The first interstellar object, `Oumuamua, was discovered in 2017 as it passed near Earth in its trajectory around the Sun. The trajectory exhibited an anomalous non-gravitational acceleration away from the Sun with no sign of cometary evaporation.

Given the lack of cometary evaporation and the favored disk geometry, the non-gravitational acceleration could have been produced by radiation pressure from sunlight on the disk, as suggested in the paper I wrote in 2018 with my former postdoc, Shmuel Bialy. In that case, the measured non-gravitational acceleration requires a large surface area to mass ratio for Oumuamua, translating to a thickness of an order of one millimeter at solid density. This thickness is one part in 100,000 of the estimated radius of Oumuamua.

This raises the possibility that Oumuamua’s disk was actually a thin solid layer that was manufactured technologically, since we are not aware of an astrophysical process that would produce an umbrella-shaped structure of these extreme dimensions. If artificial in origin, Oumuamua could have been either a light sail, or a tough surface layer that was torn apart from a spacecraft. Another possibility is that it could have been a broken piece of a Dyson sphere, an idea I put forward in a recent paper.