Other questions have dealt with where you could find a planet-like object, and what a planet has to consist of to count as a planet. But this question is directed as what orbit or path an object would have to have to count as a planet.

This question was spawned from comments in Does a star need to be inside a galaxy?

We used to think of planets as objects orbiting the sun IAU 2006 Resolution 5A

A "planet" is a celestial body that (a) is in orbit around the Sun [...].

but, depending on how you read

The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way

you might think that their definition only applies to our solar system and that they have left open the definition in relation to other parts of the universe.

They were working on what constituted a planet elsewhere IAU Defintion of a "Planet" but they seem to be looking only at the mass and other internal properties of a body, not its orbit (or lack thereof).

Wikipedia quotes these two sources to define a planet as

astronomical body orbiting a star or stellar remnant [...]

That is to say it has extrapolated beyond what its references say to include other single stars. This may be justifiable because

The extrasolar planet issue was deemed too complex to resolve at the 2006 IAU conference (Wikipedia)

but whether it should be restricted to single stars is the question.

It defines a circumbinary planet as

a planet that orbits two stars instead of one

thus contradicting its own monostellar definition of a planet.

On the other hand, it defines a rogue planet by carefully avoiding the use of the term planet

planetary-mass object that orbits a galactic center directly.

So my question is

Are - or should - objects that are in orbit round

  1. binary (or ternary, etc.) stars,
  2. galaxy centres, or
  3. nothing

be counted as planets?

  • $\begingroup$ Most research outside the solar system is not concerned or very independent of those IAU definitions for the solar system. Any planetary-mass object outside the solar system is a planet, until proven otherwise. $\endgroup$ – AtmosphericPrisonEscape Feb 20 '19 at 16:19
  • $\begingroup$ Yes, @atmosphericprisonescape, the point is that research can never prove whether or not an object is a planet unless we can agree a definition of planet. You are entitled to your own opinion on what the definition should be but we need an agreed one. If you have evidence of a body of opinion that differs from the sources I quoted please give it. If we can conclude that there is no satisfactory definition then we need to agree one. If you think that the orbit should not form part of the definition then we need to see if others agree with you. $\endgroup$ – David Robinson Feb 20 '19 at 17:41
  • $\begingroup$ "research [...] can never prove whether or not an object is a planet unless we can agree a definition of planet." Well here's where you are factually wrong. Amongst the ~4000 confirmed exoplanets, I have not seen a single one dabble with the IAU definition of 'planet'. None of the papers does it. It's not important. Because this strict definition is only useful in the solar system. $\endgroup$ – AtmosphericPrisonEscape Feb 20 '19 at 21:15
  • $\begingroup$ I might presume that, when we have the ability to detect Ceres sized (or smaller) bodies around other stars that the IAU might get around to defining exoplanets more strictly. For now, as pointed out above, that does not seem to be a priority to anyone. $\endgroup$ – Jon Custer Feb 20 '19 at 23:49
  • $\begingroup$ I am sorry, @AtmosphericPrisonEscape, but I cannot see how you can confirm an exoplanet (which I think we can agree is a planet that does not orbit the sun) without a definition of a planet. If you detect something, surely you have to decide if it is a planet before you add it to the list. It may be that you are working to an implicit definition that does not specify orbit. That is fine but if that definition is unofficially agreed then we should be able to agree that the Wiki definition is wrong, especially as it gives references that wrongly imply their definition is based on the IAU. $\endgroup$ – David Robinson Feb 21 '19 at 1:46