# Charon (Pluto's Moon) is now a planet? **URL:** \ **Category:** Factual Questions\ **Created:** [August 16, 2006, 3:38pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958 "2006-08-16T15:38:24Z")\ **Posts on this page:** 20\ **Page:** 1
**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:** [August 16, 2006, 3:38pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/1 "2006-08-16T15:38:24Z")
What the…? First. there’s controversy over whether or not Pluto should be considered a planet. Now I hear on the radio that not only is Pluto a planet, but so is its moon, Charon. Again, wtf? ---
**Author:** ![One\_And\_Only\_Wanderers](https://avatars.discourse-cdn.com/v4/letter/o/838e76/32.png) [@One\_And\_Only\_Wanderers](https://boards.straightdope.com/u/One_And_Only_Wanderers)\ **Post date:** [August 16, 2006, 3:43pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/2 "2006-08-16T15:43:08Z")
In the article I read, it said experts were considering creating a new classification of planets called “Pluton’s” This is required due to the discovery of Kuiper belt objects that are of comparible size or bigger than Pluto. Pluto would retain it’s “normal” planet status due to being grandfathered in. [BBC article](http://news.bbc.co.uk/1/hi/sci/tech/4795755.stm) ---
**Author:** ![pulykamell](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pulykamell/32/3166_2.png) [@pulykamell](https://boards.straightdope.com/u/pulykamell)\ **Post date:** [August 16, 2006, 3:44pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/3 "2006-08-16T15:44:09Z")
> [@Jackknifed Juggernaut](#): > > What the…? First. there’s controversy over whether or not Pluto should be considered a planet. Now I hear on the radio that not only is Pluto a planet, but so is its moon, Charon. Again, wtf? Oh, but there’s more. Under the new proposed definition of a planet, Ceres (an asteroid between Mars and Jupiter), would also be a planet. The definition hasn’t been voted on yet. Also, there’s another object past Pluto & Charon called 2003 UB313 (aka “Xena”) would also become a planet. ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [August 16, 2006, 3:49pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/4 "2006-08-16T15:49:33Z")
In addition, it is possible the the next largest asteroids, Pallas and Vesta, might be considered planets. The issue with them is whether they hjave become spherical enough under their own gravity. If they do, then Heinrich Wilhelm Matthäus Olbers will be the only person to have discovered two planets (since he discovered Pallas and Vesta). ---
**Author:** ![Chez\_Guevara](https://avatars.discourse-cdn.com/v4/letter/c/aeb1de/32.png) [@Chez\_Guevara](https://boards.straightdope.com/u/Chez_Guevara)\ **Post date:** [August 16, 2006, 3:52pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/5 "2006-08-16T15:52:59Z")
Charon has hitherto been known as a satellite of Pluto but, as they are similar in size, spherical, and their common centre of gravity is outside Pluto, it has been decided that Pluto/Charon is a double planet under the new system. ---
**Author:** ![Angua](https://avatars.discourse-cdn.com/v4/letter/a/8baadc/32.png) [@Angua](https://boards.straightdope.com/u/Angua)\ **Post date:** [August 16, 2006, 3:59pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/6 "2006-08-16T15:59:48Z")
The problem stems from the fact that Pluto is a bit of an oddity; its orbit is far more elliptical than the other 8 planets, its much much smaller than the other planets (0.003 times the Earth’s mass) and its distance from the Sun puts it at a radius where lots of other bodies have been found, that are known as Trans-Neptunian Objects (TNOs) Pluto. Since then a couple more TNOs larger than Pluto were also found. So this raises a question – if Pluto is classified as a planet, then shouldn’t the other large TNOs be classified as planets? The International Astronomical Union (IAU) is attempting to get round this by proposing that we have 8 classical planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune), dwarf planets such as Ceres, and then the “Plutons”, which would include Pluto, 2003 UB313 and any other large TNO. Their proposed definition of planet is > [@IAU](#): > > (1)A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet. > > (2) We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a “dwarf planet.” > > (3) We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call “plutons”. > > (4) All non-planet objects orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”. Basically, anything that’s approximately spherical in shape will be a planet; the subdivisions come from the size of the object. So, the planets discovered before 1900 would be the classical planets, Ceres and some other large asteroids would be dwarf planets, and Pluto and other large TNOs would be Plutons. [Link to IAU resolution](http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_resolution.html) ---
**Author:** ![PoorYorick](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@PoorYorick](https://boards.straightdope.com/u/PoorYorick)\ **Post date:** [August 16, 2006, 4:20pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/7 "2006-08-16T16:20:48Z")
I’m just thinking of all those poor astologers. ---
**Author:** ![AskNott](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/asknott/32/5790_2.png) [@AskNott](https://boards.straightdope.com/u/AskNott)\ **Post date:** [August 16, 2006, 4:21pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/8 "2006-08-16T16:21:30Z")
In the newspaper story, the business of Charon, a moon, being called a planet or pluton was explained by saying Pluto and Charon orbit around each other, with neither being dominant. There has been no reaction yet from James Dobson’s Family Something-or-other group about the morality of two plutons being married in an apparently equal relationship. ---
**Author:** ![Otto](https://avatars.discourse-cdn.com/v4/letter/o/bbe5ce/32.png) [@Otto](https://boards.straightdope.com/u/Otto)\ **Post date:** [August 16, 2006, 4:34pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/9 "2006-08-16T16:34:24Z")
> [@AskNott](#): > > There has been no reaction yet from James Dobson’s Family Something-or-other group about the morality of two plutons being married in an apparently equal relationship. Pluto and Charon sittin’ in a tree Oh Are Be Eye Tee Tee Eye En Gee! ---
**Author:** ![Anne\_Neville](https://avatars.discourse-cdn.com/v4/letter/a/b9e5f3/32.png) [@Anne\_Neville](https://boards.straightdope.com/u/Anne_Neville)\ **Post date:** [August 16, 2006, 4:49pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/10 "2006-08-16T16:49:46Z")
They haven’t voted on the resolution yet- the vote is scheduled for August 25. So it’s not official yet. ---
**Author:** ![pulykamell](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pulykamell/32/3166_2.png) [@pulykamell](https://boards.straightdope.com/u/pulykamell)\ **Post date:** [August 16, 2006, 4:50pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/11 "2006-08-16T16:50:31Z")
> [@Otto](#): > > Pluto and Charon sittin’ in a tree > Oh Are Be Eye Tee Tee Eye En Gee! Psst…one “tee.” ---
**Author:** ![PoorYorick](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@PoorYorick](https://boards.straightdope.com/u/PoorYorick)\ **Post date:** [August 16, 2006, 4:51pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/12 "2006-08-16T16:51:18Z")
> [@PoorYorick](#): > > I’m just thinking of all those poor astologers. Er, also the poor _astrologers_. :smack: ---
**Author:** ![Otto](https://avatars.discourse-cdn.com/v4/letter/o/bbe5ce/32.png) [@Otto](https://boards.straightdope.com/u/Otto)\ **Post date:** [August 16, 2006, 4:54pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/13 "2006-08-16T16:54:01Z")
> [@pulykamell](#): > > Psst…one “tee.” Sez you, I was having Tee for Two. ---
**Author:** ![Gorsnak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gorsnak/32/7587_2.png) [@Gorsnak](https://boards.straightdope.com/u/Gorsnak)\ **Post date:** [August 16, 2006, 5:24pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/14 "2006-08-16T17:24:15Z")
> [@Otto](#): > > Sez you, I was having Tee for Two. I thought Tee-ing generally involved a foursome. ---
**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:** [August 16, 2006, 7:40pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/15 "2006-08-16T19:40:35Z")
> [@Chez Guevara](#): > > Charon has hitherto been known as a satellite of Pluto but, as they are similar in size, spherical, and their common centre of gravity is outside Pluto, it has been decided that Pluto/Charon is a double planet under the new system. Interesting. So if the Earth’s moon was big enough to move the common center of gravity outside the Earth, it would also be considered planet? Anyone know how far deep inside the Earth it is currently? ---
**Author:** ![Fish](https://avatars.discourse-cdn.com/v4/letter/f/bc8723/32.png) [@Fish](https://boards.straightdope.com/u/Fish)\ **Post date:** [August 16, 2006, 7:51pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/16 "2006-08-16T19:51:29Z")
> [@PoorYorick](#): > > I’m just thinking of all those poor astrologers. [spelling corrected] See, the reason you had such bad luck was that Jupiter was rising while Pluto was in the house of Neptune, borrowing a cup of Mercury. While Pluto was there, he borrowed a second cup for his co-orbital partner. That just goes to prove the old saying “Charon share alike.” ---
**Author:** ![Angua](https://avatars.discourse-cdn.com/v4/letter/a/8baadc/32.png) [@Angua](https://boards.straightdope.com/u/Angua)\ **Post date:** [August 16, 2006, 8:12pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/17 "2006-08-16T20:12:35Z")
> [@Jackknifed Juggernaut](#): > > Interesting. So if the Earth’s moon was big enough to move the common center of gravity outside the Earth, it would also be considered planet? Anyone know how far deep inside the Earth it is currently? On average, the CoM of the Earth/Moon system (also called the barycentre) is about 4,670 km from the centre of the Earth. The radius of the Earth is ~6,400 km, so the CoM is within the Earth. As the Earth’s centre and the barycentre are different, the Earth wobbles a bit on its axis. [linky](http://www.reference.com/browse/wiki/Barycenter) ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [August 16, 2006, 8:14pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/18 "2006-08-16T20:14:54Z")
> [@Gorsnak](#): > > I thought Tee-ing generally involved a foursome. Pluto (or Pluto-Charon) has two other moons. 😉 ---
**Author:** ![DSYoungEsq](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DSYoungEsq](https://boards.straightdope.com/u/DSYoungEsq)\ **Post date:** [August 16, 2006, 8:32pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/19 "2006-08-16T20:32:49Z")
The whole proposal is silly, really. It relies upon trying to define “planet” with a measure of precision (that is, with a definition that does not depend upon too much interpretation or upon subjective qualifiers). It also is intended in large measure to avoid the simpler solution of taking Pluto and removing it from “planet” classification, which, it is figured, will be difficult to get the public to accept now that we’ve put it in that class for the last 70 years or so. The fact that Ceres (568 mi. in diameter) will be a “planet” and Ganymede (3200 mi. in diameter) won’t be shows how silly the idea is. No one is going to seriously consider something as small as Ceres a planet. Nor, frankly, is the public easily going to accept the concept of hundreds of planets the size of Pluto, which we will discover as soon as we end up poking around in the outskirts of the system. It makes much more sense to simply remove Pluto/Charon from planetary status, and reserve “planet” for what it originally was: really big ball of stuff way out there that orbits the sun in roughly circular motion. ---
**Author:** ![Angua](https://avatars.discourse-cdn.com/v4/letter/a/8baadc/32.png) [@Angua](https://boards.straightdope.com/u/Angua)\ **Post date:** [August 16, 2006, 8:46pm UTC](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958/20 "2006-08-16T20:46:57Z")
> [@DSYoungEsq](#): > > The fact that Ceres (568 mi. in diameter) will be a “planet” and Ganymede (3200 mi. in diameter) won’t be shows how silly the idea is. Actually, its not all that silly. Part of the definition of planet proposed by the IAU, is that a planet “is in orbit around a star, and is neither a star nor a satellite of a planet.” Ganymede is blatantly the satellite of a planet, and hence a moon, and not a planet. Ceres, on the other hand, despite being smaller than Ganymede is in orbit directly around the Sun, and under the new scheme would be termed a “dwarf planet” rather than a planet proper. [Next page](https://boards.straightdope.com/t/charon-plutos-moon-is-now-a-planet/368958.md?page=2)