Perhaps you heard about Oumuamua (don’t ask us how to pronounce that). The cigar-shaped object is the first item found by astronomers that is known to have come from outside the solar system and is continuing to pass through, not being captured by the sun’s gravitational field. A recent paper from [Shmuel Bialy] and [Abraham Loeb] from Harvard suggests that the thing could be a discarded light sail from an alien spacecraft.
Of course, it is fun to speculate that anything in space we don’t understand could be alien. However, the paper is doing more than just speculating. The rotation rate of the object suggests it is fairly flat (pancake-like, was the exact phrase used). In addition, it appears to experience “non-gravitational” acceleration — that is, it is accelerating due to some force other than gravity.
Others have suggested that the acceleration is due to material boiling off as the sun warms it. However, there’s no indication of that happening and activity like that ought to also change the spin rate which does not appear to be happening. Solar wind pressure could explain the changes, though. You might think that proves nothing since the solar wind pushes on everything. However, it is just like the wind in the atmosphere — sure it pushes on your car, your house, and a sailboat, but only the sailboat moves appreciably from it.
The paper argues that to produce the observed acceleration, the object would have to be very thin — on the order of 300 to 900 microns. Could such a thin structure survive a trip through interstellar space? The paper says yes, although it has to make some assumptions about the material involved. It doesn’t help that we are unable to get an actual image of the object as it whizzes by.
In all fairness, the paper doesn’t prove that the object is a solar sail. It simply proves that it could be. The reasoning is it could be a purposeful probe or just space debris. The purposeful probe seems unlikely because there were no radio emissions detected — at least down to the level of a common cell phone. Of course, that could be like primitive people seeing an airplane fly over and concluding it must be cold because there’s no smoke coming from a fire. However, space debris could be more likely. According to the paper:
Considering an artificial origin, one possibility is that ‘Oumuamua is a lightsail, floating in interstellar space as a debris from an advanced technological equipment. Lightsails with similar dimensions have been designed and constructed by our own civilization, including the IKAROS project and the Starshot Initiative. The lightsail technology might be abundantly used for transportation of cargos between planets or between stars. In the former case, dynamical ejection from a planetary System could result in space debris of equipment that is not operational anymore, and is floating at the characteristic speed of stars relative to each other in the Solar neighborhood. This would account for the various anomalies of ‘Oumuamua, such as the unusual geometry inferred from its lightcurve, its low thermal emission, suggesting high reflectivity, and its deviation from a Keplerian orbit without any sign of a cometary tail or spin-up torques.
Unfortunately, according to the paper, it is too late to get an image with any existing telescope (the pictures you see are artist’s conceptions) or to chase it down with chemical rockets. However, the paper’s conclusion calls for a search for similar objects, including some that were unlucky enough to get caught around Jupiter or the Sun. We could have an indication of an alien culture orbiting right next door and have simply failed to notice. (We’re not holding our breath.)
The solar wind played a part in keeping the Kepler telescope running — its solar panels acted as sails to prevent a failure from ending the mission. We don’t know if it was a solar sail passing through or not but we have to wonder, just like we wonder about FRBs.