# Is there really a Planet 9 out there in the Kuiper Belt? **URL:** \ **Category:** Factual Questions\ **Created:** [January 29, 2021, 1:49pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771 "2021-01-29T13:49:54Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Jackknifed\_Juggernaut](https://avatars.discourse-cdn.com/v4/letter/j/9de053/32.png) [@Jackknifed\_Juggernaut](https://boards.straightdope.com/u/Jackknifed_Juggernaut)\ **Post date:** [January 29, 2021, 9:24pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/21 "2021-01-29T21:24:18Z")
Most are hybrids anyway, right? So, what’s the criteria? I guess 50% volume or more of gas would be a gasball. And stars are just gasballs on fire. And black holes are noballs. 😁 ---
**Author:** ![FlikTheBlue](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fliktheblue/32/10972_2.png) [@FlikTheBlue](https://boards.straightdope.com/u/FlikTheBlue)\ **Post date:** [January 29, 2021, 9:36pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/22 "2021-01-29T21:36:51Z")
> [@Chronos](#): > > Yet another reason why I support classing objects as rockballs, gasballs, and iceballs, rather than lumping all three into one “planet” category. Mercury, Venus, Earth, Luna, Mars, and Ceres are all rockballs. Jupiter, Saturn, Uranus, and Neptune are all gasballs. Pluto is one of the largest iceballs (not the largest; Eros at least is larger, and probably several others as well that we haven’t discovered yet), but there are many, many other iceballs. Do we know for sure that Venus is a rockball? I thought it was more like a hot gasball than a rockball. ---
**Author:** ![Frodo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/frodo/32/302_2.png) [@Frodo](https://boards.straightdope.com/u/Frodo)\ **Post date:** [January 29, 2021, 9:37pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/23 "2021-01-29T21:37:27Z")
We’ve landed probes on the rocky part. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [January 29, 2021, 10:09pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/24 "2021-01-29T22:09:45Z")
> [@Jackknifed\_Juggernaut](#): > > And stars are just gasballs on fire. \* pushes glasses up nose\* Stars are not on fire. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [January 29, 2021, 10:10pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/25 "2021-01-29T22:10:28Z")
It’s just a rockball with what I like to call a hotmosphere. ---
**Author:** ![DrDeth](https://avatars.discourse-cdn.com/v4/letter/d/b487fb/32.png) [@DrDeth](https://boards.straightdope.com/u/DrDeth)\ **Post date:** [January 29, 2021, 10:28pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/26 "2021-01-29T22:28:21Z")
If Pluto is a planet, and that is fine with me, then so is Eris, then there’d be ten planets. There is either 8 or 10, not 9, unless this new one is found. Makemake is just somewhat smaller than Pluto, so maybe 11. Then there is Gonggong. And Haumea. Also seven moons larger than Pluto. See, Pluto is a LOT smaller than they thought. ---
**Author:** ![mikecurtis](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mikecurtis/32/3284_2.png) [@mikecurtis](https://boards.straightdope.com/u/mikecurtis)\ **Post date:** [January 29, 2021, 10:41pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/27 "2021-01-29T22:41:03Z")
Size isn’t the reason. [Why is Pluto no longer a planet? | Library of Congress](https://www.loc.gov/everyday-mysteries/astronomy/item/why-is-pluto-no-longer-a-planet/#:~:text=The%20International%20Astronomical%20Union%20)(IAU,neighboring%20region%20of%20other%20objects.%E2%80%9D ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [January 29, 2021, 10:41pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/28 "2021-01-29T22:41:31Z")
> [@Pleonast](#): > > If there is something there, can it be said to have cleared it’s area sufficiently to be a planet? The orbits of objects in that part of the solar system are evidence that it has. All the objects that far out that we know of have orbits that keep them out of the way of this hypothetical planet. Either those objects started with such orbits or Planet 9 pushed them into one. > [@md-2000](#): > > the article linked is not terribly clear what they mean by “aligned long axes” - I presume they mean the long axes are close to the same plane. That may be a very rare coincidence, but only if that’s the case for everything, not just for the few bodies selected The orbits are not on the same plane. Their perihelion-aphelion lines all go in the same general direction. As far as I know, all known objects in that part of the solar system have such orbits, not just a selected number. It’s not a huge number of objects, originally 6, but there’s been at least one (and I think more than one) addition. > [@Chronos](#): > > Pluto is one of the largest iceballs (not the largest; Eros at least is larger, and probably several others as well that we haven’t discovered yet), but there are many, many other iceballs. It’s Eris; Eros is a near-Earth asteroid. It turns out that Eris is slightly smaller than Pluto. By about 50 km. However, Eris is more massive (about 25% more), which means it’s composed of much more rock and less ice than Pluto. ---
**Author:** ![DrDeth](https://avatars.discourse-cdn.com/v4/letter/d/b487fb/32.png) [@DrDeth](https://boards.straightdope.com/u/DrDeth)\ **Post date:** [January 29, 2021, 10:52pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/29 "2021-01-29T22:52:06Z")
Yes, actually it is. They were presented with quite a few trans Plutonian planets, most smaller than pluto. We also found out- Pluto was much smaller than thought. So, rather than have two dozen planets, they had to come up with a reasonable scientific definition that would include most of the known planets and exclude the new, tiny ones. 1. It is in orbit around the Sun. 2. It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape). 3. It has “cleared the neighborhood” around its orbit. So they came up with those three rules, and with “cleared the orbit” Pluto failed. (And it is quite possible some of the Kuiper belt planetoids may clear their orbit, we dont know, they are very far). They had to define planet, and yes, size was why they had to. ---
**Author:** ![mikecurtis](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mikecurtis/32/3284_2.png) [@mikecurtis](https://boards.straightdope.com/u/mikecurtis)\ **Post date:** [January 29, 2021, 10:58pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/30 "2021-01-29T22:58:25Z")
Size was one of (but far from the only) reasons for the debate, but not the reason for the reclassification. ---
**Author:** ![SCAdian](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scadian/32/3292_2.png) [@SCAdian](https://boards.straightdope.com/u/SCAdian)\ **Post date:** [January 29, 2021, 11:11pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/31 "2021-01-29T23:11:18Z")
> [@Jasmine](#): > > It even has a moon of its own, Which one do you have in mind? > **[Moons of Pluto](https://en.wikipedia.org/wiki/Moons_of_Pluto)** > > The dwarf planet Pluto has five natural satellites. In order of distance from Pluto, they are Charon, Styx, Nix, Kerberos, and Hydra. Charon, the largest, is mutually tidally locked with Pluto, and is massive enough that Pluto and Charon are sometimes considered a binary dwarf planet. > The innermost and largest moon, Charon, was discovered by James Christy on 22 June 1978, nearly half a century after Pluto was discovered. This led to a substantial revision in estimates of Pluto's size, which ha... ---
**Author:** ![SCAdian](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scadian/32/3292_2.png) [@SCAdian](https://boards.straightdope.com/u/SCAdian)\ **Post date:** [January 29, 2021, 11:11pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/32 "2021-01-29T23:11:56Z")
And as far as I know, Pluto is still officially a planet. ---
**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:** [January 29, 2021, 11:20pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/33 "2021-01-29T23:20:38Z")
> [@Chronos](#): > > Yet another reason why I support classing objects as rockballs, gasballs, and iceballs, rather than lumping all three into one “planet” category. Mercury, Venus, Earth, Luna, Mars, and Ceres are all rockballs. Jupiter, Saturn, Uranus, and Neptune are all gasballs. Pluto is one of the largest iceballs (not the largest; Eros at least is larger, and probably several others as well that we haven’t discovered yet), but there are many, many other iceballs. That works for our own solar system, where there are three quite distinct categories of objects. We don’t know if that would work as a general classification for extrasolar planets, where there could be more of a continuum, or even additional categories. And then we have moons, whose only criterion is that they orbit a larger body. They vary a lot in composition so a simple classification would be difficult. ---
**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:** [January 29, 2021, 11:31pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/34 "2021-01-29T23:31:25Z")
> [@SCAdian](#): > > And as far as I know, Pluto is still officially a planet. Since 2006, the [International Astronomical Union has classified Pluto not as a planet, but as a dwarf planet.](https://solarsystem.nasa.gov/planets/dwarf-planets/pluto/overview/) This designation has always seemed to me to be absurd. Clearly Pluto is a different class of object from the inner planets, but calling it a dwarf planet was a kind of limbo. I think it was a sop to people who wanted to still call it a planet. If it’s different, call it something different. The definition of “cleared its orbit” for a planet seems to me contrived. Pluto has no more in common with Ceres, another dwarf planet, than it does with any of the rocky planets. Planet is one of those classical terms like continent that has come to designate objects quite different in concept from the original definition. Originally the Sun and Moon were also planets, and the Earth wasn’t. As we have learned more about the solar system, the definition of planet has been tweaked to keep the their number between 5 and 9. Once we find more than 9 objects (like asteroids or Kuiper Belt objects) that fit the previous definition of planet the definition gets changed to keep the number small. ---
**Author:** ![Northern\_Piper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/northern_piper/32/5304_2.png) [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)\ **Post date:** [January 29, 2021, 11:43pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/35 "2021-01-29T23:43:18Z")
> [@Jackknifed\_Juggernaut](#): > > And black holes are noballs. Goebbels doesn’t orbit the Sun. ---
**Author:** ![SCAdian](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scadian/32/3292_2.png) [@SCAdian](https://boards.straightdope.com/u/SCAdian)\ **Post date:** [January 29, 2021, 11:47pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/36 "2021-01-29T23:47:51Z")
> [@Colibri](#): > > dwarf planet I’m sorry, I didn’t quite hear you. A dwarf _what_? ---
**Author:** ![Frodo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/frodo/32/302_2.png) [@Frodo](https://boards.straightdope.com/u/Frodo)\ **Post date:** [January 29, 2021, 11:51pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/37 "2021-01-29T23:51:17Z")
So dwarf planets are planets, prairie dogs are dogs and sea lions are lions… ---
**Author:** ![SCAdian](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scadian/32/3292_2.png) [@SCAdian](https://boards.straightdope.com/u/SCAdian)\ **Post date:** [January 29, 2021, 11:59pm UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/38 "2021-01-29T23:59:45Z")
And your point? That dwarf rabbits (and perhaps even giant rabbits) are not rabbits? ---
**Author:** ![Frodo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/frodo/32/302_2.png) [@Frodo](https://boards.straightdope.com/u/Frodo)\ **Post date:** [January 30, 2021, 12:01am UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/39 "2021-01-30T00:01:34Z")
That the inclusion of the term “planet” in the name “dwarf planet” does not mean that Pluto is a planet. It’s not. ---
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [January 30, 2021, 12:36am UTC](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771/40 "2021-01-30T00:36:41Z")
> [@Frodo](#): > > That the inclusion of the term “planet” in the name “dwarf planet” does not mean that Pluto is a planet. I hate the term “dwarf planet”; it’s a sop and confusing. We should’ve kept the term “planetoid”. [Previous page](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771.md?page=1) [Next page](https://boards.straightdope.com/t/is-there-really-a-planet-9-out-there-in-the-kuiper-belt/931771.md?page=3)