# How many planets would there be if Pluto were still a planet? **URL:** \ **Category:** Great Debates\ **Created:** [October 24, 2011, 4:05am UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600 "2011-10-24T04:05:58Z")\ **Posts on this page:** 20\ **Page:** 3
**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 25, 2011, 4:58am UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/41 "2011-10-25T04:58:35Z")
> [@Colibri](#): > > If there is a formal definition on that page, please quote it directly. Here is what I get from copying out of the page which you must have completely ignored if you looked at the link. > [@](#): > > Planets 1930–2006 > 1 > Mercury > 2 > Venus > 3 > Earth > 4 > Mars > 5 > Jupiter > 6 > Saturn > 7 > Uranus > 8 > Neptune > 9 > Pluto > [@Colibri](#): > > That of course is not in any way a formal scientific definition of a planet, merely a list of them. > > A scientific definition would allow new objects to be classified when they are found. No such formal definition existed before 2006. > > Playing linguistic games does not help your point. I never claimed that is was a formal scientific definition. You are attempting to move the goalposts. So post your definition of a\* formal scientifc definition \*before I respond. And while you’re at it, why don’t you tell us how many planets you thought there were 6 years ago. ---
**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 25, 2011, 5:09am UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/42 "2011-10-25T05:09:42Z")
I suppose next your going to claim [these guys](http://www.nasa.gov/vision/universe/solarsystem/newplanet-072905.html)knew nothing about astronomy. ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [October 25, 2011, 7:21am UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/43 "2011-10-25T07:21:30Z")
> [@Alka\_Seltzer](#): > > This approach is not useful when we start looking at other solar systems. Other solar system aren’t really an issue. The current official definition for planets that was adopted in 2006 specifically only applies to our solar system. There is a separate official definition that applies to other solar systems. ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [October 25, 2011, 7:26am UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/44 "2011-10-25T07:26:00Z")
A question for the more astronomically informed. Is it possible that there are other undiscovered bodies in our solar system that meet the current definition of a planet? Obviously it would have to be quite a ways out to be still undiscovered. Does out current knowledge of the way the solar system works allow for the possibility that there may be more planets out there? Or do we know that anything that gets discovered from this point on will not meet the criteria for planethood? ---
**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:** [October 25, 2011, 3:57pm UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/45 "2011-10-25T15:57:36Z")
We can’t even completely rule out the possibility of brown dwarfs orbiting the Sun, way out. Something the size of Earth or Neptune, forget about it. ---
**Author:** ![Alka\_Seltzer](https://avatars.discourse-cdn.com/v4/letter/a/c89c15/32.png) [@Alka\_Seltzer](https://boards.straightdope.com/u/Alka_Seltzer)\ **Post date:** [October 25, 2011, 8:03pm UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/46 "2011-10-25T20:03:19Z")
> [@Little\_Nemo](#): > > A question for the more astronomically informed. Is it possible that there are other undiscovered bodies in our solar system that meet the current definition of a planet? Obviously it would have to be quite a ways out to be still undiscovered. Does out current knowledge of the way the solar system works allow for the possibility that there may be more planets out there? Or do we know that anything that gets discovered from this point on will not meet the criteria for planethood? It’s possible. As Chronos says, the outer solar system has not been well surveyed, so large objects could easily escape detection. Over the next few years we can expect to get a better idea of what’s out there, what could possibly be there and the conditions under which the solar system formed. Automatic survey telescopes such as [PAN-STARRS](http://en.wikipedia.org/wiki/PAN-STARRS) and the [LSST](http://en.wikipedia.org/wiki/LSST) are expected to discover many times the number of TNOs that are currently known. It’s not just a case of detecting them, determining their orbits puts limits on what else could be present. Objects such as Pluto and Eris are thought to have formed much closer to the sun than their current orbits, where there was sufficient material in the proto-planetary disc for them to coalesce. They were scattered into the outer solar system by the outward migration of the planets. (This is the context in which the dwarf planet term makes sense, planets bully the dwarf planets). The largest of these objects could be bigger than Mercury. The cutoff point between regular and dwarf planet is not precisely defined, but a Mercury sized TNO would probably be considered a dwarf planet under the current definition, which would be consistent but somewhat unsatisfying. Something considerably bigger might be considered a planet. [Sedna](http://en.wikipedia.org/wiki/90377_Sedna#Origin)’s highly eccentric orbit is something of a mystery, it doesn’t come close enough to the planets to have been scattered into that orbit by them. Another much larger object must have attracted it. This might be a planet or brown dwarf in the outer solar system, or it might be due to the pull from a star which had a close encounter with the solar system. Finding more objects in that region of space will help resolve this mystery. (Another explanation is that Sedna actually formed around another star, but was captured by the sun, but this is thought to be less likely). Theories of star system formation predict that a large number of bodies are ejected from due to gravitational encounters, and that stars form in clusters. It’s possible the sun may have captured planets ejected from other star systems. ---
**Author:** ![Gary\_Robson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gary_robson/32/3448_2.png) [@Gary\_Robson](https://boards.straightdope.com/u/Gary_Robson)\ **Post date:** [October 25, 2011, 10:46pm UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/47 "2011-10-25T22:46:57Z")
> [@TriPolar](#): > > This thread needs to move to GD so I can unleash my rage. [moderating] That’s a logical move, considering it’s pretty much been a debate for over 30 posts now. So moved. [/moderating] ---
**Author:** ![E-Sabbath](https://avatars.discourse-cdn.com/v4/letter/e/e19adc/32.png) [@E-Sabbath](https://boards.straightdope.com/u/E-Sabbath)\ **Post date:** [October 26, 2011, 1:37am UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/48 "2011-10-26T01:37:35Z")
Tyche is a speculated brown dwarf, at roughly, what was it, 15,000 AU? > **[Tyche (hypothetical planet)](https://en.wikipedia.org/wiki/Tyche_(hypothetical_planet))** > > Tyche /ˈtaɪki/ was a hypothetical gas giant located in the Solar System's Oort cloud, first proposed in 1999 by astrophysicists John Matese, Patrick Whitman and Daniel Whitmire of the University of Louisiana at Lafayette. They argued that evidence of Tyche's existence could be seen in a supposed bias in the points of origin for long-period comets. More recently, Matese and Whitmire re-evaluated the comet data and noted that Tyche, if it existed, would be detectable in the archive of Matese, Whi... ---
**Author:** ![GIGObuster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gigobuster/32/421_2.png) [@GIGObuster](https://boards.straightdope.com/u/GIGObuster)\ **Post date:** [October 26, 2011, 2:31am UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/49 "2011-10-26T02:31:19Z")
> [@TriPolar](#): > > I suppose next your going to claim [these guys](http://www.nasa.gov/vision/universe/solarsystem/newplanet-072905.html)knew nothing about astronomy. Well, there is this thing also called time, back then in the pluto-is-still-a-planet days of 2005, the discovery of Eris (as many call that dwarf planet in your link) was hailed as the tenth planet by those experts you point at; but, I think I have to agree with \*\*Colibri \*\*here, it was thanks to discoveries like Eris that told astronomers that there was a need now to properly define what a planet was and the experts you point at then dropped the idea that a tenth planet was discovered, it is now considered a dwarf planet just like 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 26, 2011, 3:04am UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/50 "2011-10-26T03:04:17Z")
> [@GIGObuster](#): > > Well, there is this thing also called time, back then in the pluto-is-still-a-planet days of 2005, the discovery of Eris (as many call that dwarf planet in your link) was hailed as the tenth planet by those experts you point at; but, I think I have to agree with \*\*Colibri \*\*here, it was thanks to discoveries like Eris that told astronomers that there was a need now to properly define what a planet was and the experts you point at then dropped the idea that a tenth planet was discovered, it is now considered a dwarf planet just like Pluto. So for there to be a tenth planet discovered, there must have been nine at some point, right? ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [October 26, 2011, 4:03am UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/51 "2011-10-26T04:03:26Z")
> [@naita](#): > > Yes of course. Traditions can’t possibly be from the 20th century. Well, maybe they can in some johnny-come-lately science like nuclear physics or genetics. But astronomy, especially planetary astronomy, has a history spanning several thousand years. And that much longer tradition says that we adjust what is considered a planet as new theories and data come in. As far as more planets out there, I would say the most likely place is about 50 AU out. The classic Kuiper Belt has a fairly sharp drop-off about there and one possibility is a largish planet orbiting at that point and clearing out most of the small objects. I don’t think this is very likely, but more likely than anywhere else another planet could be. ---
**Author:** ![GIGObuster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gigobuster/32/421_2.png) [@GIGObuster](https://boards.straightdope.com/u/GIGObuster)\ **Post date:** [October 26, 2011, 4:25am UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/52 "2011-10-26T04:25:17Z")
> [@TriPolar](#): > > So for there to be a tenth planet discovered, there must have been nine at some point, right? Moot point. As mentioned, it was the discordance brought by the new objects discovered recently that told the astronomers how inadequate the previous definitions were. “The tenth planet” was never officially declared that BTW. NASA alone can not go over the rulings of the International Astronomical Union (IAU). ---
**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:** [October 26, 2011, 7:49am UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/53 "2011-10-26T07:49:46Z")
> [@dtilque](#): > > Well, maybe they can in some johnny-come-lately science like nuclear physics or genetics. But astronomy, especially planetary astronomy, has a history spanning several thousand years. And that much longer tradition says that we adjust what is considered a planet as new theories and data come in. How do you reconcile this with your statement > [@](#): > > If tradition were the only criterion, the answer would be 7 and Pluto would never have been added to the list of planets. Nor would Uranus and Neptune. Or the Earth. The Sun and Moon would still be planets, though. Pre Pluto’s demotion, planets where defined by tradition. Orbiting bodies were not added based on a set of predefined criteria, or removed by properly defining new criteria, but by gut feeling, which then affected tradition. And that’s my limit for pointless semantics-discussions. 😃 ---
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [October 26, 2011, 10:14am UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/54 "2011-10-26T10:14:30Z")
I strongly recommend Mike Brown’s memoir _How I Killed Pluto and Why It Had It Coming_, by the Caltech astronomer who discovered Eris and several other Kuiper Belt objects to get the whole discussion rolling (and he’s a damned entertaining writer, too). Brown tells the full story of the 2006 vote by the IAU about just what definitions to use, and how the definition has changed a number of times in the past. Brown adds that his own preference might be to categorize the objects that revolve around the Sun into 4 groups: the terrestrial planets (Mercury thru Mars), the asteroids, the outer planets (Jupiter thru Neptune), and the Kuiper Belt objects (Pluto on out). ---
**Author:** ![Alka\_Seltzer](https://avatars.discourse-cdn.com/v4/letter/a/c89c15/32.png) [@Alka\_Seltzer](https://boards.straightdope.com/u/Alka_Seltzer)\ **Post date:** [October 26, 2011, 11:50am UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/55 "2011-10-26T11:50:50Z")
> [@TriPolar](#): > > So for there to be a tenth planet discovered, there must have been nine at some point, right? Pluto’s status as a planet has been disputed since it’s discovery in 1930. It was originally thought to be a similar mass to the Earth, in accordance with the [Planet X](http://en.wikipedia.org/wiki/Planet_X#Planet_X) hypothesis (now disproven, the mass of Neptune had been overestimated by 0.5%). It was assumed to be a large, dark object, when it is actually a small, highly reflective one. Some astronomers did not accept this, saying it had more in common with an asteroid or comet. Estimates of it’s size were revised downwards throughout the 20th century. In 1978, the discovery of it’s moon Charon allowed it’s mass to be calculated for the first time. It was found to be 1/500th the mass of the Earth. It wasn’t until 1992 that another astronomers found another body orbiting beyond Neptune. Since then, several hundred have been detected, confirming what has long been suspected, that Pluto is just one of a large population of TNOs. The discovery of Eris, 27% more massive than Pluto, brought matters to a head. There is also Neptune’s moon [Triton](http://en.wikipedia.org/wiki/Triton_(moon)), which was captured early in the history of the solar system. It has a very similar composition to Pluto, but is about 60% more massive. Those complaining about Pluto’s demotion are really missing the point. We haven’t lost anything, we’ve simply learned more about the solar system. We’ve gained Eris, Sedna, [Haumea](http://en.wikipedia.org/wiki/Haumea_(dwarf_planet)) (ellipsoid in shape, twice as long in one axis than another due to it’s rapid rotation), Makemake, Orcus, Varuna, Quaoor… ---
**Author:** ![BigT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bigt/32/12044_2.png) [@BigT](https://boards.straightdope.com/u/BigT)\ **Post date:** [October 26, 2011, 1:39pm UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/56 "2011-10-26T13:39:56Z")
The only reason I hate the demotion is that Pluto was the perfect name for the last planet. Is it at least the first or largest of the Kuiper Belt objects? Then it could be the lord of the dark planetoids. ---
**Author:** ![GIGObuster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gigobuster/32/421_2.png) [@GIGObuster](https://boards.straightdope.com/u/GIGObuster)\ **Post date:** [October 26, 2011, 2:47pm UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/57 "2011-10-26T14:47:12Z")
> [@BigT](#): > > The only reason I hate the demotion is that Pluto was the perfect name for the last planet. Is it at least the first or largest of the Kuiper Belt objects? Then it could be the lord of the dark planetoids. Curiously, that is very similar to a following resolution from the IAU: [http://www.iau.org/public\_press/news/detail/iau0804/](http://www.iau.org/public_press/news/detail/iau0804/) > [@](#): > > The International Astronomical Union has decided on the term plutoid as a name for dwarf planets like Pluto at a meeting of its Executive Committee in Oslo. > > Almost two years after the International Astronomical Union (IAU) General Assembly introduced the category of dwarf planets, the IAU, as promised, has decided on a name for transneptunian dwarf planets similar to Pluto. The name plutoid was proposed by the members of the IAU Committee on Small Body Nomenclature (CSBN), accepted by the Board of Division III, by the IAU Working Group for Planetary System Nomenclature (WGPSN) and approved by the IAU Executive Committee at its recent meeting in Oslo, Norway. > > Plutoids are celestial bodies in orbit around the Sun at a semimajor axis greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit. Satellites of plutoids are not plutoids themselves, even if they are massive enough that their shape is dictated by self-gravity. The two known and named plutoids are Pluto and Eris. It is expected that more plutoids will be named as science progresses and new discoveries are made. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [October 27, 2011, 6:24am UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/58 "2011-10-27T06:24:31Z")
> [@naita](#): > > How do you reconcile this with your statement OK, you caught me. I’ll admit I misused the term “tradition” the first time. Misused in the same way you did, which is why I did it. In retrospect, I should have put it in quotes, but that didn’t occur to me at the time. > [@](#): > > Pre Pluto’s demotion, planets where defined by tradition. The term you want there is “concensus”, not “tradition”. ---
**Author:** ![Doubticus](https://avatars.discourse-cdn.com/v4/letter/d/e56c9b/32.png) [@Doubticus](https://boards.straightdope.com/u/Doubticus)\ **Post date:** [October 27, 2011, 8:19pm UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/59 "2011-10-27T20:19:09Z")
As our observational abilities changed, so did the definition of “planet.” Future discoveries are likely to require additional revisions. Such is science. ---
**Author:** ![GIGObuster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gigobuster/32/421_2.png) [@GIGObuster](https://boards.straightdope.com/u/GIGObuster)\ **Post date:** [October 29, 2011, 12:29am UTC](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600/60 "2011-10-29T00:29:25Z")
> [@Doubticus](#): > > As our observational abilities changed, so did the definition of “planet.” Future discoveries are likely to require additional revisions. Such is science. What is funny is that Mork was [right](http://www.tv.com/shows/mork-and-mindy/mork-runs-away-29161/) years before that! 🙂 " **Mork** : Oh yes! I’ve been to all the planets in your solar system. **Exidor** : Mars? Mercury? Pluto? **Mork** : Oh never go to Pluto! It’s a Mickey Mouse planet." [Previous page](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600.md?page=2) [Next page](https://boards.straightdope.com/t/how-many-planets-would-there-be-if-pluto-were-still-a-planet/600600.md?page=4)