Evidence for ET?

[Paleophyte]

That is the question.

Impact with another object is ridiculously improbable and would have produced debris. It's also been accelerating over a fair bit of time rather than a one-off bump.

Outgassing of volatiles wasn't observed. It may be too distant to make out the gasses, though that seems unlikely.

Both of the above should have altered the rate of its tumble, but that wasn't observed either.

It's possible that it's a light sail or fragment of a light sail, though the reddish colour makes that unlikely. The better answer is our old stand-by, "We don't know." It has made Oumuamua a bit more interesting though. Regardless of what it is, it's unlikely that our first detection of a large extra-solar object is wildy anomalous, so we'll likely be visiting something similar with a probe in the next decade or two.

Thrust of outgassing would not appreciably effect the rate of tumble if the thrust vector directs through the elongated object near the middle.  Since the expelled gas is not attached to the object, the departure of the gas would also have no effect on the angular momentum of the remaining mass of the object.

It would be interesting to determine how much density and optical depth would the plume of expelled gas need to have to be detectable from earth, and how large must the plume be to be resolvable separately from object itself as a coma.   Couple that with reasonable estimate of the velocity of the outgas would put a constaint on the amount of material that can be outgased without being detected from earth.  That in turn would provide a constraint on the maximum amount of orbital energy change outgassing can cause without the outgas being detectable from earth.

A solar sail able to appreciably affect the orbit of a solid object of reasonable density a good fraction of a km long would be enormous and at least severa Times larger in dimension than the object itself.   Unless the sail is a near perfect, non-scattering mirror reflector from IR to UV, we would have been able to detect the sail more easily than the object itself???

The thrust vector is unlikely to be that close to axial all the time. There should have been some evolution of the tumble if outgassing had been significant enough to produce linear acceleration.

I suspect the answer to how much gas could be expelled without being detected gets complicated pretty quickly. I'll vary considerably depending on the molecules involved and their tendency to fluoresce. Even if the gas weren't directly visible from earth as a coma or tail its presence should have modified the spectral absorption lines. That wasn't observed either.

We don't actually know the size of the object. It's just a point of light in our most powerful telescopes. The dimensions were inferred from the light curve, distance, and some assumptions about the albedo and density. Assuming an albedo of about 0.1, similar to outer solar system objects, you get an asteroid ~1000 m long and ~200 m wide. If it's a light sail then you'd hope it's designed with a much higher albedo (~1) and likely isn't pushing anything other than its own miniscule mass. That gives you dimensions closer to ~300 m by ~80 m and a tenth of a millimeter thick. That implies that it's folded up a bit too, possibly damaged or discarded. If it were a flat sheet then the light curve would plunge to near zero as it tumbled edge on to us.

I don't think that it is a light sail. The colour and shape are wrong for a start. More likely some mundane explanation. Still, I'd very much like to go and check it out.

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Here's the original paper (PDF Download) along with a less technical article in Scientific American. And here's a rebuttal.

TL;DR Oumuamua is weird and we don't know why.