@snder it doesn't say how they know.. Why nooot..
Link is in there https://www.pnas.org/content/early/2020/01/07/1904573117 Turns out it's not ratio of isotopes & decay rates, it's via transmutation to Neon-21 by cosmic rays. Suppose Neon is a gas, but trapped in the Silicon carbide(moissanite) so you measure cumulative exposure to a certain types of radiation after it formed as a grain.
@snder Apparently they figured it was part of a larger aggregate to. From the abstract, i think they mean He-3 gets "kicked further", i.e. it only gets re-absorbed into the material if the He-3 has to travel far enough through the material. So if it was in a bigger object before, there is more He-3. See Lithium is another element they used. (also grains were previously thought to be from outside the solar system due to isotope ratios not matching ours)
Also cosmic radiation might vary though..
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