<blockquote>
  <p>Or am I missing something here?</p>
</blockquote>

<p>Yes. You are missing the word "Sun". The IAU did not want to tackle the problem of how to categorize exoplanets. There are only eight planets in the entire universe, plus a good number of (five and counting) dwarf planets. An expoplanet orbiting Alpha Centauri is not a planet or a dwarf planet, per the IAU definition. Some hypothesize that the early solar system contained five giant planets, but one of them was expelled. That rogue giant planet, if it exists, is not a planet per the IAU definition.</p>

<p>You are treating the word "orbit" as if it's a mutually exclusive term. It's not. The Earth's Moon, for example, orbits the Earth and it orbits the Sun. But since the Moon is a satellite of the Earth, it's not a planet or a dwarf planet. Side note: the concept of what distinguishes a "satellite" from a non-satellite is not defined.</p>

CC BY-SA 3.0
2015-08-25T13:27:23.380Z
Some sites note the inclusion of point d) thanks to charon? Pluto and Charon technically being a Binary Dwarf System? But I guess the Earth System might also be seen as a binary planetary system?
cstruter
CC BY-SA 3.0
2015-08-25T13:34:34.330Z
@cstruter - The IAU currently deems Charon to be a satellite, but has left the door open to a reclassification of Charon as a dwarf planet.
David Hammen
CC BY-SA 3.0
2015-08-25T13:34:35.093Z
I just thought that orbit would imply its parent? Since obviously we can get more technical and include what the sun orbits as well etc etc etc
cstruter
CC BY-SA 3.0
2015-08-25T13:44:26.440Z
@cstruter: It's hard to argue that the Earth's Moon does not orbit the Sun. For example, see http://www.wired.com/2012/12/does-the-moon-orbit-the-sun-or-the-earth/ . The easy way out of this false dilemma is to say that the Moon orbits both the Earth and the Sun.
David Hammen
CC BY-SA 3.0
2015-08-25T13:47:01.813Z
Or rather the Earth & Moon orbits a common point of gravity which orbits the sun?
cstruter
CC BY-SA 3.0
2015-08-25T19:46:36.023Z
@cstruter: The center of gravity of the Earth/Moon system is inside Earth, about 1000 miles below the surface. (Some suggest that that is, or should be, relevant to some criterion for something or other.)
Keith Thompson
CC BY-SA 3.0
2015-08-26T06:38:07.333Z
Yup, the barycenter, something like 4700km down?
cstruter
CC BY-SA 3.0
2015-08-25T13:19:58.550Z
2015-08-25T13:30:59.903Z
2015-08-25T13:30:59.903Z
David Hammen
<p>I am a bit confused with point 2.d of the IAU's resolution on dwarf planets.</p>
<blockquote>
<p>(2) A &quot;dwarf planet&quot; is a celestial body that</p>
<p>(a) is in orbit around the Sun</p>
<p>(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape</p>
<p>(c) has not cleared the neighbourhood around its orbit</p>
<p><strong>(d) is not a satellite.</strong></p>
</blockquote>
<p><a href="http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf" rel="nofollow noreferrer">http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf</a></p>
<p>I notice that a lot of websites exclude this point all together? Some of
them even rather combine it with point 2.a ? Like over <a href="http://lasp.colorado.edu/education/outerplanets/kbos_dwarfplanets.php" rel="nofollow noreferrer">here</a></p>
<p>Feels a bit redundant, since orbits the sun implies not orbiting a planet?</p>
<p>Or am I missing something here?</p>

<dwarf-planets>
IAU Dwarf planet definition Point 2.b - is not a satellite
cstruter
# IAU Dwarf planet definition Point 2.b - is not a satellite

(Asked by: username_0 on 2015-08-25)

I am a bit confused with point 2.d of the IAU's resolution on dwarf planets.

> (2) A "dwarf planet" is a celestial body that
>
> (a) is in orbit around the Sun
>
> (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
>
> (c) has not cleared the neighbourhood around its orbit
>
> **(d) is not a satellite.**

<http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf>

I notice that a lot of websites exclude this point all together? Some of
them even rather combine it with point 2.a ? Like over [here](http://lasp.colorado.edu/education/outerplanets/kbos_dwarfplanets.php)

Feels a bit redundant, since orbits the sun implies not orbiting a planet?

Or am I missing something here?

**Tags:** dwarf-planets

---

## 1 answers

### Answer by username_1 on 2015-08-25. Score: 2 | Accepted answer

> Or am I missing something here?

Yes. You are missing the word "Sun". The IAU did not want to tackle the problem of how to categorize exoplanets. There are only eight planets in the entire universe, plus a good number of (five and counting) dwarf planets. An expoplanet orbiting Alpha Centauri is not a planet or a dwarf planet, per the IAU definition. Some hypothesize that the early solar system contained five giant planets, but one of them was expelled. That rogue giant planet, if it exists, is not a planet per the IAU definition.

You are treating the word "orbit" as if it's a mutually exclusive term. It's not. The Earth's Moon, for example, orbits the Earth and it orbits the Sun. But since the Moon is a satellite of the Earth, it's not a planet or a dwarf planet. Side note: the concept of what distinguishes a "satellite" from a non-satellite is not defined.

> **Comments:**
> **username_0**: Some sites note the inclusion of point d) thanks to charon? Pluto and Charon technically being a Binary Dwarf System? But I guess the Earth System might also be seen as a binary planetary system?  
> **username_1**: @cstruter - The IAU currently deems Charon to be a satellite, but has left the door open to a reclassification of Charon as a dwarf planet.  
> **username_0**: I just thought that orbit would imply its parent? Since obviously we can get more technical and include what the sun orbits as well etc etc etc  
> **username_1**: @cstruter: It's hard to argue that the Earth's Moon does not orbit the Sun. For example, see http://www.wired.com/2012/12/does-the-moon-orbit-the-sun-or-the-earth/ . The easy way out of this false dilemma is to say that the Moon orbits both the Earth and the Sun.  
> **username_0**: Or rather the Earth & Moon orbits a common point of gravity which orbits the sun?  
> **username_2**: @cstruter: The center of gravity of the Earth/Moon system is inside Earth, about 1000 miles below the surface. (Some suggest that that is, or should be, relevant to some criterion for something or other.)  
> **username_0**: Yup, the barycenter, something like 4700km down?  

---

