Scientists have been puzzling over Oumuamua ever since the mysterious space object was observed tumbling past the sun in late 2017. Given its high speed and its unusual trajectory, the reddish, stadium-sized whatever-it-is had clearly come from outside our solar system. But its flattened, elongated shape and the way it accelerated on its way through the solar system set it apart from conventional asteroids and comets.

Now a pair of Harvard researchers are raising the possibility that Oumuamua is an alien spacecraft. As they say in a paper to be published Nov. 12 in The Astrophysical Journal Letters, the object “may be a fully operational probe sent intentionally to Earth vicinity by an alien civilization.”

The researchers aren’t claiming outright that aliens sent Oumuamua. But after a careful mathematical analysis of the way the interstellar object sped up as it shot past the sun, they say Oumuamua could be a spacecraft pushed through space by light falling on its surface — or, as they put it in the paper, a “lightsail of artificial origin.”