One Alien’s Trash is Another Alien’s Treasure
On the scientific debate around the first observed interstellar guest
Fair warning: lots of technical content in this post. I have provided an abundance of hyperlinks that might provide some useful background.
1. SETI: The Search for Extraterrestrial Intelligence
If you want to try to find alien life in the universe (I would advise against this as a career choice because it’s very hard and not super rewarding) you have a number of options.
Aliens might broadcast signals on the electromagnetic spectrum. You could try to listen for radio signals with large telescopes. Despite being the dominant strategy, this has turned up basically nothing in the last 60 years or so. Aliens might also broadcast optical signals with laser pulses. You could scan the sky for these, as well.
Alien life might be producing particular gas molecules or other biosignatures at concentrations that are unlikely to be found naturally, either from living processes or through industrial production. You could look at the spectra of exoplanets with a different kind of very powerful telescope. This will likely yield a lot of false positives, though, since many molecules that life can produce might be produced through natural means as well. Also, the spectroscopy of exoplanets is incredibly sensitive and challenging.
Aliens might heavily modify their star systems by constructing megastructures, building Dyson spheres around stars, or otherwise doing weird things to them that we can't imagine. If an alien civilization were harvesting large amounts of energy from a star, this would be observable in the star’s spectrum, and you might be able to see this with a telescope. There’s one star that’s famously kind of weird, but it seems more likely that out of the very large numbers of stars we can currently observe, there would be some weird ones by chance, due to confusing dust clouds or debris fields. There have been some fairly convincing natural explanations for this star’s flickering, in any case.
Alien civilizations might produce debris in space. We’ve already produced a decent amount of space debris ourselves (and sent a few objects on trajectories leading out of our solar system). It’s therefore quite reasonable to assume that space-faring aliens might produce large quantities of space trash. You could look for these entering our solar system as interstellar objects with yet another kind of very powerful telescope.
You could try to reach out to any listening aliens by loudly broadcasting our own messages. Ultimately, it would take a really long time to hear back.
And of course… Some think that aliens might visit our planet. You could look for aliens living among us, flying around in saucers, and abducting people. I am incredibly skeptical that this has happened. The vast majority of scientists agree with me there. I’m not gonna get into it, but I’m sure you can find other people who have written about this.
For this article, I’m going to focus on one of these bullet points: observing alien space trash.
2. The Trash We Expect
People often criticize the search for extraterrestrial intelligence for being non-falsifiable. First you look for radio messages. You don’t find any after a couple years. So you look harder, with more powerful telescopes, and you scan more frequently, and you still don’t hear anything. Is this evidence that there is no extraterrestrial intelligence? Well, not necessarily. It could be that aliens just don’t really use radio to communicate anymore, or that there are so few alien civilizations that we wouldn’t expect to hear their radio signals by chance, or that they’ve just disguised their messages so well that they’re indistinguishable from noise, or that they’re just careful to block all the radio messages that would happen to go to other star systems, or… you get the idea.
However, there are similar problems in a lot of open-search questions in physics. For example, dark matter, supersymmetry, string theory, proton decay: all these fields have analogous problems where you don’t really know how to find something, so you keep trying to look in different parts of the phase space where you expect it might be hiding. Notably, this sometimes works. The Higgs Boson involved a similar search. So did gravitational waves, which were found after years of researchers coming up empty-handed!
By their very nature, big open questions such as the nature of consciousness, the origin of life, the unification of physical laws, and SETI are challenging, long-term endeavors. Researchers need to get better at making their theories testable and falsifiable, and to their credit, I think many do. And when it comes to alien trash, the search should be quite testable. You could imagine that once you’ve observed, say, a few dozen interstellar objects and they all have been found to have benign explanations, you might start to look elsewhere. Of course, there’s still a receding horizon of possibilities, but you can bring down the upper limits to the size and density of space junk in the solar system with repeated observations.
So let’s do our best to think about what we might expect this search to look like. How might we differentiate alien trash from normal rocks?
Well, like all trash, alien space trash would likely fragment into small pieces. It’s reasonable to assume that there are vastly larger amounts of tiny fragments of space trash than there are more massive objects. Also, if there was a lot of interstellar trash passing through our solar system larger than about 1 km in size, it’s likely we would have already already observed it with our current telescopes. The fact that we haven’t probably places an upper bound on the size of space trash that we might expect to find.
How might we distinguish alien-produced trash from naturally-produced junk? Well, we might theorize about what this trash would be made of. As humans, we have a large number of materials that we make things out of, but it seems like the presence of, for example, metal alloys not found in nature would be good evidence that what we’re seeing is space trash and not just a rock. So if we observe a 10 meter-long chunk of titanium that has hexagonal patterns on the side, I think we can be fairly confident that there was once extraterrestrial life somewhere in our galaxy. But it will probably not be that easy at first. From our telescopes, we will see a speck of light.
3. Scout
As I hinted in my last blog post, this one would be about ‘Oumuamua, the first observed interstellar visitor, discovered in 2017 by the Pan-STARRS observatory in Hawaii. Its name means “scout”.
It’s actually not difficult to tell that an observed object is interstellar. Simply put, it is traveling too fast to be bound by the sun’s gravity. By the time we observed it, unfortunately, it was traveling away from us, going fast enough to escape our solar system. There were a number of very strange properties (I am borrowing this framing mechanism) of ‘Oumuamua.
Weird thing #1: It has a very large aspect ratio, of at least 5:1, but possibly higher. This is (as far as my search of the literature could determine) a larger aspect ratio than any natural object observed in our solar system. Scientists know this because it’s tumbling about an axis such that the maximum and minimum brightness of the speck that we see changes by a certain factor with a periodicity of about eight hours. Scientists have subjected this light curve to various models to see which size and shape of objects would conform best. The best fits are either a cigar-shaped object with an aspect ratio of at least 8:1, or a flat disc with an aspect ratio of at least 6:1. However, it could be even more flat! Imagine holding a dinner plate in front of you. Now, rotate it in your hands while looking at it. If it’s perfectly aligned to your eyes, there will be a moment when it appears very thin, almost a single line. However, if it’s rotating about a slightly different axis, it will never appear as a single flat line from your perspective. Since it’s unlikely that the rotational axis of ‘Oumuamua is perfectly aligned with our vantage point from Earth, we don’t actually have an upper bound for how flat or elongated it is. It could be very flat or elongated.
Weird thing #2: It’s dramatically shinier than any observed objects from our solar system. We didn’t detect heat in the form of infrared radiation from its surface, which sets an upper limit on its mass, and therefore lets us determine the minimum reflectivity of its surface. It has an abnormally high albedo.
Weird thing #3: ‘Oumuamua accelerated away from the sun (this is normal) without leaving behind an observable cometary tail (this is surprising). For a typical comet, the observed acceleration would have required a thrust generated by a considerable amount of mass leaving the object that would have been easily visible from Earth.
Weird thing #4: The sun, like many of the stars in our neighborhood, is moving relative to the stars around it and to the center of the Milky Way galaxy. ‘Oumuamua, before it encountered our sun, had a unique frame of reference in that it was at rest relative to the average motion of stars in our neighborhood of the galaxy, the so-called “local standard of rest.” The interaction of our solar system with ‘Oumuamua has been analogized to a buoy sitting at rest in an ocean with our solar system playing the part of a ship speeding by, dragging the buoy towards our ship and then away from it, from our own frame of reference.
Any of these properties—as well as several other unique features that are perhaps less explicable in a blog post due to being very astrophysics-heavy—would be surprising on their own, but together, they pose an interesting puzzle.
Physicist Avi Loeb, who chaired the astronomy department at Harvard University for a decade, has proposed an explanation to that puzzle. He believes that it is plausible, and perhaps even likely, that ‘Oumuamua is a piece of alien space junk. An object like a solar sail would neatly fit all the observed characteristics of ‘Oumuamua to a tee. It’s very flat, it’s shiny, it would accelerate due to solar radiation pressure (this is how light sails work), and it would make sense for an object of intentional design to be at the local standard of rest.
I’ve read his papers and arguments, and I’ve read the papers and arguments of his supporters and detractors. Ultimately, it’s a sophisticated and complex scientific puzzle on a number of levels, mixing together astrophysical observations, anthropic logic, the Fermi paradox, and competing theories about the formations of solar systems. So how can someone who’s not a top-10 astrophysicist in the world begin to evaluate the epistemics of the debate?
4. Claims and Evaluations
Loeb has muddied his reputation by doubling down. It’s considered uncouth for serious physicists to talk about aliens. And of course, extraordinary claims should require extraordinary evidence. But Loeb, for his part, has delivered.
He is unafraid of his theory being falsified; he has called loudly and repeatedly for the international scientific community to direct their resources towards looking for more ‘Oumuamua-like objects (if we saw one with Pan-STARRS so soon, it’s likely that with further effort, we can find more objects of a similar nature). He has expressed impatience for future sky surveys to begin, rather than fear that they’ll invalidate his ideas.
He has also called for a rendezvous mission to chase down ‘Oumuamua to inspect it.
He has put out pre-print after pre-print defending his ideas with cold, hard math, from modeling the reflectivity of a rotating disc to ruling out the emission of various gases from ‘Oumuamua based on spectroscopic results.
Loeb is also very much not a crackpot. He has published works debunking UFO claims. He continues to publish prolifically on many other astrophysics topics that are well received. His works are often profound, clearly reasoned, and insightful. That is to say, there’s a reason he’s a physics professor at Harvard. However, he doesn’t shy away from speculative research. He is interested in getting at root of the Fermi Paradox and thinks that physicists should be more engaged in the search for extraterrestrial life.
However, there are also reasons to doubt Loeb.
Since ‘Oumuamua, Loeb has done more controversial work, including a search for fragments of another interstellar object that may have impacted Earth (depending on partially-classified trajectory data from the US Defense Department). This research I am quite skeptical of. Loeb for his part seems to be treating this more recent work as a funding opportunity. For what it’s worth, he appears to be in agreement with some of his detractors that it’s a bit of a wild goose chase.
There’s a big spotlight effect for interesting astronomical findings, and Loeb might just be fixating on the one observation of particularly high weirdness. You could imagine that “the first interstellar object” is one of dozens of astronomical observations that merits interest to SETI (first observed objects in different size ranges, first directly-observed black hole, first observed exoplanet, etc), and one of these is bound to have a weirdness level a couple standard deviations higher than you’d expect.
While some other “serious physicists” back Loeb, many very much do not. I’ve personally spoken to a couple physics PhD students who seem at least a little peeved about Loeb’s work.
Unfortunately, those opposed to Loeb’s theories on ‘Oumuamua have not had great results in presenting a compelling alternative. Several early explanations for an object matching ‘Oumuamua’s properties were rapidly debunked by consensus.
The leading alternative today is that ‘Oumuamua is a chunk of ice, cleaved off an exoplanet or expelled from a forming solar system. Due to its observed properties, it would need to be made from nitrogen, hydrogen, or water.
In particular, the radiolytic production of hydrogen from a water-ice body could explain its acceleration. This work, unlike that of Loeb, was rapidly published in Nature, where colleagues quickly patted the authors on the back. However, Nature was just as quickly forced to publish a comment to the piece raising serious criticism of their calculations. The original authors replied to this comment.
I read this back-and-forth when it came out, and I’m fairly perplexed. In general, it feels like it should be appropriate for a double standard to be applied in extreme cases such as this, where one side (Loeb) is claiming “Aliens”... and the other side is claiming something banal, like “radiolytic hydrogen production from water-ice”.
On the other hand, Loeb has written dozens of journal articles defending himself in a fairly consistent manner, whereas his opponents have a theory that seems to be built on a framework of eggshells. Now, “winning a debate” is very much not the same thing as “being right”, but Loeb has unquestionably “won the debate,” in that his opponents have not spent a lot of time presenting a serious alternative, while Loeb has been extremely diligent in defending every angle of his theory.
5. Time to Pull Out the Bayes Calculator
Bayesian inference is a great tool for evaluating competing hypotheses, but unfortunately it can only help us a little. I tried to give a fairly conservative estimate of the various relevant pieces of information that have come to light, while also trying very hard to avoid the “multiple stage fallacy”. However, the conclusion is highly dependent on the prior I give on the possibility that there are alien technosignatures in our galaxy to be found. I need to put myself into my mental state before I knew that ‘Oumuamua existed.
The prior I’m interested in, specifically, is “what are the odds that when we find the first object that has entered our solar system from interstellar space, it is a technosignature versus an ice chunk (or another object of natural origin)?”
If I think there’s, say, a 3% chance that the first interstellar object we observe might be a piece of space trash, then I should believe after seeing ‘Oumuamua that it’s very likely to be one. However, if I think there’s only a 0.01% chance of such a thing, then even after seeing ‘Oumuamua, I should be skeptical that it’s anything but a weird chunk of ice.
I think 0.01% is about as low as one should reasonably go. Something like 80% of astrophysicists/astrobiologists think that alien life probably exists somewhere in the universe. Even if we take a fairly conservative estimate of the likelihood of intelligent life in our galaxy (say 1%), the conditional odds that intelligent life would have generated some meaningful density of space trash is probably somewhere north of 5%. Let’s say I’m about 20% confident that we’ll see the space trash before we see a ton of other natural objects flying around the interstellar medium (which we had not previously observed due presumably to a very low base rate on that sort of thing). That gets us to a prior of 0.01%, which I’m very hesitant to go below.
I also think somewhere around 3% is about as high as one should reasonably go. That would imply aggressive estimates of 50% (intelligent life in our galaxy), 10% (they make a decent amount of space trash) and 60% (we see that trash before seeing natural objects).
Well, depending on my prior, I should get somewhere between 1% and 80% likelihood that Loeb’s theory is correct that ‘Oumuamua is the first observed alien technosignature. If you put an alien death ray to my head and forced me to make a forecast, I’d say… 10%. So, Loeb is probably wrong. But the chance he’s right is definitely high enough that the search for interstellar objects should be consuming a lot more of humanity’s—or at the very least, SETI’s— attention.
You can use that Bayes calculator at this website (shoutout Adele Lopez for making a cool tool) if you want to try it for yourself. If you’ve never heard of this kind of thought process, feel free to watch this video if you’re interested.
Luckily, as I discussed in my last blog post, we won’t have to continue to speculate indefinitely. The Vera Rubin Observatory is going to start a sky survey as soon as July of this year. They hope to increase how often we are able to observe objects of the size of ‘Oumuamua by more than 10x, so we will likely be cataloguing several interstellar objects per year over the next few years.
I, for one, am excited to see what they look like!
I, for one, welcome our high aspect ratio overlords.
I thought this was going to be about https://boardgamegeek.com/boardgame/418059/seti-search-for-extraterrestrial-intelligence which does feature ʻOumuamua