Fermi's paradox demonstrates it's almost certainly not a light sail. For it to be a light sail which was observed by chance by our totally unprepared observatories after a window of less than a century but also consistent with all our observations of the universe, we would need to hypothesize that we got extraordinarily lucky to observe a super-rare artifact, or that countless (or one vast) interstellar, pan-galactic civilization throwing out light sails in every direction constantly which also collectively choosing for millions/billions of years to avoid any kind of stellar engineering or direct colonization or Dyson sphere-like artifacts or even atmosphere composition tweaks visible on exoplanet spectrographs all carefully preserving the illusion of a 100% dead cosmos in every inch of our Hubble volume, or that alternately, interstellar civilizations are super common and in fact routinely throw out solar sails all the time, but some sort of interstellar-capable Great Filter kills them almost instantly before they can have any visible effects, leaving just a bunch of spreading artifacts. (And this Great Filter can't be any of the usual ones like asteroids or nukes or pandemic or development of intelligence.)

Or it's a small piece of a larger, rarer piece of equipment that broke up. Or something we don't really have the imagination for. It's far too early to make absolute statements about what is and isn't possible wrt ETI.

Or that this was a relatively nearby civilization, and the light sail was intentionally headed toward our solar system (like a few of our proposed interstellar missions to Alpha Centauri).

That falls under the 'we got ludicrously lucky' category: in the entire galaxy of billions of stars over billions of years we just happened to arise exactly simultaneous with the other civilization in time to see it? (Also, if it was coming from a nearby solar system, why did it pass right through, do nothing, and is at rest with respect to the galaxy? I also think if its trajectory led straight to a nearby star system with any potential for life, someone would've noted that.)

It could be a weapon, but they missed

> even atmosphere composition tweaks visible on exoplanet spectrographs

I'm fairly certain we can't detect that yet even if they had.

Exoplanet atmospheres can be studied by the transit spectroscopy method. This is how water has been detected, for example. Still no oxygen or any other signatures of life. Lots of hostile atmospheres, though: https://en.wikipedia.org/wiki/Extraterrestrial_atmosphere#Ex...

I don’t think we could see a rocky planet like earth though.

All that tells us is that it wasn’t observed by chance.

thats not true. if the probability of whether or not its a sail could be calculated, it would have been in the paper ;)

It probably is true. And it might surprise you, but papers leave things out all the time... Even things that can be calculated. Shocking, I know. And that wouldn't be in the paper anyway because the Fermi Paradox & Great Filter is not hard science but still half philosophy.