# Pluto may be a Giant Comet? **URL:** \ **Category:** Factual Questions\ **Created:** [May 28, 2018, 4:20pm UTC](https://boards.straightdope.com/t/pluto-may-be-a-giant-comet/814978 "2018-05-28T16:20:33Z")\ **Posts on this page:** 17\ **Page:** 2
**Author:** ![JKellyMap](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jkellymap/32/15880_2.png) [@JKellyMap](https://boards.straightdope.com/u/JKellyMap)\ **Post date:** [May 30, 2018, 2:25pm UTC](https://boards.straightdope.com/t/pluto-may-be-a-giant-comet/814978/21 "2018-05-30T14:25:02Z")
> [@Rucksinator](#): > > But aren’t Earth and Luna particularly different, in that Earth rotates around a star, and Luna rotates around a thing that rotates about a star? So maybe Earth is a “First degree rockball” while Luna is a “Second degree rockball”. Or maybe Luna could be a “rockball once removed”. > > BTW, is it possible for a rockball (or gassball or iceball) to be the center of a system? Is there a reason that only a star can be at the center of an orbital system? Is a [black/white dwarf](https://en.wikipedia.org/wiki/Black_dwarf) (remnant of a star after it stops out emitting heat or light) much like a gasball? ---
**Author:** ![JKellyMap](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jkellymap/32/15880_2.png) [@JKellyMap](https://boards.straightdope.com/u/JKellyMap)\ **Post date:** [May 30, 2018, 2:28pm UTC](https://boards.straightdope.com/t/pluto-may-be-a-giant-comet/814978/22 "2018-05-30T14:28:41Z")
> [@Steve\_MB](#): > > Er, what now? There are multiple known examples of rockballs and gasballs and iceballs at the center of orbital systems. The poster obviously mean “where that system doesn’t also orbit a star.” ---
**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:** [May 30, 2018, 2:37pm UTC](https://boards.straightdope.com/t/pluto-may-be-a-giant-comet/814978/23 "2018-05-30T14:37:39Z")
> [@Chronos](#): > > In composition, Pluto is much more like a comet than it is like the Earth. In size, it’s not particularly like either, being much smaller than Earth and much larger than a comet. Intermediate objects exist in both directions: There are many icy bodies known smaller than Pluto but larger than comets, and there are objects like Ganymede which are in between Pluto and Earth in size, and composed of a mix of roughly equal parts rock and ice. > > I’ve said it before, but I personally favor ditching the “planet” label entirely, and instead referring to nonstar bodies large enough to collapse into spherical as either gasballs, rockballs, or iceballs, as appropriate. By this standard, Mercury, Venus, Earth, Luna, Mars, and Ceres are all rockballs; Jupiter, Saturn, Uranus, and Neptune are gasballs, and Io, Europa, Callisto, Titan, Triton, Pluto, Eris, Quaoar, Sedna, and too many others to list are all iceballs. I’ll grant that I’m not entirely sure which category to put Ganymede into (any human-made classification system will have some gray areas), but it’s at least a lot cleaner than trying to pretend that Jupiter and Mercury have anything meaningful in common, or that Earth and Luna are particularly different. The problem with this, of course, is that the number of bodies in the categories besides gasballs is far too large for the general public to relate to. When you’ve got “too many to list” then the term becomes meaningless to anyone but scientists. Another solution would be to just recognize that “planet,” like “continent,” is a popular term and not even try to define it in scientific terms, because there is no non-arbitrary scientific definition. If the general public wants to call Pluto a planet, why not? ---
**Author:** ![Rucksinator](https://avatars.discourse-cdn.com/v4/letter/r/898d66/32.png) [@Rucksinator](https://boards.straightdope.com/u/Rucksinator)\ **Post date:** [May 30, 2018, 2:42pm UTC](https://boards.straightdope.com/t/pluto-may-be-a-giant-comet/814978/24 "2018-05-30T14:42:51Z")
> [@Steve\_MB](#): > > Er, what now? There are multiple known examples of rockballs and gasballs and iceballs at the center of orbital systems. > [@JKellyMap](#): > > The poster obviously mean “where that system doesn’t also orbit a star.” Nope. I was actually unaware of that. Obviously, I am not an astronomer. It only just recently occurred to me that such a thing could be possible, but I don’t recall hearing about one. ---
**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:** [May 30, 2018, 3:10pm UTC](https://boards.straightdope.com/t/pluto-may-be-a-giant-comet/814978/25 "2018-05-30T15:10:08Z")
> [@Rucksinator](#): > > Nope. I was actually unaware of that. Obviously, I am not an astronomer. It only just recently occurred to me that such a thing could be possible, but I don’t recall hearing about one. What they are saying is that a “_x_ball at the center of an orbital system” can describe all planets with moons. But you mean an _x_ball that has moons that did not form around a star. And the answer is, I don’t know of any that have been actually discovered, but don’t see why not. [Brown dwarf stars can have planets](https://www.technologyreview.com/s/517556/first-planet-discovered-orbiting-a-brown-dwarf/), so I don’t see why [a really large gas giant that isn’t quite big enough to be called a brown dwarf](https://en.wikipedia.org/wiki/Sub-brown_dwarf) can’t have them. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [May 30, 2018, 3:11pm UTC](https://boards.straightdope.com/t/pluto-may-be-a-giant-comet/814978/26 "2018-05-30T15:11:46Z")
> [@Rucksinator](#): > > Nope. I was actually unaware of that. Obviously, I am not an astronomer. It only just recently occurred to me that such a thing could be possible, but I don’t recall hearing about one. \*\* Steve MB\*\* is obviously referring to Jupiter and Saturn. Each is the center of a system consisting of rings and dozens of moons. There probably are “star systems” where the central object is a brown dwarf or gas giant, not a star. I don’t believe we’ve discovered any, but that’s just a limitation of our observing equipment. ---
**Author:** ![Rucksinator](https://avatars.discourse-cdn.com/v4/letter/r/898d66/32.png) [@Rucksinator](https://boards.straightdope.com/u/Rucksinator)\ **Post date:** [May 30, 2018, 3:18pm UTC](https://boards.straightdope.com/t/pluto-may-be-a-giant-comet/814978/27 "2018-05-30T15:18:12Z")
Could there be a system similar to the solar system except, instead of having a star in the center, there is a giant hunk of rock? ---
**Author:** ![Rucksinator](https://avatars.discourse-cdn.com/v4/letter/r/898d66/32.png) [@Rucksinator](https://boards.straightdope.com/u/Rucksinator)\ **Post date:** [May 30, 2018, 3:26pm UTC](https://boards.straightdope.com/t/pluto-may-be-a-giant-comet/814978/28 "2018-05-30T15:26:12Z")
> [@Rucksinator](#): > > But aren’t Earth and Luna particularly different, in that Earth rotates around a star, and Luna rotates around a thing that rotates about a star? So maybe Earth is a “First degree rockball” while Luna is a “Second degree rockball”. Or maybe Luna could be a “rockball once removed”… After reading another thread, it occurred to me that my assumption that “Luna” is the Earth’s moon might not be correct. If so, please correct me and know that this is the (main) reason why my post above didn’t make sense. ---
**Author:** ![RTFirefly](https://avatars.discourse-cdn.com/v4/letter/r/c77e96/32.png) [@RTFirefly](https://boards.straightdope.com/u/RTFirefly)\ **Post date:** [May 30, 2018, 3:33pm UTC](https://boards.straightdope.com/t/pluto-may-be-a-giant-comet/814978/29 "2018-05-30T15:33:26Z")
> [@kanicbird](#): > > From what I see from the demotion of Pluto I think so far\* in the solar system there should only be 4 planets - the gas/ice giants The original definition of ‘planet’ applied to the five objects in the night sky, other than the moon,that didn’t maintain a fixed positional relationship to one another. ISTM that whatever definition of ‘planet’ we have, these five - Mercury, Venus, Mars, Jupiter, Saturn - belong in it. They were THE planets for thousands of years. Then along came Galileo & Co., and it became clear that one big property the five planets had in common was that they orbited the sun, not the earth. And in fact the earth orbited the sun as well. So six planets. Then when Uranus and Neptune were discovered, they (a) orbited the sun, and (b) were bigger than anything else in the solar system besides Jupiter, Saturn, and the sun itself. You couldn’t exactly say they weren’t planets. Eight now. As \*\*Colibri \*\*says, the definition of ‘planet’ has been tinkered with to keep the number of planets from getting large. I disagree that anything was special about 10; if there had only been four objects in the asteroid belt, kids in 1900 would have grown up with a 12-planet solar system, IMHO. But any definition of planet that would have overwhelmed the original eight was going to be right out, so when it became clear that there were a shitload of asteroids, the handful of asteroids that had been declared planets got un-declared. And that’s basically what happened with Pluto and the Kuiper Belt Objects. Pluto-killer Mike Brown could have been the discoverer of the tenth planet - for a while. But he realized that the same thing was likely to play out with KBOs that did with the asteroids: that if Pluto and the KBO he’d discovered were both called planets, then once again we’d likely have a shitload of planets, not just 10 or 11 or 12. So that’s why we have eight planets. There’s the original five, plus Earth, Uranus, and Neptune, because there’s no way to keep them out without tossing out some of the original five. But you can toss out the asteroids and the KBOs without tossing out any of the eight planets. ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [May 30, 2018, 4:20pm UTC](https://boards.straightdope.com/t/pluto-may-be-a-giant-comet/814978/30 "2018-05-30T16:20:27Z")
> [@Rucksinator](#): > > Could there be a system similar to the solar system except, instead of having a star in the center, there is a giant hunk of rock? The reason this isn’t as likely is that this system has to form out of something. Regular solar systems form out of gas and dust clouds that are mostly hydrogen, so that’s why the Sun is mostly hydrogen. And that’s also why there are gas giants that are also mostly hydrogen. Jupiter and Saturn very likely contain more rocks than the Earth does, but since they have so much more hydrogen they’re mostly gas. If the inner rockball planets once had that much gas that was all lost very early in the formation of the solar system. So maybe proto-Earth was once a lot more like Jupiter, but it lost almost all of that gas. Anyway, if you try to imagine a small cloud of space dust and gas collapsing into a denser state, there’s a continuum on how much gas gets collected. You can have super-massive stars that are millions of times bigger than the sun, massive stars that are just much bigger than the sun, “regular” stars that are like the sun, and smaller stars. [List of nearest stars and brown dwarfs - Wikipedia](https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown_dwarfs) These smaller red dwarf stars are much more common than brighter stars like the sun, the only reason we don’t see them with the naked eye when we look up at the night sky is that they’re much less bright than stars like the sun. Barnard’s Star is only 6 light years away, but it was only discovered in 1916, because it is a red dwarf and you can’t see it with the naked eye. So then there are brown dwarfs, which are even less massive than red dwarfs. The distinction is that red dwarfs are massive enough to fuse hydrogen, while brown dwarfs aren’t. And that grades all the way down to planets like Jupiter. So there’s no reason that there couldn’t be gas giant type planets out there with smaller bodies orbiting them. And there are rocky bodies out there in interstellar space as well. The reason there’s probably not many systems of rocky bodies in interstellar space is that the gravitational forces holding the systems together wouldn’t be that strong, compared to stars, red dwarfs, brown dwarfs, and gas giants. So those rocky systems are going to be torn apart very easily, most before they even form. So the system will consolidate into one body, and the rest of the bodies will get ejected. And in any case, if these bodies formed without a central star to vaporize the volatiles, they’ll just form as ices. So any random bodies out in interstellar space are probably mostly iceballs rather than rockballs. Or, if they’re massive enough, they’ll be gasballs if the body is warm enough that the volatiles are gasses. ---
**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:** [May 30, 2018, 4:23pm UTC](https://boards.straightdope.com/t/pluto-may-be-a-giant-comet/814978/31 "2018-05-30T16:23:32Z")
> [@RTFirefly](#): > > I disagree that anything was special about 10; if there had only been four objects in the asteroid belt, kids in 1900 would have grown up with a 12-planet solar system, IMHO. I didn’t mean there was something inherently special about 10, just that that’s been the de facto limit that’s been employed recently. The first 4 asteroids to be discovered were at one time classified as planets, and weren’t demoted for about 40 years until a lot more asteroids were discovered. So during that time there were in fact 12 “planets.” (I wonder what the astrologers did about it?) And the major moons of Jupiter and Saturn at one time were referred to as “secondary planets.” ---
**Author:** ![Steve\_MB](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/steve_mb/32/5339_2.png) [@Steve\_MB](https://boards.straightdope.com/u/Steve_MB)\ **Post date:** [May 30, 2018, 5:09pm UTC](https://boards.straightdope.com/t/pluto-may-be-a-giant-comet/814978/32 "2018-05-30T17:09:55Z")
> [@Lemur866](#): > > Anyway, if you try to imagine a small cloud of space dust and gas collapsing into a denser state, there’s a continuum on how much gas gets collected. You can have super-massive stars that are millions of times bigger than the sun, massive stars that are just much bigger than the sun, “regular” stars that are like the sun, and smaller stars A slight fact-check digression – [stellar masses](https://en.wikipedia.org/wiki/Stellar_mass) max out at somewhere around 150-300 solar masses, because anything bigger would generate energy so fast it would quickly blow itself apart. > [@](#): > > So then there are brown dwarfs, which are even less massive than red dwarfs. The distinction is that red dwarfs are massive enough to fuse hydrogen, while brown dwarfs aren’t. And that grades all the way down to planets like Jupiter. So there’s no reason that there couldn’t be gas giant type planets out there with smaller bodies orbiting them. And there are rocky bodies out there in interstellar space as well. The reason there’s probably not many systems of rocky bodies in interstellar space is that the gravitational forces holding the systems together wouldn’t be that strong, compared to stars, red dwarfs, brown dwarfs, and gas giants. So those rocky systems are going to be torn apart very easily, most before they even form. So the system will consolidate into one body, and the rest of the bodies will get ejected. And in any case, if these bodies formed without a central star to vaporize the volatiles, they’ll just form as ices. So any random bodies out in interstellar space are probably mostly iceballs rather than rockballs. Or, if they’re massive enough, they’ll be gasballs if the body is warm enough that the volatiles are gasses. That said, there are probably rocky planets out there that got ejected from their original solar systems, probably during the demolition-derby stage of solar system formation. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [May 31, 2018, 9:32am UTC](https://boards.straightdope.com/t/pluto-may-be-a-giant-comet/814978/33 "2018-05-31T09:32:04Z")
There are at least four planetary mass objects in interstellar space listed on Wikipedia; PSO J318.5-22 with around 6 times Jupiter’s mass > **[PSO J318.5−22](https://en.wikipedia.org/wiki/PSO_J318.5-22)** > > PSO J318.5−22 is a rogue planet, an extrasolar object of planetary mass that does not orbit a parent star. It is approximately 80 light-years away and belongs to the Beta Pictoris moving group. The object was discovered in 2013 in images taken by the Pan-STARRS PS1 wide-field telescope. PSO J318.5-22's age is inferred to be 12 million years, the same age as the Beta Pictoris group. Based on its calculated temperature and age, it is classified under the brown dwarf spectral type L7. The team lea... Cha 110913-773444 with around 8 times Jupiter’s mass > **[Cha 110913−773444](https://en.wikipedia.org/wiki/Cha_110913-773444)** > > Cha 110913−773444 (sometimes abbreviated Cha 110913) is an astronomical object surrounded by what appears to be a protoplanetary disk. It lies at a distance of 529 light-years from Earth. There is no consensus yet among astronomers whether to classify the object as a sub-brown dwarf (with planets) or a rogue planet (with moons). > Cha 110913−773444 was discovered in 2004 by Kevin Luhman and others at Pennsylvania State University using the Spitzer Space Telescope and the Hubble Space Telescope, as... and 2MASS J1119–1137, a binary system with two planets around 4 times as massive as Jupiter. [https://en.wikipedia.org/wiki/2MASS\_J1119–1137](https://en.wikipedia.org/wiki/2MASS_J1119%E2%80%931137) The last one at least is definitely a system of planetary mass objects orbiting each other with no star involved, assuming that the masses quoted are correct. ---
**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 1, 2018, 1:29am UTC](https://boards.straightdope.com/t/pluto-may-be-a-giant-comet/814978/34 "2018-06-01T01:29:10Z")
> [@](#): > > Quoth **Colibri** : > > The problem with this, of course, is that the number of bodies in the categories besides gasballs is far too large for the general public to relate to. When you’ve got “too many to list” then the term becomes meaningless to anyone but scientists. The number of rockballs isn’t unmanageable: I listed all of them in our Solar System, six (or maybe seven) in total. There are lots of rocks, of course, but most of them aren’t balls. There are too many iceballs to list, but that’s going to be a problem no matter how you categorize things. It doesn’t help to say “there are lots of KBOs” or “there are lots of Plutoids” instead of “there are lots of iceballs”. And yes, **Rucksinator** , by “Luna”, I mean the world where Neil Armstrong came in peace for all mankind. It orbits the Sun (albeit in an orbit which is extremely perturbed by Earth). ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [June 1, 2018, 11:15am UTC](https://boards.straightdope.com/t/pluto-may-be-a-giant-comet/814978/35 "2018-06-01T11:15:49Z")
But can a comet have [dunes](https://www.bbc.com/news/science-environment-44317367)? ---
**Author:** ![aldiboronti](https://avatars.discourse-cdn.com/v4/letter/a/9fc348/32.png) [@aldiboronti](https://boards.straightdope.com/u/aldiboronti)\ **Post date:** [June 1, 2018, 4:05pm UTC](https://boards.straightdope.com/t/pluto-may-be-a-giant-comet/814978/36 "2018-06-01T16:05:11Z")
> [@Jasmine](#): > > I remain outraged at the ongoing campaign to denigrate and demean Pluto. Pluto is our ninth planet, PERIOD. It was recognized as our ninth planet for many decades and then cast aside by haters who manipulated their definition of “planet” to arbitrarily exclude that deserving body. Just let be known to all: :mad: > > **I LOVE Pluto!** > > That is all. Some scientists agree and I recall reading last year that there was a movement afoot to [restore Pluto’s status as a planet.](https://hub.jhu.edu/2017/03/16/make-pluto-a-planet-again/) Fingers crossed! ---
**Author:** ![tim314](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tim314/32/3468_2.png) [@tim314](https://boards.straightdope.com/u/tim314)\ **Post date:** [June 1, 2018, 4:11pm UTC](https://boards.straightdope.com/t/pluto-may-be-a-giant-comet/814978/37 "2018-06-01T16:11:15Z")
> [@JKellyMap](#): > > Is a [black/white dwarf](https://en.wikipedia.org/wiki/Black_dwarf) (remnant of a star after it stops out emitting heat or light) much like a gasball? I believe they’ve been reclassified as cull obsidian.😉 [Previous page](https://boards.straightdope.com/t/pluto-may-be-a-giant-comet/814978.md?page=1)