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The April 2, 2023 Inverse article 9 Years Ago, Astronomers Found Two Rogue Planets — But They Didn’t Realize It Until Now says:

Planets are typically part of a planetary system and are gravitationally bound to their star, or stars in the case of a binary star. Planets can migrate toward and away from their star when conditions are right, but they stay bound to the star, even if separated by a vast distance. But sometimes, a planet is kicked out of its system due to a supernova explosion, a stellar interloper, or some other event. And in some cases, a planet can form on its own outside of any solar system. These are rogue planets or free-floating planets (FFPs.)

Question: What are the cases in which free floating planets (FFP's) can form on their own, outside of any solar system?

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    $\begingroup$ Probably talking about Sub brown dwarf There is, it seems some disagreement about nomenclature, with some calling such objects "rogue planets" and others not. $\endgroup$
    – James K
    Commented Apr 10, 2023 at 8:56

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There is no clear separation in mass between planets found orbiting stars and "free-floating" planetary-mass objects (although inferring the mass of the latter from their observable properties is highly age- and model-dependent).

Some "free-floating planets" may have been ejected from their birth systems. This is a hazard that is theoretically predicted in any dense, clustered phase of the star forming environment. Others may have been born as isolated objects and simply represent the low-mass tail of the initial mass function.

Note, that I would be one of those against labelling something as a "planet" just because its mass is below the somewhat arbitrary threshold of 13 Jupiter masses (the deuterium-burning threshold). I think a low-mass lump of gas that contracts in isolation is just a low-mass brown dwarf. Others disagree.

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  • $\begingroup$ Dust is also found in interstellar space. Thus an object forming in interstellar space might contain a lot of dust and solid matter, which might make it more like a planet than a mass of gas. Any partially solid object in interstellar space which is massive enough to become a spheroid through its gravity is a planetary mass object or planemo. So it seems perfectly proper to speak of rogue planemos in inteterstellar space. $\endgroup$
    – M. A. Golding
    Commented Apr 11, 2023 at 17:40
  • $\begingroup$ @M.A.Golding Dust is 1% of the mass in interstellar gas. A planetary-mass object forming in isolation would have a minimal solid content and could not form via the "core-accretion" scenario thought to be responsible for the giant planets in our Solar System. Another reason not to call such objects planets. $\endgroup$
    – ProfRob
    Commented Apr 11, 2023 at 19:41

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