On October 19, 2017, astronomers at the University of Hawaii spotted a strange object travelling through our solar system, which they later described as “a red and extremely elongated asteroid.” It was the first interstellar object to be detected within our solar system; the scientists named it ‘Oumuamua, the Hawaiian word for a scout or messenger. The following October, Avi Loeb, the chair of Harvard’s astronomy department, co-wrote a paper (with a Harvard postdoctoral fellow, Shmuel Bialy) that examined ‘Oumuamua’s “peculiar acceleration” and suggested that the object “may be a fully operational probe sent intentionally to Earth’s vicinity by an alien civilization.” Loeb has long been interested in the search for extraterrestrial life, and he recently made further headlines by suggesting that we might communicate with the civilization that sent the probe. “If these beings are peaceful, we could learn a lot from them,” he told Der Spiegel.

I recently spoke by phone with Loeb, who was frustrated that scientists saw ‘Oumuamua too late in its journey to photograph the object. “My motivation for writing the paper is to alert the community to pay a lot more attention to the next visitor,” he told me. During our conversation, which has been edited and condensed for clarity, we discussed why Loeb thinks we need to consider the possibility that ‘Oumuamua was sent by aliens, the dangers of unscientific speculation, and what belief in an advanced extraterrestrial civilization has in common with faith in God.

Your explanation of why ‘Oumuamua might be an interstellar probe may be hard for laypeople to understand. Why might this be the case, beyond the fact that lots of things are possible?

There is a Scientific American article I wrote where I summarized six strange facts about ‘Oumuamua. The first one is that we didn’t expect this object to exist in the first place. We see the solar system and we can calculate at what rate it ejected rocks during its history. And if we assume all planetary systems around other stars are doing the same thing, we can figure out what the population of interstellar objects should be. That calculation results in a lot of possibilities, but the range is much less than needed to explain the discovery of ‘Oumuamua.

There is another peculiar fact about this object. When you look at all the stars in the vicinity of the sun, they move relative to the sun, the sun moves relative to them, but only one in five hundred stars in that frame is moving as slow as ‘Oumuamua. You would expect that most rocks would move roughly at the speed of the star they came from. If this object came from another star, that star would have to be very special.

What are some of the other strange facts?

When it was discovered, we realized it spins every eight hours, and its brightness changed by at least a factor of ten. The fact that its brightness varies by a factor of ten as it spins means that it is at least ten times longer than it is wide. We don’t have a photo, but, in all the artists’ illustrations that you have seen on the Web, it looks like a cigar. That’s one possibility. But it’s also possible that it’s a pancake-like geometry, and, in fact, that is favored.

What would be the meaning of a pancake-like geometry—

Wait. The most unusual fact about it is that it deviates from an orbit that is shaped purely by the gravitational force of the sun. Usually, in the case of comets, such a deviation is caused by the evaporation of ice on the surface of the comet, creating gases that push the comet, like the rocket effect. That’s what comets show: a cometary tail of evaporated gas. We don’t see a cometary tail here, but, nevertheless, we see a deviation from the expected orbit. And that is the thing that triggered the paper. Once I realized that the object is moving differently than expected, then the question is what gives it the extra push. And, by the way, after our paper appeared, another paper came out with analysis that showed very tight limits on any carbon-based molecules in the vicinity of this object.

What is the significance of that?

It means that there is no evidence of gas that relates to the evaporation of ice. We don’t see the telltale signatures of cometary tail. Moreover, if it was cometary activity, then we would expect the spin period of this object to change, and we don’t see that. All of these things are indicative of the fact that it is nothing like a comet that we have seen before in the solar system. And it is also nothing like an asteroid. Its brightness varies by a factor of ten, and the maximum you typically observe is a factor of three. It has a much more extreme geometry, and there is some other force pushing it. The question is, what’s providing this force, and that was the trigger for our paper.

The only thing that came to my mind is that maybe the light from the sun, as it bounces off its surface, gives it an extra push. It’s just like a wind bouncing off a sail on a sailboat. So we checked that and found that you need the thickness of the object to be less than a millimetre in order for that to work. If it is indeed less than a millimetre thick, if it is pushed by the sunlight, then it is maybe a light sail, and I could not think of any natural process that would make a light sail. It is much more likely that it is being made by artificial means, by a technological civilization.

I should say, just as background, I do not view the possibility of a technological civilization as speculative, for two reasons. The first is that we exist. And the second is that at least a quarter of the stars in the Milky Way galaxy have a planet like Earth, with surface conditions that are very similar to Earth, and the chemistry of life as we know it could develop. If you roll the dice so many times, and there are tens of billions of stars in the Milky Way, it is quite likely we are not alone.

So this civilization would be out of the solar system and in the galaxy?

In the galaxy. It may be dead by now, because we don’t take good care of our planet. Imagine another history, in which the Nazis have a nuclear weapon and the Second World War ends differently. You can imagine a civilization that develops technology like that, which would lead to its own destruction.

It’s possible that the civilization is not alive anymore, but it did send out a spacecraft. We ourselves sent out Voyager I and Voyager II. There could be a lot of equipment out there. The point is that this is the very first object we found from outside the solar system. It is very similar to when I walk on the beach with my daughter and look at the seashells that are swept ashore. Every now and then we find an object of artificial origin. And this could be a message in a bottle, and we should be open-minded. So we put this sentence in the paper.