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.


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

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

Or am I missing something here?


1 Answer 1


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.

  • $\begingroup$ 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? $\endgroup$
    – cstruter
    Commented Aug 25, 2015 at 13:27
  • $\begingroup$ @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. $\endgroup$ Commented Aug 25, 2015 at 13:34
  • $\begingroup$ 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 $\endgroup$
    – cstruter
    Commented Aug 25, 2015 at 13:34
  • $\begingroup$ @cstruter: It's hard to argue that the Earth's Moon does not orbit the Sun. For example, see 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. $\endgroup$ Commented Aug 25, 2015 at 13:44
  • $\begingroup$ Or rather the Earth & Moon orbits a common point of gravity which orbits the sun? $\endgroup$
    – cstruter
    Commented Aug 25, 2015 at 13:47

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