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This XKCD what-if talks about rainbows on planets in a binary star system. It points out that there are two types:

  • circumbinary planets, where the planet orbits far from and around both stars
  • [the other kind of planet], where the planet orbits one of the stars and the other star is distant.

enter image description here

What is the proper name for [the other kind of planet] ?

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  • $\begingroup$ The term I usually see is "non-circumbinary". I wonder if there is a better term. $\endgroup$
    – called2voyage
    May 24, 2016 at 21:24
  • $\begingroup$ I would have thought "circumstellar", as it orbits a single star and it sounds better than "circumunary". $\endgroup$ May 25, 2016 at 17:29
  • $\begingroup$ @zibadawatimmy "circumstellar" is also used for "circumsystem" objects though which orbit around multiple stars. $\endgroup$
    – called2voyage
    May 25, 2016 at 17:47

2 Answers 2

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I don't know about names for the planets, specifically, but the orbits are called S-type and P-type:

  • S-type: The planet orbits around one star, and the host star has a binary companion (i.e., "the other kind" in the XKCD comic)

  • P-type: The planet orbits around both stars of the binary

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    $\begingroup$ I propose the name "circumunary planet". It would be usable for both planets in systems like ours and in binary systems. You could refer to "the circumunary planet of Alpha Centauri B in the Alpha Centauri binary star system" for example, and you could also say "Earth is a circumunary planet of the sun in the solar system, a unary star". $\endgroup$
    – called2voyage
    May 24, 2016 at 22:15
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    $\begingroup$ All planets of unary stars would be circumunary by definition, but planets in binary star systems could be either circumunary or circumbinary. $\endgroup$
    – called2voyage
    May 24, 2016 at 22:16
  • $\begingroup$ Also, is it possible there are some objects which are gravitationally bound to a binary system but which technically revolve around neither star? $\endgroup$
    – called2voyage
    May 24, 2016 at 22:19
  • $\begingroup$ @called2voyage: Objects can be in or in semi-stable orbits around the various Lagrangian points. $\endgroup$ May 24, 2016 at 22:55
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    $\begingroup$ in a binary system a Lagrange point (L4 or 5) could be stable, around the smaller of the 2 stars, not the larger (obviously). The star ratio of mass has to be about 25 to 1 though. astronomy.stackexchange.com/questions/11389/… $\endgroup$
    – userLTK
    May 25, 2016 at 15:18
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Astronomy currently only gives us the names circumbinary (left picture) and Trojan planet (unpictured, planet is around a Lagrange point), but from circumbinary we can extrapolate other names:

  • circumunary - revolves around one star
  • circumtrinary/circumternary - revolves around a trinary/ternary star system

Technically, there is another term that actually has some use: circumsystem. This is used for objects that revolve around a multiple star system regardless of the number of stars within the system.

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  • $\begingroup$ The problem with this nomenclature is you want to distinguish between planets in binary systems and those that are not. I realise that - the "planet Alpha Cen Bb" makes this obvious, but that doesn't help when referring to them as a class. $\endgroup$
    – ProfRob
    Dec 17, 2020 at 19:04
  • $\begingroup$ @RobJeffries I realize "circumunary planets of binary systems" is a bit of a mouthful, but at least more concise than "planets which orbit only one star of a binary system". $\endgroup$
    – called2voyage
    Dec 17, 2020 at 19:59
  • $\begingroup$ But not more concise than "s-type exoplanet". $\endgroup$
    – ProfRob
    Dec 17, 2020 at 20:02
  • $\begingroup$ @RobJeffries True, though I thought strictly speaking those were considered types of orbits not types of planets, so wouldn't that be "exoplanets in s-type orbits" in the strictest sense? $\endgroup$
    – called2voyage
    Dec 17, 2020 at 20:04

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