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In this question about rogue planet/sub brown detection there are a couple of comments about classification of non-stellar, low mass, free floating objects. Part of one of the answers is -

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.

The question is, what observational differences would there be between these two classes of objects? What would show us that an observed object formed at the center of the contracting cloud and not out in the accretion disk?

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    $\begingroup$ The low mass tail probably has a minimum mass cutoff. $\endgroup$
    – eshaya
    Sep 26, 2023 at 14:45

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I think this is almost impossible for an individual object and in fact there may be almost no structural difference at all. Objects formed in a disk may form via "core accretion" around a solid core, or in some circumstances may form by direct collapse without a solid core. The latter would look just like low-mass objects that form like stars.

If you had a population of "rogue planets" then one diagnostic of the principal mechanism would be to look at their spatial distribution and kinematics.

Most rogue planets are inferred to be young, large and hot, otherwise they would not be observable. If they have been ejected from a dense star forming region in the recent past then it ought to be possible to trace them back to their birth site. If their reversed trajectory does not intercept something that was likely to have been a dense star forming region in the recent past, we might conclude that isolated formation is more likely.

The problem with this is that it is still possible for the object to form in relative isolation, then fall into a dense region, possibly be captured and then get ejected.

I suspect only very careful N-body modelling would tell us whether there would be a distinction in the spatial and kinematic properties of the two scenarios and I'm not aware of any.

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  • $\begingroup$ @planetmaker Check this out bbc.com/news/science-environment-66974738. This was published recently. I switched the accepted answer because these objects are most likely core collapse objects and we can tell the difference based on their location and relative motion inside the cluster. $\endgroup$
    – Futoque
    Oct 2, 2023 at 21:32
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For objects larger than roughly Jupiter that is very hard to impossible to do: their mass, density, size and atmospheric composition is (nearly) identical for both scenarios.

Even though the overall metallicity allows to distinguish the body from gravitational collaps (lower metallicity) from the one which grew through accretion to a size that it also could accrete gas (higher metallicity), the metal part of the gas will sediment to the core in both cases. The influence of the size of the small solid core is tiny on the gravitational and angular momentum and hard to distinguish - we have not been able to do so with Jupiter without large margins of error.

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    $\begingroup$ If one were to observe a very young object, there might be a metallicity difference in the atmosphere? Would it be a large difference? $\endgroup$
    – Futoque
    Sep 26, 2023 at 17:41
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    $\begingroup$ But note that objects formed in an accretion disk may also form by direct collapse without a solid core. That is why establishing whether Jupiter (which clearly formed in the protosolar disk) has a solid core is interesting. $\endgroup$
    – ProfRob
    Sep 26, 2023 at 18:55

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