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Given that the term planet strictly (according to the IAU) refers to a body around the sun, rogue planets can't be called that, so I assume they must be called rogue exoplanets?

But do they even qualify for the name exoplanet? Given that they do not orbit a star?

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  • $\begingroup$ I don't think it's classed as either a planet or an exoplanet. It's classed as a rogue planet. Rogue isn't necessarily just an adjective here - it's part of the designation. Sure, it may not be technically accurate, but, then again, things like planetary nebulae have nothing to do with planets, yet the name remains as it is. $\endgroup$
    – voithos
    Jun 14, 2011 at 23:52

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They are not rogue exoplanets according to the IAU, but "sub-brown dwarfs;" only objects actually orbiting stars or stellar remnants are "planets." This is a temporary working definition instituted in 2003; it isn't set in stone. See here the IAU working group's full explanation.

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  • $\begingroup$ the problem is that physically, there is a huge difference between a 10 M_Jup planet and a 10 M_Jup sub-brown dwarf. They have totally different formation mechanisms, chemical composition, and magnetic fields. Of course, it will be huge challenge to distinguish between them based on microlensing observations. $\endgroup$
    – Jeremy
    Jun 17, 2011 at 14:06
  • $\begingroup$ @Jeremy What makes you say that they'd have different magnetic fields? I thought the flux-freezing model was pretty much the same for stars and planets. $\endgroup$
    – spencer nelson
    Jun 17, 2011 at 19:54
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    $\begingroup$ mostly due to the different composition; e.g. metallic cores can generate dynamos more easily than gas. $\endgroup$
    – Jeremy
    Jun 19, 2011 at 2:04
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Remember the searches for "conventional" dark matter?

After all the WIMPs, someone had to come up with MACHOs. Extensive searches were made for dark objects roaming the interstellar space, by watching for microlensing events within our Galaxy (or involving stars in the satellite galaxies such as the Magellanic Clouds).

In the papers describing these efforts, one class of proposed objects were "rogue planets" (not rogue exoplanets) and, apparently, no one had any problem with the phrase.

Of course, that is a moot point, since they did not find anything smaller than brown dwarfs.

However, some of these objects have been recently discovered (10-Jupiter-Mass objects) and the article I just read uses the phrase "unbound planets"

"Bound and unbound planets abound" by Joachim Wambsganss. Nature, v.473, pp.289-291 (19 May 2011)

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Strigari et al in http://adsabs.harvard.edu/abs/2012MNRAS.423.1856S refer to unbound planets as "nomads" and notes that "in the literature a subset of these are sometimes called free-floating or rogue planets" (Abstract).

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