The interstellar object known as Oumuamua exhibits characteristics we cannot readily explain as natural. In particular, it displays acceleration not due to gravity. Normally this would be explained as cometary outgassing due to temperature increase during solar approach, but our telescopes showed no hint of that. One explanation for the observed acceleration would be a very thin, very reflective object that is tumbling or rotating; such an object would be effected by radiative pressure from the sun. The characteristics necessary for this to work out are an object 0.3mm thick with a 20m radius (as a perfect reflector). It would need to be larger than that if it is less reflective, but those dimensions are eyebrow-raising.
