Because Occam's Razor.

Absorption spectrums have never let us down, and are used all the time.

For this one place in the universe to have an exotic particle with an adsorption spectrum is pretty much impossible. To have an adsorption spectrum that is an exact mach for Barium is just adding a second impossible on top of the first.

Given the choice between something surprising and a doubly-impossible phenomena, the choice is clear.

> Absorption spectrums have never let us down

Devil's advocate: we have never had the chance to actually verify an exoplanet absorption spectrum. Could be that they are letting us down every time.

In this solar system, yes, absorption spectrums seem to be reliable. Anywhere else is extrapolation.

How many years until we verify? 100? 1,000? 10,000?

Sounds like something we could check empirically. Some cosmic rays are protons from outside of the solar system. We could capture those and compare them with local protons to see if they're actually the same kind of particle. (We can check if two particles are the same kind or not by seeing what happens if we try to put them in the same place. If they're both protons, they should refuse to be put into the same state due to the Pauli exclusion principle.) As far as I know the experiment hasn't actually been done, though.

Those cosmic rays are an absolute gold mine of particle physics potential. Some of them easily exceed any accelerator we could build without going solar system scale.

Edit-the best I can think is have every satellite built with a particle detector as a standard. No idea if those could be scaled down.

Edit2-seems doubtful.

So we know of two meteors that hit earth from outside the solar system. (One was IDed by the US DOD as traveling well in excess of the solar escape velocity.) They’re what Avi Loeb is bootstrapping a search for now. If those can be recovered all sorts of things can be tested and identified. It’s possible interstellar meteors are already in inventories and even studied but we don’t know what to look for to ID their source as such, yet.

But more directly, absorption spectra are so intimately tied to the QM of the molecules and elements involved that they’re probably unique. (Hunch/not seen a proof.) This is also the most experimentally verified theory known.

We also haven't verified the absorption spectrum of every atom on earth. All science is extrapolation.

To my knowledge, that's only confirmed for Earth, Moon, Mars and a couple nearby asteroids. The rest is just extrapolation of our limited data. P.S. Those razors would better be kept in the bathroom as they only give an illusion of knowledge.

Is your argument that translational symmetry in physics is wrong?

*cough* phosphine gas in Venus atmosphere

Atomic spectra are very complex, but not so complex as to cause confusions between them.

There is not a single absorption line, but a very large number of such lines, at different frequencies between which the relationship is not simple.

When several different absorption lines are measured and match the spectrum of a certain atom or ion, the chances of any other phenomenon causing correlated absorptions for all those different frequencies are negligible.

Molecular spectra are much more complex than atomic spectra and they have many closely spaced absorption lines, so there may be some confusions between different molecules when the observations are made in a limited frequency band, in which some molecules behave similarly, even if they would behave differently in other frequency bands that are not accessible for observations.

The conditions are exotic for a planetary atmosphere, trivial for a physics or chemistry lab. If such particles existed, they would have been discovered long ago.