In 2017, the Pan-STARRS telescope in Hawaii spotted an interstellar object passing by Earth for the first time. Shortly after, Harvard’s Prof Avi Loeb was met with a backlash from the scientific community for suggesting it could be of alien origin. Now, several years on he has written a book, Extraterrestrial: The First Sign of Intelligent Life Beyond Earth, outlining why we still can’t out rule the possibility, and why scientists should always keep an open mind.


How do we know that 'Oumuamua wasn’t just a regular comet?

Astronomers suspected that it must be a comet because most of the objects in the Solar System are at the periphery have ice on the surface. The ice would warm up by the sunlight hitting it, and then it will end up in vapour and shrouded with dust. So, we end up with this cometary tail surrounding these icy rocks and most of the objects lost by other stars would come from the periphery because of passing stars tear them away from their parent star. So, the first suggestion was it must be a comet.

And the only problem was that it didn't look like a comet. It didn't have a cometary tail. There was no gas surrounding it. And in fact, the Spitzer Space Telescope looked very sensitively around it and couldn't find any carbon-based molecules or dust. So, it's not a comet.

What other theories have been proposed about its origin?

One, was that it may be a hydrogen iceberg, a chunk of frozen hydrogen. So, then the hydrogen evaporates from the just like a comet, but hydrogen is transparent so you can't see the cometary tail. That would explain why we don't see it. But I wrote the paper after that with my colleague showing that a hydrogen iceberg would evaporate very quickly as a result of absorbing starlight along its journey. And it's not at all clear that it can be produced in the first place in molecular clouds. We demonstrated that as well. So that doesn't seem likely. And then there was a suggestion that maybe it's a collection of dust particles held together in a loose configuration, just a very porous material, like a cloud of dust, 100 times less dense than air.

My issue with that is when it gets close to the Sun, like 'Oumuamua was, it would be heated by hundreds of degrees and a cloud of dust that is 100 times less dense than air would not have the material strength, in my view, to sustain this heating. And then the third possibility suggested was maybe it's very elongated because it's shrapnel, it's a piece of debris of something bigger that got disrupted when it passed close to a star. The problem with that scenario is that you end up with shrapnel, which is elongated usually because of the gravitational tidal force, and the object was most likely pancake shaped, not cigar shaped. Also, the chance of it coming close to a star is very small. So, I said to myself look, after two years, that's the best that the community could come up with as an alternative to it being artificial? In my view, it being artificial is more likely and there is no clear evidence that it's not artificial.

More like this

Read more about space:

So, what was it?

Now, the object was tumbling every eight hours, and showed variations in its brightness by a factor of 10 or more. Since we are looking at the reflected sunlight from the object this implied that the area it occupies in the sky varied by a factor of 10 as it was tumbling. Think about a piece of paper that is razor thin. The chance of you seeing it edge on is very small. The factor of change is quite extreme and it means that the object was at least 10 times longer than it was wide projected on the sky. And that's what led to this cartoon version of the object looking like a cigar, even though we didn't really have an image of it because it was too small for our telescopes to resolve.

But actually the best fit to the variation in the light was that of a pancake shaped object. So, it was a flat object. And then it exhibited an extra push away from the Sun, which could not be due to the evaporation of gas like you have in a rocket. So, the only way to explain it in my mind was that it was due to a reflection of sunlight. But for that to be effective, you needed the object to be very thin, sort of like a sail that you find on a boat where the wind pushes it, except here it's the sunlight pushing it. But nature doesn't make lightsails. They are artificial if real.

What is a lightsail?

So light is made of particles in principle that are called photons. And you can think of them as billiard balls bouncing and when they bounce off a mirror, they just give a little push. The idea of a light sail is to take advantage of that push. So, for example, you could make the sail sufficiently thin, such that the reflection of light gives it enough push to move it forward. And of course, sunlight is not very powerful, but in principle, if you have a very powerful laser beam, you can reach very high speeds.

Of course, in the case of 'Oumuamua it was not moving that fast and it's quite possible that it was completely dysfunctional because it was tumbling. And what would you expect from a piece of equipment floating in space for billions of years? Think about New Horizons, Voyager One and Voyager Two, when they become a billion years old. They would not be functional anymore. So, there should be a lot of trash in space that is not working anymore.

Were you surprised at the reaction the idea has received from the scientific community?

People are opposed to the mere idea of having the option of it being artificial on the table. That's in my mind strange. I left a seminar room where there was a talk about 'Oumuamua and a colleague of mine that has worked on rocks in the Solar System for decades said "‘Oumuamua is weird I wish it never existed". To me, it was appalling. How can scientists say that? Because, you know, when you're faced with anomalies that take you out of your comfort zone, it's actually a very good thing because it means that you're learning something new.

If you want to stay in your comfort zone, then just don't look, just stay ignorant. You can enjoy life. You can eat good food. You can speak with friends. Just ignore all the facts around you. Many people do that, by the way. But as a scientist, you have an obligation to follow the evidence and see where it leads you. And if you are denying that privilege, you are not true to your obligation. And that's the problem I have with the scientific community.

What would you like to happen next?

So, here is my point. Let's deploy cameras in around the orbit of the Earth, around the Sun. Lots of them, so that when the next interstellar object is spotted, one of the cameras will be close enough to take a close-up photo. And that's what I really want. When I go to the kitchen and find an ant, I get alarmed because I know that there should be many more ants out there. The same should be true about 'Oumuamua. We found one after a few years of surveying the sky with pan-STARRS. If we continue to survey a few more years, we'll find another one.


And then there is the Vera Rubin observatory [currently being built in Chile] that will come into play in less than three years. It will have much greater sensitivity and could find an 'Oumuamua-type object every month. So, we will have a lot of opportunities to check if I'm right or wrong. I don't understand why, even if you're conservative and you say it's never aliens. Why have a prejudice to start with? Let's just let's just take a photograph. Maybe one out of a few would be a plastic bottle on the beach. You know, most of the time on the beach we find the rock and every now and then a plastic bottle that tells us there is a civilisation out there.


Jason Goodyer
Jason GoodyerCommissioning editor, BBC Science Focus

Jason is the commissioning editor for BBC Science Focus. He holds an MSc in physics and was named Section Editor of the Year by the British Society of Magazine Editors in 2019. He has been reporting on science and technology for more than a decade. During this time, he's walked the tunnels of the Large Hadron Collider, watched Stephen Hawking deliver his Reith Lecture on Black Holes and reported on everything from simulation universes to dancing cockatoos. He looks after the magazine’s and website’s news sections and makes regular appearances on the Instant Genius Podcast.