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In this Wikipedia article, it details about a T9 class brown dwarf named WISEPA J174124.26+255319.5.

Apparently, this brown dwarf has 0.95 Jupiter masses, and 0.4 Jupiter radii (with rather low error). Its mass is already weird in the first place, clearly violating the 13 Jupiter mass boundary massively, and furthermore I calculated based on these measurements that it apparently has a density about twice that of earth's.

For something made majorly of hydrogen and helium, I find this extremely, extremely odd.

What is going on with this brown dwarf? Is this a misclassification, or am I just missing something?

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    $\begingroup$ Also it has the temperature of 474 Celsius, just slightly hotter than Venus. So it may be classified as Ultra-cool brown dwarf. $\endgroup$
    – user47732
    Jan 9 at 6:20
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    $\begingroup$ The paper arxiv.org/pdf/2105.05256.pdf $\endgroup$
    – James K
    Jan 9 at 6:20
  • $\begingroup$ The wikipedia article in question has been modified thanks to this Q&A. The mass, radius, and age estimates were removed shortly after @ProfRob provided his answer. The talk page links to ProfRob's answer. $\endgroup$
    – David Hammen
    Jan 13 at 12:47

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I had a look at the original source of the measurements quoted on the Wikipedia page (Zhang et al. 2021) and the problem is simply systematic errors in the models that are being used to infer the fundamental parameters. I.e, the masses and radii (and ages) of the brown dwarfs in that paper are systematically in error, which is a clear conclusion of the paper.

The paper is about modelling the infrared spectra and trying to determine the parameters of the brown dwarfs using a suite of theoretical models generated over a grid of surface gravities, effective temperatures and metallicities. The radii basically come from the "luminosity" of the spectrum combined with an effective temperature (Stefan's law) and the mass then arises from the surface gravity and radius.

The conclusion of the paper, which comes from considering the properties of a population of late T-dwarfs that were modelled in this way, is that the radii are systematically underestimated and that the masses are "unphysically" small - thanks to the underestimated radius and an underestimated gravity. I.e., they should not be used or quoted as the masses and radii of these objects. The problem is almost certainly the assumptions that go into the atmospheric models being used to fit the brown dwarf spectra.

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    $\begingroup$ Ah that makes much more sense. Turns out wikipedia was just grabbing form the paper in a misleading way. Thank you for clearing this up! :D $\endgroup$
    – Max0815
    Jan 9 at 12:18
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    $\begingroup$ If people can make such mistakes (and they do), imagine the chaos ChatGBT could cause if it was allowed to update wikipedia articles. $\endgroup$
    – David Hammen
    Jan 13 at 12:49

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