Three years ago, a telescope at an observatory in Hawaii spotted an unusual and unprecedented object making its way through our solar system. It was given the name Oumuamua. That's Hawaiian for "scout." Some astronomers think Oumuamua was indeed a scout of sorts, perhaps an interstellar probe or vessel sent through our solar system by some type of intelligent alien life. The scientist making the most promising case for that explanation is Harvard theoretical physicist Avi Loeb. He's out with a book on Oumuamua called "Extraterrestrial: The First Sign Of Intelligent Life Beyond Earth." Loeb spoke with GBH All Things Considered host Arun Rath. This transcript has been edited for clarity.

Arun Rath: What was it that caught your attention about Oumuamua at first?

Avi Loeb: Astronomers first assumed that this was a rock ejected from the environment of another star and, judging by the rocks we have witnessed within the solar system, they expected it to be a comet or an asteroid. Now a comet is covered with ice, and when it gets close to the sun, you end up getting a cometary tail behind it of evaporated gases. But there was no cometary tail around this object, and so it was definitely not a comet. Then astronomers said, OK, then it's an asteroid, just bare rock without ice on its surface. The problem with that was that it exhibited an extra push away from the sun, the kind of thing you see from the rocket effect in a comet, but there was no cometary tail to give it that push. So what is the force acting on it? That was the puzzle. Actually, as it turns out, in September 2020 there was another object that was discovered that didn't show any cometary tail and also exhibited an extra push. That push came from sunlight bouncing off its surface. It turned out to be a rocket booster from a mission in 1966. That rocket booster, obviously, was made by us, artificially. So there is this interesting possibility that Oumuamua was also made artificially. The question is who made it and why?

Rath: So that old rocket, that empty booster, kind of turned into a solar sail.

Loeb: Yeah, because it was thin enough for the reflected sunlight to propel it to give it some push. In the case of Oumuamua, we calculated that it could potentially be a solar sail, a light sail. We are currently developing this technology where you push a sail just like the sail on a sailboat, but you push it with light by bouncing light off its surface. The advantage of that for space exploration is that the spacecraft does not need to carry the fuel with it.

Rath: Could that have happened by some natural means? Maybe there just happens to be an unusual hollow rock reflecting in a way that helps it work as a solar sail and it picks up its acceleration.

Loeb: That's an excellent question. And in fact, the mainstream astronomers that wanted to explore the possibility that Oumuamua was produced by a natural source suggested everal possibilities. One was a hydrogen iceberg, that when it evaporates, we can't see it because hydrogen is transparent. The problem with that is that the object would not survive the journey based on the evaporation that will result from absorbing starlight. Then another suggestion was that maybe it's a collection of dust particles, sort of a dust bunny that is being pushed by sunlight and is 100 times less dense than air. The problem, we thought, is that such an object would be very loosely bound and could get easily disrupted. I should say that any of the suggestions that were made for the object to come from a natural origin involve something that we have never seen before. My point is, if it's something that we have never seen before, then why not contemplate an artificial origin? When I walk down the beach, most of the time I see rocks or seashells that are naturally produced. But every now and then I do see a plastic bottle, and that indicates the existence of a civilization out there.

Rath: We have already had our closest encounter with this object. It's now speeding away from us, right?

Loeb: Yeah, it's moving away, it was moving away when we discovered it, and in fact, it would have been much better if we were to discover it in July 2017 rather than October 2017. In July, it was approaching us. And then we could have sent, in principle, a spacecraft that would take a close-up photo.

Rath: When we've been contemplating the possibility of extraterrestrial life, we've talked about things like the Fermi paradox, the idea that if there should be so many alien civilizations, why haven't we heard from them at this point? But I've also seen more recently discussions about the vast galactic scales that make it very difficult — so that even if there were other civilizations, it would be difficult to contact each other, even if we were existing at the same time. How would you place this object or the significance of this object in that context?

Loeb: Well, there are different types of communication. You can imagine radio communication, and for that, you need the civilization to be alive at the time that it transmits the signal. But if you imagine a letter in the mail like the post office delivers, the person that sends the letter might not be alive when the letter arrives at the destination. So civilizations that produce debris in space, space trash or probes like the Voyager 1, Voyager 2 or New Horizons, such things could go into space and stay there for millions or billions of years. In principle, you can fish them out long after the civilization died, and that is equivalent to doing space archeology.

Rath: Given the vastness of space, which most of us can't even comprehend, do you think that we have a good chance of finding another relic like this? Or could this have been our only chance?

Loeb: We should expect to find many more. We monitored the sky with the Panoramic Survey Telescope and Rapid Response System, at Haleakala Observatory, Hawaii, for the first time over a few years and discovered more and more. That means that if we aren't particularly lucky, if we are not privileged in terms of looking exactly at the right time in the right place, there should be many more out there. When I go to the kitchen and I find an ant, I usually get alarmed because there must be many more ants walking around if I find one small spot on the cabinet. The same is true here. We should find many more. That opens a window for figuring out what the nature of these objects is by taking more data in the future.

Rath: As a scientist, what would your feeling be about either confirmation of extraterrestrial life or, more excitingly, maybe first contact? Stephen Hawking made the case that if there is alien life, we wouldn't want to encounter it. What are your feelings?

Loeb: I think when you enter a room full of strangers, it would be prudent not to speak, but rather listen, because you never know what the risks are of speaking out loud. That's the approach we should take. But on the other hand, we should really listen rather than dismiss the possibility that there might be others out there. You know, when my daughters were infants, they tended to think that they were unique and special. But when they went to kindergarten, they found other kids that have qualities sometimes better than theirs. So the only way for our civilization to mature would be to find others. I think that the likelihood for that is very high, because we know that the Earth's solar system is not special in any way. About half of the sun-like stars have an Earth-sized planet, roughly at the same separation. That means that the chemistry of life as we know it could potentially take place on those planets. If you arrange for similar circumstances, you should get a similar outcome. It would be arrogant to think that we are special and unique. I advocate for modesty in this context.