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> All three stars are co-moving with similar chemistry, which implies they are bound ...

I am not an orbital mechanic, but the word "chemistry" is jarring to me here. Is it just a strained metaphor or is there some sense it which it fits? Maybe the romantic chemistry of mutually attracting bodies?

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I think they mean "chemical composition", e.g. how much hydrogen / helium / other elements there are in the star, deduced from their spectra

What could the chemical composition of the planets have to do with observations of their inertia? How could that matter other than by their mass?

I think the idea is that if they have the same chemical composition this is good evidence that they formed from the same primordial gas cloud. If they just happened to be passing by each other, there would be no reason for the composition to be so similar.

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