications "m-class planet" ;-)


I hope "Eris" stays the name. Even more suitable now. 0:)


How "round" do the planets have to be? Only 3 of the traditional 9 have the same equatorial and polar radius. Therefore, 6 are not perfectly round but round to a lesser extent. Information on the radii can be found here: http://www.nasm.si.edu/research/ceps/etp/ss/ss_planetdata.html.

J.R. Stoodley

The fact that the Moon used to be considered a planet helps warm me up to the idea that it could be considered one.

Danny's point is important though. We will need to have a precise definition of "how round is round" for Jimmy's proposed definition. The exact cutoff point will probably have to be fairly "arbitrary." You just have to live with such things in science.

francis 03

Realist's framework seems pretty sharp to me, although I would allow any kind of fusion to qualify a body as a star. I would suggest that the label "planetoid" should be applied to any body that constitutes less than some specified percentage of the total mass in the vicinity of its solar orbit. This is essentially an elaboration of the "cleared its orbit" criterion, but it clarifies why Ceres and Pluto aren't planets while Jupiter is. What the exact cutoff point should be I don't know. Is there some natural benchmark?

In the long run, it seems to me that "planet" is going to wind up being way too broad a term for scientific usage anyway. We will have to have lots of classes, of which our current rocky-gaseous-icy trichotomy will be only the broadest level.

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