# Charon a planet? **URL:** \ **Category:** Factual Questions\ **Created:** [August 16, 2006, 3:41pm UTC](https://boards.straightdope.com/t/charon-a-planet/368959 "2006-08-16T15:41:41Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Jophiel](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jophiel/32/66_2.png) [@Jophiel](https://boards.straightdope.com/u/Jophiel)\ **Post date:** [August 16, 2006, 3:41pm UTC](https://boards.straightdope.com/t/charon-a-planet/368959/1 "2006-08-16T15:41:41Z")
In [This thread](http://boards.straightdope.com/sdmb/showthread.php?t=382980&highlight=charon) about seeing Charon from Pluto’s surface, the OP mentions that the two are tidally locked but I assumed that Pluto was still the axis around which Charon moved. However, the International Astronomers Union, in trying to noodle out what counts as a planet says that (per the Associated Press) > [@The AP](#): > > That is the conclusion, to be announced today, of an international panel formed to devise a scientific definition of a planet and settle an increasingly intense dispute over whether Pluto qualifies. The panel suggests retaining Pluto and immediately adding three new planets to the nine that are familiar to any schoolchild: Ceres, currently considered a large asteroid; Charon, now considered a moon of Pluto; and Xena, a recently discovered object that is larger than Pluto. Why Charon and not any other moon? Needless to say, all the moons in the solar system revolve around the sun in a fashion. ---
**Author:** ![Jophiel](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jophiel/32/66_2.png) [@Jophiel](https://boards.straightdope.com/u/Jophiel)\ **Post date:** [August 16, 2006, 3:43pm UTC](https://boards.straightdope.com/t/charon-a-planet/368959/2 "2006-08-16T15:43:12Z")
I should have mentioned in the OP that the proposed official definition of a planet is now that it (A) revolves around a star without being a star itself and (B) has become a sphere by force of its own gravity. ---
**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, 3:45pm UTC](https://boards.straightdope.com/t/charon-a-planet/368959/3 "2006-08-16T15:45:00Z")
Apparently it’s because the centre of mass of the Pluto-Charon system is outside Pluto, so it’s a two-planet system. The Moon (i.e., the Earth’s natural satellite) would otherwise qualify as a planet, but the centre of mass of the Earth-Moon system is inside the Earth. ---
**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, 3:46pm UTC](https://boards.straightdope.com/t/charon-a-planet/368959/4 "2006-08-16T15:46:16Z")
> [@Jophiel](#): > > Why Charon and not any other moon? Needless to say, all the moons in the solar system revolve around the sun in a fashion. Because Charon is comparible in size with Pluto? (about half the diameter) ---
**Author:** ![Jophiel](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jophiel/32/66_2.png) [@Jophiel](https://boards.straightdope.com/u/Jophiel)\ **Post date:** [August 16, 2006, 3:53pm UTC](https://boards.straightdope.com/t/charon-a-planet/368959/5 "2006-08-16T15:53:38Z")
> [@Giles](#): > > Apparently it’s because the centre of mass of the Pluto-Charon system is outside Pluto, so it’s a two-planet system. The Moon (i.e., the Earth’s natural satellite) would otherwise qualify as a planet, but the centre of mass of the Earth-Moon system is inside the Earth. Ahhh… I knew about the size issue but thought that the axis of rotation between the two was still through the Plutoian (that a word?) poles. The couple sites I looked at didn’t mention the center of mass being outside of Pluto. ---
**Author:** ![Freddy\_the\_Pig](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@Freddy\_the\_Pig](https://boards.straightdope.com/u/Freddy_the_Pig)\ **Post date:** [August 16, 2006, 7:36pm UTC](https://boards.straightdope.com/t/charon-a-planet/368959/6 "2006-08-16T19:36:09Z")
> [@Jophiel](#): > > Ahhh… I knew about the size issue but thought that the axis of rotation between the two was still through the Plutoian (that a word?) poles. The couple sites I looked at didn’t mention the center of mass being outside of Pluto. It’s not a matter of axis of rotation, it’s a matter of center of gravity. The center of mass of the Earth-Moon system is inside the Earth, and both Earth and Moon revolve around it, but in different planes. The plane of the Moon’s revolution is inclined between 18 and 29 degrees relative to Earth’s equator. (Or, to put it another way, five degrees relative to the plane of the Earth’s orbit around the Sun.) I don’t know the comparable figures for Pluto and Charon, but they have no bearing on Charon’s status as a planet.