# What's a planet? **URL:** \ **Category:** Factual Questions\ **Created:** [June 23, 2006, 12:54pm UTC](https://boards.straightdope.com/t/whats-a-planet/361851 "2006-06-23T12:54:04Z")\ **Posts on this page:** 15\ **Page:** 2
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [June 24, 2006, 1:54am UTC](https://boards.straightdope.com/t/whats-a-planet/361851/21 "2006-06-24T01:54:25Z")
> [@Podkayne](#): > > Better make that “does not support nuclear fusion of [sup]1[/sup]H.” > > Larger brown dwarfs go through a short phase of deuterium ([sup]2[/sup]H) burning. Then would the ones into He burning phases not count as stars anymore? No [sup]1[/sup]H… ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [June 24, 2006, 4:03am UTC](https://boards.straightdope.com/t/whats-a-planet/361851/22 "2006-06-24T04:03:37Z")
> [@Podkayne](#): > > Better make that “does not support nuclear fusion of [sup]1[/sup]H.” > > Larger brown dwarfs go through a short phase of deuterium ([sup]2[/sup]H) burning. Due to gravitational compression during formation? ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [June 24, 2006, 7:26am UTC](https://boards.straightdope.com/t/whats-a-planet/361851/23 "2006-06-24T07:26:04Z")
> [@Chronos](#): > > Depending on exactly how “planet” is defined, I can argue for any number from 8 to 18 “planets” in the Solar System (ironically, in one case, shrinking the definition of “planet” actually increases the total number). The longest list I can come up with is Mercury, Venus, Earth, Luna, Mars, Ceres, Vesta, Io, Europa, Ganymede, Callisto, Saturn, Uranus, Neptune, Pluto, Xena, Quaoar, Sedna: This list comes from the definition that a planet is an object large enough to form into a sphere gravitationally (with enough leeway on “sphere” to include Vesta), but small enough that dR/dm \> 0 and hydrogen does not fuse in its core, on which the strongest gravitational force is exerted by a star. dR/dm ??? I’m trying to figure out how you exclude Jupiter from this list, but I don’t understand this part of the definition. Defining a planet as being any object, irrespective of its orbit, that masses at least as much as Pluto produces a list of 17 objects. Personally, I’d go with this kind of definition, but put the minimum mass higher so that there are only 8 planets. Yes, this is arbitrary, but any definition of planet we come up with is going to have arbitrary factors in it. > [@](#): > > Really, though, I think the real problem with classifying Pluto is that we’ve taken at least two completely different types of objects, or three if we include Pluto, Xena, and the like, and lumped them into a single category. Actually, 4 types of objects. Jupiter and Saturn are clearly gas giants, with the bulk of their mass being made up of gases. Uranus and Neptune seem to be a different kind of animal, with much higher proportions of rocks and ices. They are frequently called ice giants. > [@Stupid\_Owl](#): > > Well, as I hope all of you already know, when Ceres was first discovered everyone was quite positive that it was a planet. It wasn’t until the interest sparked by Ceres’ discovery lead to the identification of several other masses in similar Solar orbits that Ceres planetary status was revoked in favor of just marking off several million kilometers of solar orbit as ‘The Asteroid Belt’. Actually, the situation was a bit different than that. Ceres was discovered at the beginning of the 19th century; the next three, Pallas, Juno, and Vesta, were discovered in relative quick succession over the next few years. Then there was a hiatus of some 30 years (I think) where there were no more asteroids discovered. During that time, the 4 known asteroids were all called planets. But then many more asteroids were discovered and it became obvious that these were a different class of object than the classic planets. The same situation applies here, except that the hiatus was more than 60 years between the discovery of Pluto and the next KBO. During that time, Pluto was enshrined in the list of planets by popular culture, making it difficult to evict. The situation is unfortunate, but we need to bite the bullet and expunge this imposture from the ranks of the planets. ---
**Author:** ![NineToTheSky](https://avatars.discourse-cdn.com/v4/letter/n/b5a626/32.png) [@NineToTheSky](https://boards.straightdope.com/u/NineToTheSky)\ **Post date:** [June 24, 2006, 10:03am UTC](https://boards.straightdope.com/t/whats-a-planet/361851/24 "2006-06-24T10:03:42Z")
> [@Stupid\_Owl](#): > > I should add that, by the original definition, only moving points of light in the night sky regularly visible to the naked eye are planets; that would be: Mercury, Venus, Mars, Jupiter, and Saturn. Therefore there would be only five planets. Wouldn’t our Earth qualify? Making six? (Agreed, you’d have to stand well back). ---
**Author:** ![Podkayne](https://avatars.discourse-cdn.com/v4/letter/p/edb3f5/32.png) [@Podkayne](https://boards.straightdope.com/u/Podkayne)\ **Post date:** [June 24, 2006, 11:02am UTC](https://boards.straightdope.com/t/whats-a-planet/361851/25 "2006-06-24T11:02:31Z")
> [@Mathochist](#): > > Then would the ones into He burning phases not count as stars anymore? No [sup]1[/sup]H… Stars that burn helium have moved off the main sequence, but they are still stars by any definition because they continue to burn hydrogen in a shell around the helium-burning core. As massive stars burn heavier and heavier elements, they become like onions, burning the heaviest element they are currnently capable of fusing in the core, then the next lightest in a shell around the core, then the next lightest in a shell around that, etc., and the outermost shell always burns hydrogen. (People who know more about stellar evoution, feel free to correct me, but I believe that’s the basic jist of it.) When we reach a real problem with the definition of “star” is when the object reaches the end of its lifetime and becomes a white dwarf, neutron star, or black hole. Which of these objects, if any, should still be called a “star”? White dwarfs are commonly called stars. Neutron stars usually aren’t (unless the word “neutron” comes before the “star.”) Black holes, never, except by poets. As a class all three are known as compact objects. And **Chronos** , what’s up with this “Luna” business? Anglo Saxon and common professional parliance ain’t good enough for ya? ---
**Author:** ![Stupid\_Owl](https://avatars.discourse-cdn.com/v4/letter/s/eb8c5e/32.png) [@Stupid\_Owl](https://boards.straightdope.com/u/Stupid_Owl)\ **Post date:** [June 24, 2006, 11:17am UTC](https://boards.straightdope.com/t/whats-a-planet/361851/26 "2006-06-24T11:17:33Z")
> [@NineToTheSky](#): > > Wouldn’t our Earth qualify? Making six? (Agreed, you’d have to stand well back). Ha ha ha. Well, Earth _is_ luminous and consistenly visible to the naked eye, but it’s not so much a _point_ of light as an uninterupted horizon occupying the entire lower half of the sky. I’m not exactly sure what standing ‘well back’ would entail; but I have been to the top of the Empire State Building, and as I recall the earth did not resolve to a disk from that vantage point. > [@Ludovic](#): > > The classical definition of a planet is a “wanderer” that moves about the “fixed” layer of stars throughout the year. The Moon and the Sun were lumped together with the classical “planets”. By this definition, not only would you also have to include Vesta but also Uranus, as it is also sometimes bright enough to see with the naked eye. > [@Squink](#): > > It’s a good thing you stuck ‘regularly’ in there or you’d have to include Vesta as a planet. It’s only 340 miles in diameter, but it does sometimes brighten to magnitude 5.2 at opposition. I explicitly chose to add ‘regularly’, because otherwise _any_ NEO that passed within visual range (Apophis will do so in 2029) would be a ‘planet’ for the duration of its visibility. This might, in fact, be a more useful definition of ‘planet’- “A planet is any (slowly) moving point of light visible in the night sky with the naked eye.”- but it would also mean that one could no more catalogue the planets than catalogue the clouds. Seriously, though, I strongly agree with Chronos’ point that we should refer to different classes of spacial bodies with completely different terms, rather than having four or more subcatagories of ‘planet’. On a side note- In an old essay of Asimov’s, he observed that there was a dramatic gap in the range of masses of spacial bodies in the solar system between Earth and Uranus, and conjectured that this may suggest that planetary formation hits a runaway point if the mass is much greater than Earth’s. Has anyone here heard of any recent developments in such a theory? ---
**Author:** ![Stupid\_Owl](https://avatars.discourse-cdn.com/v4/letter/s/eb8c5e/32.png) [@Stupid\_Owl](https://boards.straightdope.com/u/Stupid_Owl)\ **Post date:** [June 24, 2006, 11:28am UTC](https://boards.straightdope.com/t/whats-a-planet/361851/27 "2006-06-24T11:28:25Z")
I should add, that in light of the fact that we are forced to refer to such a wide variety of things as ‘planets’, due to the lack of suitable alternatives- it is fairly ridiculous that we are expected to juggle asteroid, meteoroid, meteor, and meteorite, which all pretty much refer to the same object in different states. “When it’s in the machine, it’s ice cream. While your pouring it, it’s soft serve. Once it’s in the cone, it’s a scoop. After you’ve eaten it, then it’s crap.” ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 25, 2006, 10:52pm UTC](https://boards.straightdope.com/t/whats-a-planet/361851/28 "2006-06-25T22:52:59Z")
> [@](#): > > dR/dm ??? I’m trying to figure out how you exclude Jupiter from this list, but I don’t understand this part of the definition. If you add a little bit of mass to, say, Saturn, Saturn’s radius will increase slightly. If you add a little bit of mass to a brown dwarf, its radius will actually decrease slightly, due to the greater gravity. Somewhere in between is a threshhold, where dR/dm = 0, and this threshhold was one of the contenders for the definition of a brown dwarf (though nowadays, the deuterium-burning definition seems to be the consensus). I may be misremembering, but I seem to recall reading that this threshhold was very close to Jupiter’s size, and that Jupiter was in fact slightly over it. If, then, we use that definition, Jupiter is no longer a mere “planet”, but a brown dwarf. And, of course, if we also call brown dwarfs “stars”, then you could make a case for Ganymede et al to be planets. > [@](#): > > Neutron stars usually aren’t (unless the word “neutron” comes before the “star.”) Some of the professors here work with neutron celestial-objects-formerly-known-as-stars, and I’ve seen and heard the word “starquake” thrown around in seminars and the like, without being specified as “neutron starquake”. But I’ll agree that black holes are “stars” only poetically. Black dwarfs, I’m not sure about: I don’t think I’ve ever seen one referred to as a “star” in technical contexts, and they’re not high enough in the non-technical consciousness to be discussed poetically, but really, a black dwarf isn’t inherently different from a white dwarf in any way, just colder. Oh, and I used “Luna” rather than “Moon” for that object which shows naked-eye phases because I didn’t want to confuse it with other “moons” like Europa or Triton, the more so since I was considering the possibility that Luna _isn’t_ a “moon”. In cases where there’s no such ambiguity, I do generally use “the Moon” or “our Moon”. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [June 26, 2006, 6:28am UTC](https://boards.straightdope.com/t/whats-a-planet/361851/29 "2006-06-26T06:28:21Z")
> [@Chronos](#): > > If you add a little bit of mass to, say, Saturn, Saturn’s radius will increase slightly. If you add a little bit of mass to a brown dwarf, its radius will actually decrease slightly, due to the greater gravity. Somewhere in between is a threshhold, where dR/dm = 0, and this threshhold was one of the contenders for the definition of a brown dwarf (though nowadays, the deuterium-burning definition seems to be the consensus). I may be misremembering, but I seem to recall reading that this threshhold was very close to Jupiter’s size, and that Jupiter was in fact slightly over it. OK, I’m familiar with the concept and have always heard that Jupiter was about at the threshold. But I’ve never heard of this as being a boundary between planets and brown dwarfs. > [@](#): > > Some of the professors here work with neutron celestial-objects-formerly-known-as-stars, and I’ve seen and heard the word “starquake” thrown around in seminars and the like, without being specified as “neutron starquake”. But I’ll agree that black holes are “stars” only poetically. I think neutron stars are generally considered to be stars, since they formerly supported fusion. Black holes did not necessarily have fusion, although many are former stars. > [@](#): > > Black dwarfs, I’m not sure about: I don’t think I’ve ever seen one referred to as a “star” in technical contexts, and they’re not high enough in the non-technical consciousness to be discussed poetically, but really, a black dwarf isn’t inherently different from a white dwarf in any way, just colder. Not sure exactly what a black dwarf is. A white dwarf that’s cooled off? Or a red dwarf that’s run out of fuel and doesn’t have enough mass to go the red giant/white dwarf route? I’m not sure either of these two actually exist yet. The coolest white dwarfs are still about 5 or 6000 K and red dwarfs have not had enough time to run out of fuel yet. But either one will be classed with stars, since they formerly supported fusion. ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [June 26, 2006, 9:36am UTC](https://boards.straightdope.com/t/whats-a-planet/361851/30 "2006-06-26T09:36:43Z")
> [@ltfire](#): > > I’ve always wondered if there was a lot of childish giggling going on when they named Uranus. Don’t blame the discoverer. Herschel wanted to call it after the [King](http://en.wikipedia.org/wiki/Uranus_(planet)). ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 26, 2006, 8:52pm UTC](https://boards.straightdope.com/t/whats-a-planet/361851/31 "2006-06-26T20:52:32Z")
> [@](#): > > Not sure exactly what a black dwarf is. A white dwarf that’s cooled off? Or a red dwarf that’s run out of fuel and doesn’t have enough mass to go the red giant/white dwarf route? Either, actually. But there’s a slim chance that some white dwarfs might have cooled off by now, and no chance whatsoever that a red dwarf might have, so the term is more commonly used for the former. ---
**Author:** ![scotandrsn](https://avatars.discourse-cdn.com/v4/letter/s/a4c791/32.png) [@scotandrsn](https://boards.straightdope.com/u/scotandrsn)\ **Post date:** [June 26, 2006, 9:50pm UTC](https://boards.straightdope.com/t/whats-a-planet/361851/32 "2006-06-26T21:50:25Z")
> [@dtilque](#): > > Defining a planet as being any object, irrespective of its orbit, that masses at least as much as Pluto produces a list of 17 objects. Actually, 18. You have to include the KBO 2002UB313, which is _bigger_ than Pluto. ---
**Author:** ![scotandrsn](https://avatars.discourse-cdn.com/v4/letter/s/a4c791/32.png) [@scotandrsn](https://boards.straightdope.com/u/scotandrsn)\ **Post date:** [June 26, 2006, 10:12pm UTC](https://boards.straightdope.com/t/whats-a-planet/361851/33 "2006-06-26T22:12:04Z")
> [@scotandrsn](#): > > … KBO 2002UB313… Further research indicates it’s the one they’re calling “Xena” for now… ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [June 27, 2006, 4:59am UTC](https://boards.straightdope.com/t/whats-a-planet/361851/34 "2006-06-27T04:59:14Z")
> [@scotandrsn](#): > > Further research indicates it’s the one they’re calling “Xena” for now… Right, and it’s one of the 17 objects massing at least as much as Pluto: the 9 traditional planets, the 7 largest satellites, and “Xena”. ---
**Author:** ![scotandrsn](https://avatars.discourse-cdn.com/v4/letter/s/a4c791/32.png) [@scotandrsn](https://boards.straightdope.com/u/scotandrsn)\ **Post date:** [June 27, 2006, 5:13am UTC](https://boards.straightdope.com/t/whats-a-planet/361851/35 "2006-06-27T05:13:40Z")
> [@dtilque](#): > > Right, and it’s one of the 17 objects massing at least as much as Pluto: the 9 traditional planets, the 7 largest satellites, and “Xena”. Whoops! :smack: The list I was skimming included the Sun… [Previous page](https://boards.straightdope.com/t/whats-a-planet/361851.md?page=1)