Short summary: it's an entertaining but rather far fetched proposal in an arxiv preprint not published anywhere but mentioned in a Scientific American op ed. Implausible for many reasons including its spectrum which is not the shiny spectrum you'd expect from a solar sail but the red of tholins mixed with rock and metal as you'd expect from an asteroid / comet.
Oumuamua is the interstellar asteroid that passed Earth last year. Study of its orbit shows evidence of a small outwards push that gets less the further it is from the sun. You would expect that of an asteroid that is a kind of run down comet. This solar sail proposal is very far fetched. The paper works out that if it is a thin solar sail it would behave similarly to the way it does - an extra force acting on it that drops off as the square of its distance from the sun. This is the main basis on which they suggest it could be a solar sail.
Image to show the idea, NASA artist’s impression of a solar sail in use - I think exploring an orange dwarf, as our sun is white (despite its name “yellow dwarf” our sun is actually a white star). I found it in this article: Can a satellite hover over the Sun?
Faint outgassing from a nearly exhausted comet / asteroid would do the same thing. The evidence is strongly in favour of this because of the presence of organics red coloured like tholins, rock and metals. It doesn't seem too likely that a solar sail would have the spectrum of a typical asteroid from the outer solar system.
Surely a solar sail would be made of some shiny material to reflect as much sunlight as possible?
It also is tumbling end on end which would suggest it is not an active solar sail as that would both reduce the amount of push from the sunlight and make pointing instruments more complex for no good reason.
And although it flew fairly close to Earth - that doesn't mean much because it is only because it did that that we were able to detect it. There may be many of these fly through our solar system too far away to detect. There is an observer bias in effect here.
And if it was targeted to observe Earth it did a poor job of it. If we sent a solar sail to study an Earth-like world around another star it would skim as close above the cloud tops as our navigation of it permitted, to get really close up measurements, spectra and photographs. It didn't do this.
Though the authors touch on the idea of an active solar sail, their more likely solar sail hypothesis is a derelict one. But this would mean that the galaxy is full of trillions of derelict solar sails that are coincidentally coloured similarly to asteroids in the outer solar systems, to give a chance that one of them with those unlikely characteristics would fly so close to us.
Although an entertaining hypothesis it's not very plausible.
It is a paper uploaded to arXiv.org e-Print archive. It is not published anywhere yet. It has had a referee who did some minor corrections, but no real peer review, though its authors do have many previous papers to their name.
For some reason one of the authors was able to get a kind of op-ed article about this published in Scientific American. Perhaps it’s because they are a high reputation author. But it is just entertaining speculation. I am surprised SA ran it. It is more like the sort of thing I'd expect from New Scientist.
This is how it is covered in Universe Today with email from the researcher and an article in Scientific American. Very speculative. Suggests it may be a derelict solar sail from another civilization or possibly one sent to study our solar system
The paper is here
This is an article about trying to use a solar sail to study the asteroid, they decided we don't have the ability to do this yet.
They say it is metallic, rocky and with signature of tholins. Doesn't sound like a solar sail to me. At all.
Whatever it is, it seems just chance that this one passed close enough to Earth for us to spot it. Many are probably flying through our solar system every year at vast distances too far for us to see them.
We should discovery many morre. Once it is not just one object but one of several in a family of objects, we can study their properties, whether they have a common origin, get closer up images, and maybe send a probe to visit one of them.
We should be able to make a good start on finding them once we have the next generation sky search telescope LSST in the early 2020s which will scan the entire night sky every few nights with four times the reach and so, 64 times the search volume of our best sky survey telescopes to date. It should help detect things like this early on.
For more about this and the various hypotheses that have been developed to explain it, and even a suggestion of the star it may have come from, see also my: Could the interstellar asteroid Oumuamua be an alien probe?