(NEXSTAR) — In a new interview, Harvard physicist Avi Loeb says there’s a “serious possibility” that an interstellar object that passed through Earth’ solar system could be an alien spacecraft.

Scientists, who discovered the object in 2017, call the mysterious entity ‘Oumuamua. The scientific consensus determined ‘Oumuamua to be of natural origin, but Loeb disagrees. In his new book “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth,” he argues that the interstellar mystery object was likely similar to a light sail — a method of spacecraft propulsion — created by extraterrestrials.

In a new interview with Salon, Loeb defended his hypothesis, asserting that “perhaps ‘Oumuamua was artificial, definitely not made by us.”

“I would say, given everything we know, I would give a high likelihood that it could have been artificially made,” he said.

The trouble is the only way to know for certain ‘Oumuamua’s origins is by taking an image of it, but the object is already too far away.

“So we missed the opportunity,” he said. “It’s like having a guest for dinner, by the time you realize it’s weird, it’s already out of the front door into the dark street. That was the first guest, and we should look for more.”

Loeb goes on to stress the importance of not resting on one’s laurels.

“The way to make progress is not to stick to your notions and maintain a prejudice,” he said. “… It’s like stepping on the grass and saying look it doesn’t grow. Science is not about that, science is about finding the truth.”