<p>Mathematically, the motion of the Pluto-Charon system can be decomposed into two parts: The motion of Pluto-Charon about the Sun, and the motion of Pluto and Charon about one another. If one sets the reference point to be the center of Pluto, the path the Pluto-Charon system would appear to follow about the Sun would be an <a href="http://en.wikipedia.org/wiki/Deferent_and_epicycle" rel="nofollow noreferrer">epicycle</a>, which is a far more complicated means of describing the pair's trajectory through space. By instead setting this reference point to be the <a href="http://en.wikipedia.org/wiki/Barycentric_coordinates_%28astronomy%29" rel="nofollow noreferrer">barycenter</a>, the trajectory followed by the Pluto-Charon system is conveniently a Keplerian ellipse.</p>

<p>Charon is 11.6% the mass of Pluto, and is (on average) 19,571 km away from Pluto (Source <a href="http://en.wikipedia.org/wiki/Pluto" rel="nofollow noreferrer">1</a>, <a href="http://en.wikipedia.org/wiki/Charon_%28moon%29" rel="nofollow noreferrer">2</a>). The barycenter is the point at which the masses of the two bodies "balance", and for the Pluto-Charon system it lies at a distance of $$\frac{m_{\text{Charon}}}{m_{\text{Pluto}}+m_{\text{Charon}}}\times\text{distance}=\frac{0.116}{1.116}\times\text{distance}=0.104\times 19,571\,\text{km}=2,034\,\text{km}$$ from the center of Pluto. As Pluto is only 1,153 km in radius, the barycenter lies ~900 km above its surface. The Pluto-Charon system is the only known (minor or major) planet for which this is the case, although the Earth-Moon system will likely satisfy this criteria billions of years in the future (see <a href="https://physics.stackexchange.com/questions/26304/is-the-moon-a-planet">Is the Moon a Planet?</a> on Physics SE). There are some instances of <a href="http://en.wikipedia.org/wiki/Binary_asteroid" rel="nofollow noreferrer">binary asteroids</a>, however, for which the barycenter lies outside the surface of both bodies.</p>

CC BY-SA 3.0
2013-09-30T20:30:06.453Z
The IAU needs to update the dwarf planet list to include Charon so we can start officially calling the Pluto-Charon system a binary dwarf planet system.
called2voyage
CC BY-SA 3.0
2013-09-30T19:39:52.553Z
2013-10-22T03:28:40.917Z
2017-04-13T12:40:15.377Z
Guillochon
<p>Even though Pluto is not known as a planet anymore, theroetically it has/had a moon, called Charon. I've heard about something that their size are so close to each other that while Charon rotates around Pluto, it also rotates around a central point.</p>

<p>How does it work exactly?</p>

CC BY-SA 4.0
2024-01-16T00:23:56.443Z
Has a moon, or five. In 1994 a smaller asteroid was found orbiting a lsrger one, and ever since a "moon" does not have to orbit a "planet". So Charon (and now, others) is a moon of Pluto, period.
Oscar Lanzi
CC BY-SA 3.0
2013-09-30T20:05:49.220Z
Of course, it also has Styx, Hydra, Nix and Kerberos - which also have an effect, just a much smaller one.
Rory Alsop
CC BY-SA 3.0
2013-10-02T14:48:33.480Z
@RoryAlsop Never heard of them. My knowledge is a bit old maybe
Zoltán Schmidt
<solar-system><natural-satellites><pluto><charon>
Effect of Charon on Pluto
Zoltán Schmidt
# Effect of Charon on Pluto

(Asked by: username_0 on 2013-09-30)

Even though Pluto is not known as a planet anymore, theroetically it has/had a moon, called Charon. I've heard about something that their size are so close to each other that while Charon rotates around Pluto, it also rotates around a central point.

How does it work exactly?

**Tags:** solar-system, natural-satellites, pluto, charon

> **Comments:**
> **username_2**: Of course, it also has Styx, Hydra, Nix and Kerberos - which also have an effect, just a much smaller one.  
> **username_0**: @username_2 Never heard of them. My knowledge is a bit old maybe  
> **username_1**: Has a moon, or five. In 1994 a smaller asteroid was found orbiting a lsrger one, and ever since a "moon" does not have to orbit a "planet". So Charon (and now, others) is a moon of Pluto, period.  

---

## 1 answers

### Answer by username_3 on 2013-09-30. Score: 9 | Accepted answer

Mathematically, the motion of the Pluto-Charon system can be decomposed into two parts: The motion of Pluto-Charon about the Sun, and the motion of Pluto and Charon about one another. If one sets the reference point to be the center of Pluto, the path the Pluto-Charon system would appear to follow about the Sun would be an [epicycle](http://en.wikipedia.org/wiki/Deferent_and_epicycle), which is a far more complicated means of describing the pair's trajectory through space. By instead setting this reference point to be the [barycenter](http://en.wikipedia.org/wiki/Barycentric_coordinates_%28astronomy%29), the trajectory followed by the Pluto-Charon system is conveniently a Keplerian ellipse.

Charon is 11.6% the mass of Pluto, and is (on average) 19,571 km away from Pluto (Source [1](http://en.wikipedia.org/wiki/Pluto), [2](http://en.wikipedia.org/wiki/Charon_%28moon%29)). The barycenter is the point at which the masses of the two bodies "balance", and for the Pluto-Charon system it lies at a distance of $$\frac{m\_{\text{Charon}}}{m\_{\text{Pluto}}+m\_{\text{Charon}}}\times\text{distance}=\frac{0.116}{1.116}\times\text{distance}=0.104\times 19,571\,\text{km}=2,034\,\text{km}$$ from the center of Pluto. As Pluto is only 1,153 km in radius, the barycenter lies ~900 km above its surface. The Pluto-Charon system is the only known (minor or major) planet for which this is the case, although the Earth-Moon system will likely satisfy this criteria billions of years in the future (see [Is the Moon a Planet?](https://physics.stackexchange.com/questions/26304/is-the-moon-a-planet) on Physics SE). There are some instances of [binary asteroids](http://en.wikipedia.org/wiki/Binary_asteroid), however, for which the barycenter lies outside the surface of both bodies.

> **Comments:**
> **username_4**: The IAU needs to update the dwarf planet list to include Charon so we can start officially calling the Pluto-Charon system a binary dwarf planet system.  

---

