# What a planet is, revisited. Also Pluto. **URL:** \ **Category:** In My Humble Opinion\ **Created:** [July 16, 2015, 4:40am UTC](https://boards.straightdope.com/t/what-a-planet-is-revisited-also-pluto/725299 "2015-07-16T04:40:01Z")\ **Posts on this page:** 16\ **Page:** 4
**Author:** ![Measure\_for\_Measure](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/measure_for_measure/32/557_2.png) [@Measure\_for\_Measure](https://boards.straightdope.com/u/Measure_for_Measure)\ **Post date:** [July 25, 2015, 10:37pm UTC](https://boards.straightdope.com/t/what-a-planet-is-revisited-also-pluto/725299/61 "2015-07-25T22:37:00Z")
> [@Measure\_for\_Measure](#): > > Nice discussion of Sedna, which is 3 times as far from the sun as Neptune and ~42-45% the diameter of Pluto. It is outside of the Kuiper belt and in the inner part of the Oort cloud. They think it’s round though. Discussion of what a planet is and how it could be defined follows. [http://web.gps.caltech.edu/~mbrown/sedna/index.html](http://web.gps.caltech.edu/~mbrown/sedna/index.html) > > Possible ways of defining planet: historic, “Historic-plus”, gravitational rounding (the author prefers something like planetoid or sphereoid), and population classification. That last one is tied to the “Cleared its neighborhood” criteria and is currently accepted scientific usage. “Historic-plus” is the conventional definition: planets are defined as bodies that are equal to or bigger than Pluto. If I understand the argument correctly, the population based approach consists of matching like with like. And Pluto is indeed a Kuiper belt object: its orbit is off the solar plane and even crosses Neptune’s orbit at times. But that’s an odd way to define a planet, unless you believe that planets only exist in our solar system. And that doesn’t appear to be common usage either among astronomers or the general public. I can imagine fixing a cutoff of size, mass, atmosphere, geologic characteristics or (as I prefer) shape to designate a planet. But location is fairly odd. But! I see that a claim I made upthread was incorrect. [According to Steven Soter, a resident research associate at the American Museum of Natural History](http://www.politifact.com/punditfact/statements/2015/jul/21/other-space/plutos-advocates-say-even-earth-would-not-count-pl/) (quoted by Politifact of all sources) if Earth was in Pluto’s orbit it would dynamically dominate its orbital zone, if only barely. Earth would do about as good a job as Mars or Mercury - which are designated planets. ## (Note “Dynamically dominate” is a little better than “Clear its neighborhood”, which technically Earth doesn’t even do in its own orbit, though it does a better job than Pluto by orders of magnitude. See upthread discussion.) Another argument says that if we admit Pluto, we will have to admit hundreds of other objects. I think I showed in post 40 that this is not necessarily the case. That said and to repeat definitions are arbitrary and characteristics matter more than labels; this thread was intended as a decent entry point into planetary science for non-experts. I’ve learned a lot anyway. I presume that those familiar with the subject roll their eyes a little. My WAG is that roundness isn’t a criteria because it is difficult to measure relative to its scientific importance. Meanwhile, scientists debate which trans-Neptune object they will send the New Horizons spacecraft towards: the decision is expected next month. All 3 potential targets are 20-55 kilometers across. If all goes well one of them would be visited in 2018-2019. ---
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [July 26, 2015, 3:03pm UTC](https://boards.straightdope.com/t/what-a-planet-is-revisited-also-pluto/725299/62 "2015-07-26T15:03:34Z")
> [@Flyer](#): > > … I have a strong tendency to feel that the original of anything is the best. The original definition of Pluto was that it was a regular planet. It was a planet before it was discovered, and it is a planet now. Calling it something else does not and CANNOT change the fact that it is still a planet, and always will be. Then biological taxonomy is the thing for you! Once someone’s named a species, that’s IT, even if everything they thought about it was wrong (other than it being the first specimen of a previously unnamed species). This path leads to confusion, too. > [@UltraVires](#): > > I tend to agree with this. Why can’t any definition of a planet simply grandfather in Pluto to include it as a planet? I call Pluto “one of the historical 9 planets.” I think that gives it proper homage. But I also call it a dwarf planets. For some silly reason, I’d like to know the names of all the planets, and that list above just isn’t going to cut it! And seriously, there _is_ a significant distinction between the 8 planets and the dwarf planets, and it doesn’t hurt to give that distinction a name. But, it’s all semantics. We’re not arguing science, just nomenclature, so it really doesn’t matter. > [@Stranger\_On\_A\_Train](#): > > (BTW pronounced “kAr-on” not “shAy-ron” or “chair-On” like the newsheads keep repeating). Well … > [@wikipedia : Charon](#): > > There is minor debate over the preferred pronunciation of the name. The practice of following the classical pronunciation established for the mythological ferryman Charon (IPA [ˈkɛ:rən]) is used by major English-language dictionaries, such as the Merriam-Webster and Oxford English Dictionary.[13][14] These indicate only one pronunciation of “Charon” when referring specifically to Pluto’s moon: with an initial “k” sound. Speakers of many languages other than English, and many English-speaking astronomers as well, follow this pronunciation.[15] > However, Christy himself pronounced the ch as sh (IPA [ʃ]), after his wife Charlene. Because of this, as an acknowledgement of Christy and sometimes as an in-joke or shibboleth, the initial sh pronunciation is common among astronomers when speaking English,[16][15][17][18] and this is the prescribed pronunciation at NASA and of the New Horizons team.[19][20] So, as it turns out, those newsheads aren’t necessarily wrong after all, unless they’re talking about the mythical figure. But I find that I was pronouncing them both wrong in my head all these years, thinking the stress was on the second syllable, and that the ‘o’ was long. Ignorance fought! ---
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [July 26, 2015, 3:14pm UTC](https://boards.straightdope.com/t/what-a-planet-is-revisited-also-pluto/725299/63 "2015-07-26T15:14:09Z")
> [@kanicbird](#): > > Pluto is better off as king of the Kepler Belt then the lonely outsider of the planets. > > I don’t see it as a demotion anymore but a far better place for Pluto to be. Thread winner for appropriate application of whimsy. 🙂 ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [July 26, 2015, 3:49pm UTC](https://boards.straightdope.com/t/what-a-planet-is-revisited-also-pluto/725299/64 "2015-07-26T15:49:28Z")
> [@Learjeff](#): > > Then biological taxonomy is the thing for you! Once someone’s named a species, that’s IT, even if everything they thought about it was wrong (other than it being the first specimen of a previously unnamed species). This path leads to confusion, too.! And that’s how you ended up with the “male” seahorse being the one that gives birth 🙂 ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [July 26, 2015, 4:14pm UTC](https://boards.straightdope.com/t/what-a-planet-is-revisited-also-pluto/725299/65 "2015-07-26T16:14:54Z")
> [@Learjeff](#): > > Then biological taxonomy is the thing for you! Once someone’s named a species, that’s IT, even if everything they thought about it was wrong (other than it being the first specimen of a previously unnamed species). This path leads to confusion, too. That’s far from the case. Species are reclassified into different genera all the time, which often causes a change in the specific (second) name because its Latin gender is different from the new genus. Also, the specific name may have to be changed entirely if it is preoccupied by another species in the new genus. And while mistakes made in the original naming of species are generally retained, inadvertent spelling errors can be corrected at a later date. Every year, I have to update my species lists because of changes and revisions issued by the American Ornithologists’ Union. It drives me crazy. ---
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [July 26, 2015, 6:17pm UTC](https://boards.straightdope.com/t/what-a-planet-is-revisited-also-pluto/725299/66 "2015-07-26T18:17:14Z")
> [@kanicbird](#): > > Pluto is better off as king of the Kepler Belt then the lonely outsider of the planets. > > I don’t see it as a demotion anymore but a far better place for Pluto to be. > [@Colibri](#): > > That’s far from the case. Species are reclassified into different genera all the time, which often causes a change in the specific (second) name because its Latin gender is different from the new genus. Also, the specific name may have to be changed entirely if it is preoccupied by another species in the new genus. And while mistakes made in the original naming of species are generally retained, inadvertent spelling errors can be corrected at a later date. > > Every year, I have to update my species lists because of changes and revisions issued by the American Ornithologists’ Union. It drives me crazy. Thanks for the correction. I was thinking of the “type specimen” system (where the first one found and preserved is named and all other examples are referenced to that – even if the first is in poor shape or missing a lot), but clearly even with that system things change. Perhaps they should just give each species a UUID, and then it’d be easy to update name references. You’d still have to update things when one species turns out to be (judged to be) two, or vice-versa. Since there’s no objective criteria for what constitutes a species, I imagine this happens enough to be a nuisance. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [July 26, 2015, 7:36pm UTC](https://boards.straightdope.com/t/what-a-planet-is-revisited-also-pluto/725299/67 "2015-07-26T19:36:47Z")
> [@Learjeff](#): > > Thanks for the correction. I was thinking of the “type specimen” system (where the first one found and preserved is named and all other examples are referenced to that – even if the first is in poor shape or missing a lot), but clearly even with that system things change. In older literature, sometimes a describer failed to designate a single type specimen from among a group of specimens, which can be a problem with cryptic species in which specimens of two species may have both been included in the type description. A few years ago I had to designate what’s called a lectotype for a subspecies which was described on the basis of 14 specimens selecting a specific one. > [@](#): > > Perhaps they should just give each species a UUID, and then it’d be easy to update name references. You’d still have to update things when one species turns out to be (judged to be) two, or vice-versa. Since there’s no objective criteria for what constitutes a species, I imagine this happens enough to be a nuisance. Actually, a [DNA bar-coding system](https://en.wikipedia.org/wiki/DNA_barcoding) is being developed for species, but this depends on sections of the genetic code rather than designating an UUID. ---
**Author:** ![DrDeth](https://avatars.discourse-cdn.com/v4/letter/d/b487fb/32.png) [@DrDeth](https://boards.straightdope.com/u/DrDeth)\ **Post date:** [July 26, 2015, 9:24pm UTC](https://boards.straightdope.com/t/what-a-planet-is-revisited-also-pluto/725299/68 "2015-07-26T21:24:31Z")
> [@Stranger\_On\_A\_Train](#): > > Charon (BTW pronounced “kAr-on” not “shAy-ron” or “chair-On” like the newsheads keep repeating). Greek only has Hard Cs, no “sh” sounds from a C. We should just change the spelling to K for all the words. Same with Elvish. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [October 29, 2019, 4:45pm UTC](https://boards.straightdope.com/t/what-a-planet-is-revisited-also-pluto/725299/69 "2019-10-29T16:45:32Z")
\*\* [Surprisingly round asteroid may actually be the smallest dwarf planet](https://www.newscientist.com/article/2221288-surprisingly-round-asteroid-may-actually-be-the-smallest-dwarf-planet/) _SPACE_ 28 October 2019 By Leah Crane\*\* Hygiea … discovered in 1849, … is much more spherical than we originally thought. In order to be a dwarf planet, an object must satisfy four criteria: it has to orbit the sun, it cannot be a moon, it must not have swept up all the smaller objects in its orbit, and it must have enough mass that its own gravity has pulled it into a spherical shape. We already knew that Hygiea satisfied the first three, and the new images taken by Miroslav Brož at Charles University in Prague and his colleagues show that it satisfies the fourth. Has link to other discussions on “roundness,” definitions… ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [October 29, 2019, 9:52pm UTC](https://boards.straightdope.com/t/what-a-planet-is-revisited-also-pluto/725299/70 "2019-10-29T21:52:13Z")
> [@Flyer](#): > > I only half-jokingly offer this definition– > > Planet: Something of which this solar system has nine. > > […] Great. You just made Ruth Bader Ginsburg a planet. Well, she probably does deserve the status… ---
**Author:** ![BlueInfinity](https://avatars.discourse-cdn.com/v4/letter/b/9e8a1a/32.png) [@BlueInfinity](https://boards.straightdope.com/u/BlueInfinity)\ **Post date:** [October 29, 2019, 10:21pm UTC](https://boards.straightdope.com/t/what-a-planet-is-revisited-also-pluto/725299/71 "2019-10-29T22:21:00Z")
I thought another criteria for a planet was[clearing the neighborhood of its orbit.](https://en.wikipedia.org/wiki/Clearing_the_neighbourhood). That’s why Ceres isn’t a planet, it shares the neighborhood of its orbit with umpteen other asteroids. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [October 29, 2019, 11:16pm UTC](https://boards.straightdope.com/t/what-a-planet-is-revisited-also-pluto/725299/72 "2019-10-29T23:16:56Z")
Yeah, this: > [@](#): > > According to simulations by the researchers, it seems as if it also formed differently from the others – in an enormous collision that completely broke apart a larger body, allowing the fragments to coalesce into a sphere, rather than by slowly changing shape over time. is not hydrostatic equilibrium. It is a rubble pile. ---
**Author:** ![Two\_Many\_Cats](https://avatars.discourse-cdn.com/v4/letter/t/8dc957/32.png) [@Two\_Many\_Cats](https://boards.straightdope.com/u/Two_Many_Cats)\ **Post date:** [October 30, 2019, 12:31am UTC](https://boards.straightdope.com/t/what-a-planet-is-revisited-also-pluto/725299/73 "2019-10-30T00:31:27Z")
Six suns?? On Pluto?? ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [October 30, 2019, 12:39am UTC](https://boards.straightdope.com/t/what-a-planet-is-revisited-also-pluto/725299/74 "2019-10-30T00:39:09Z")
What a planet is not is something decided by the IAU. If they want to invent new categories for celestial bodies they can come up with their own words. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [October 30, 2019, 1:24am UTC](https://boards.straightdope.com/t/what-a-planet-is-revisited-also-pluto/725299/75 "2019-10-30T01:24:20Z")
> [@Darren\_Garrison](#): > > Yeah, this: > > is not hydrostatic equilibrium. It is a rubble pile. My understanding is that Hygiea was thought to have partially melted because of that collision, which means it would have been in a rough hydrostatic equilibrium until it resolidified. Anyway, it’s not a dwarf planet until so declared by the IAU. They came up with the category, so they own it. Note that they haven’t added any to the list since the original five, even though there are many likely candidates. My guess is that no one there is enthused about the category, so no action. It certainly isn’t a scientificly useful category. ---
**Author:** ![GargoyleWB](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gargoylewb/32/199_2.png) [@GargoyleWB](https://boards.straightdope.com/u/GargoyleWB)\ **Post date:** [October 30, 2019, 3:25pm UTC](https://boards.straightdope.com/t/what-a-planet-is-revisited-also-pluto/725299/76 "2019-10-30T15:25:00Z")
> [@Colibri](#): > > Every year, I have to update my species lists because of changes and revisions issued by the American Ornithologists’ Union. It drives me crazy. Dear Members, Please be informed that members of family Columbidae, aka pigeons, have been reclassified as planets due to their general roundness and tendency to clear their local area of neighbors. Henceforth, the family name Planetidae shall be used. –President, American Ornithologists Union [Previous page](https://boards.straightdope.com/t/what-a-planet-is-revisited-also-pluto/725299.md?page=3)