Asteroids and comets only seem to refer to masses within the solar system, and it seems unclear whether planetesimal does.

  • $\begingroup$ With regard to planetesimals themselves, that is the right word. Google scholar found a paltry four articles that use either exoplanetesimal or exo-planetesimal. Compare that to the tens of thousands it found that just use planetesimal. $\endgroup$ – David Hammen Mar 8 '15 at 9:45
  • $\begingroup$ @DavidHammen The problem with planetesimal is that it's ambiguous. Though it's sometimes defined as a small objects "whose internal strength is dominated by self-gravity and whose orbital dynamics is not significantly affected by gas drag," other times its definition requires it's in the solar system and other times the name is only "applied to small bodies during the process of planet formation." $\endgroup$ – Kelmikra Mar 8 '15 at 13:41

Just add the prefix "exo-". At least exocomet and exomoon seem to be established.


I have to disagree with the accepted answer. The terms Planets, moons, asteroids, planetesimals are generic and are not implied to mean objects in the Solar system. So, these terms are completely correct when dealing with the generic objects.

When specifically emphasizing the fact that a certain objects is not in Solar system, then you may want to add 'exo' for disambuigation, but this should be reserved for this purpose only.

A planet is a planet no matter where. Same with a house, which is a house no matter whether it's in your home town or elsewhere on the Earth.

Note in edit: My answer is based on the practice amongst professional astronomers and not on what some dictionaries say. I reckon the latter are somewhat behind the times, when all planets, asteroids etc known where those in orbiting the Sun. Wikipedia is a great resource, but anybody can change its contents and, not surprisingly, some of its pages are quite biased.

  • $\begingroup$ Wikipeia says, "asteroids are minor planets, especially those of the inner Solar System" and that "a minor planet is an astronomical object in direct orbit around the Sun that is neither a planet nor originally classified as a comet." According to this, asteroids need to orbit around the Sun. $\endgroup$ – Kelmikra Mar 9 '15 at 21:33
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    $\begingroup$ @Kyth'Py1k: Perhaps that's just because all known asteroids are within the Solar System (unless I've missed some recent discovery). I have little doubt that if we discovered an asteroid-like (or planet-like) object orbiting, say, Alpha Centauri A, we'd call it an asteroid (or planet), Wikipedia notwithstanding. $\endgroup$ – Keith Thompson Mar 9 '15 at 23:26
  • $\begingroup$ @KeithThompson Dictionary.com also says they must orbit around the sun, and Merriam Webster says they need to be between Mars and Jupiter. Still, you could be correct. $\endgroup$ – Kelmikra Mar 9 '15 at 23:33
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    $\begingroup$ @Kyth'Py1k it is often non-scientists who write the dictionaries and they cannot be expected to keep up with the pace of scientific discovery. The prefix exo is only used so the reader/listener can be under no doubt that you are talking about an object orbiting another star. [And as an aside, prefixing any statement with "Wikipedia says..." is a sure way to rile a scientist. Wikipedia is just a bunch of people, some knowledgeable, some not.] $\endgroup$ – Rob Jeffries Mar 10 '15 at 10:23
  • $\begingroup$ @Kyth'Py1k "...nor originally classified as a comet" Damn it! That excludes Ceres. The IAU should've hired a lawyer, someone who tries to apply math to language, before they tried to make their definitions of a "direct orbit" to no good place. An exo-thing is just a bump or discoloration in the light curve of a star, or at most it is a dot. Something we can't reach in a lifetime. But a non-exo planet/comet is something nearby at which we can throw things. Huge difference. "Exo" basically means out of this world. And we've seen stuff orbiting Saturn which kind of fulfills that criterium. $\endgroup$ – LocalFluff Mar 11 '15 at 5:26

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