If you were hoping, based on the title, that I was going to describe the time I saw strange lights moving at inexplicable speeds across the sky, then I am about to disappoint you. This column is actually about my experience in the public spotlight talking publicly about the connection between UFOs and extraterrestrial life. It was quite a ride.
Extraordinary claims require extraordinary evidence
On May 30, 2021, I wrote an op-ed in the New York Times titled "I'm a Physicist Who Studies Aliens. U.F.O's Don't Impress Me." I don't get to write titles for the op-ed pieces that I write for the Times — or most other places for that matter — but, as provocative as it was, I think it captured the essence of my point. As a scientist involved in the search for life and "techno-signatures" on exoplanets, I think a lot about what constitutes a good data set for that search. In other words, what kind of data would allow me to make the extraordinary claim that my colleagues and I have detected life and a civilization on another world?
The answer had better be "some really damn good data." By that, I mean we would need to take measurements that gave us strong and unambiguous evidence for the conclusion that a particular signal comes from a technologically advanced civilization. My main point in the op-ed was that no matter how intriguing those navy UFO sightings may be — and they are interesting — they don't provide the extraordinary evidence that we need to conclude that aliens are visiting us. My arguments are in the op-ed if you want to see them. What I want to focus on here is what happened after that argument appeared in the press.
The UFO brigade
Within an hour or so, my email and Twitter feed began to light up. By the end of the day, I was getting more messages about the piece than almost anything I had ever written before. Some of the messages affirmed the argument I was making. The majority, however, wanted me to know how wrong I was. These fell into two categories.
There was a fair amount of "the-government-knows-but-won't-tell-us" kind of narrative. Lots of these messages were pretty mean.
Some people wanted me to know that UFOs — or as the government calls them, Unidentified Aerial Phenomena (UAPs) — didn't need to be connected to aliens for them to be of interest. I had however made this exact point in my piece.
I have no problem with people wanting to have those navy sightings (and others) studied scientifically and openly. My colleagues on the NASA techno-signature grant made this point in an excellent Washington Post op-ed. I think the process of vetting those sightings would greatly help show the public exactly how science works. These days, we have a real problem with science denial, and anything that lets folks understand "what science knows and how it knows it" would be helpful.
Credit: IgorZh / 280582371 via Adobe Stock
But many folks (on Twitter and elsewhere) held that the connection between UFOs and aliens had already been made. I got floods of links to one video or website after another, the vast majority of which were people describing something they had seen in the sky. As I said in the op-ed, there really isn't much science you can do with personal testimony. One can't get accurate measurements of velocity or distance or mass or any of the other basic data that a physicist would need to tell if something really was moving in a way that's impossible for human technology.
Some folks reached out because they had seen a UFO themselves. I totally understand that these people would want someone to take their reports seriously. I would never tell them that they did not have their experiences. What I can say, however, is that there's nothing a scientist can do to transform the description of that experience into data that we would need to reach the extraordinary conclusion that they had seen evidence for extraterrestrial life.
The truth is out there
But a significant fraction of what I saw coming across Twitter and elsewhere was just pure vehemence. These folks were absolutely certain that UFOs were alien visitors. There was a fair amount of "the-government-knows-but-won't-tell-us" kind of narrative. Lots of these messages were pretty mean. I got the sense that, for these folks, no public investigation — no matter how open and transparent — would be satisfying unless it reached the conclusion that they already believed. This, of course, is the opposite of science.
So, it was an interesting week. My brief time in the UFO limelight (I did many interviews on places like CNN, BBC, etc.) showed me a lot about how people view the question. Since I am so deeply involved with techno-signature science, I felt it was important to try to explain how the science of life and the universe works as a science.
But I don't really want to spend a whole lot more time in that limelight. It was kind of exhausting, in large part because of the vehemence of the true believers. I will follow whatever happens after the government's report comes out with interest. But my bet (and every researcher makes a bet when they choose their research topics) is that the data I need to know about life elsewhere in the universe will come from telescopes, not jet fighters.
Research that aims to identify exoplanets that might contain life usually use oxygen as a biomarker. But a new study published in AGU Advances explains that this can be very misleading: Oxygen can easily accumulate in an exoplanet's atmosphere without any biological origin.
Oxygen is considered a biomarker because photosynthesis — the process by which plants use sunlight to fix carbon dioxide into sugar — produces oxygen as a waste product. Thus, a planet with oxygen in its atmosphere is considered a strong candidate to host some kind of lifeform.
The team, led by Joshua Krissansen‐Totton of UC Santa Cruz, developed a model of planetary formation that allowed them to tinker with variables that could affect how an Earth-like planet develops. Using their model, the researchers were consistently able to produce three scenarios in which an Earth-like planet has levels of oxygen in its atmosphere similar to ours, but life was not part of the formula.
Three oxygenated worlds with no life
The planetary evolution model. Arrows show the flow of certain substances and heat energy between different layers of the Earth and its atmosphere.Krissansen-Totton et al./ AGU Advances
In the first scenario, an exoplanet has very high levels of carbon dioxide and water in the atmosphere. Under these conditions, a strong greenhouse gas effect means there will be no water on the exoplanet's surface. When hit by ultraviolet (UV) light, water vapor in the upper atmosphere can occasionally split into hydrogen and oxygen. The lighter hydrogen gas escapes into space, leaving the heavier oxygen gas behind.
In the second scenario, an exoplanet "waterworld" contains anywhere from 10 to 230 times as much water as the Earth has today. Under these conditions, the oxygen cycle — which involves the circulation of oxygen through the atmosphere, lifeforms, and rocks — essentially doesn't exist. Pressure from the massive oceans on the crust would shut down the geological activity necessary to recycle oxygen, leaving it in the atmosphere.
In the third scenario, an exoplanet "desertworld" has conditions exactly the opposite of those in the "waterworld." This type of exoplanet has very little water, no more than a third of what Earth has in its oceans. Under these conditions, the molten surface of a young exoplanet can freeze while the limited water supply is still found only as steam (vapor) in the atmosphere. This prevents oxygen from being absorbed by the crust. Then, as with the first scenario, UV light breaks up water into hydrogen and oxygen.
Implications for the hunt for E.T.
An infographic illustratingthe three planets described above and how they might form.Illustration by J. Krissansen-Totton
None of the three scenarios assures an oxygen-rich atmosphere; they simply allow for oxygen to occur abiotically. Professor Krissansen‐Totton described the utility of the model in a press release:
"This is useful because it shows there are ways to get oxygen in the atmosphere without life, but there are other observations you can make to help distinguish these false positives from the real deal. For each scenario, we try to say what your telescope would need to be able to do to distinguish this from biological oxygen."
Such telescopes should be in orbit by 2030. Now the scientists using them know what to look for.
How will humans discover the existence of extraterrestrial civilizations?
Unless aliens decide to visit Earth, the most likely answer is by scanning the skies for "technosignatures," which are observational evidence of technological or industrial activity in the Universe.
In a recent paper published in the journal Acta Astronautica, a team of NASA-funded researchers outlined some of the most promising ways scientists and space agencies could search for technosignatures. The paper included a somewhat surprising proposition: Humanity's "first contact" with aliens is likely to be with a much more advanced civilization.
In other words, there could be many alien civilizations throughout the Universe, or even in our galaxy, but if they're similar to us in terms of technological advancement, we probably can't spot them yet. The same goes for those human-like civilizations spotting us.
That's because the "cosmic footprints" of our civilization and theirs would be relatively small, compared to highly advanced alien civilizations. The researchers call this concept "contact inequality."
"It seems unlikely that civilizations with a relatively low level of technological development would enter into contact with each other, since that would require either very high sensitivities or highly visible engineering," reads the paper. "Less advanced civilizations lack the sensitivity needed to detect other civilizations unless they have built very large or luminous structures."
ʻOumuamua,' the first known interstellar object detected passing through the Solar System. Some scientists have proposed that Oumuamua might be an extraterrestrial artifact.
Robert Weryk using Pan-STARRS 1
So, unless near-future technology like the James Webb Space Telescope enables scientists to find biomarkers on other planets, we're far more likely to discover more sophisticated civilizations.
How? The paper outlines a series of strategies that are either currently being practiced, could be practiced in conjunction with other astronomical projects, or could be developed in the near future.
A few of those strategies include searching for:
- Dyson spheres — gigantic structures that, in theory, could orbit stars and generate vast amounts of energy for civilizations.
- Near-Earth objects — space agencies could search the Moon, Mars or other space bodies for evidence of extraterrestrial artifacts, such as crashed probes.
- Abnormal spectra in planetary atmospheres — if aliens are conducting industrial activity on another planet, that planet's atmosphere would likely contain evidence of such activity.
- Night-time illumination
- Radio and laser signals
When thinking about the best ways to search for alien life, it helps to reverse the perspective: How would aliens know humans exist? With this question in mind, researchers who deal with technosignatures consider all of the signals humans are sending into space.
Some of our technosignatures are transmitted intentionally, like the Arecibo message humans sent toward the globular star cluster M13 in 1974. Others are unintentional, like night-time illumination and pollution-driven atmospheric changes.
The 'ichnoscale'
To put the concept of technosignatures into perspective, researchers have developed a framework called ichnoscale. Ichnoscale ranks the size of a technosignature relative to what human technology is currently capable of producing.
The scale also ranks the number of potential targets throughout the Universe. For example, searching for a crashed alien probe on the Moon would represent one target, while scanning the stars for Dyson spheres would have millions of targets.
Together, these measurements help scientists estimate the most likely ways to discover evidence of alien civilizations. Of course, there's no guarantee that any one strategy will work, or that aliens even exist.
That's one reason why efforts to search for technosignatures have received little funding. But the researchers propose that many of these strategies could be tacked onto other astronomical missions at little cost.
Socas-Navarro et al.
And even if the searches turn up nothing, the researchers said the results would still provide "enormous ancillary benefits on solar system research and advance our knowledge about the objects being scrutinized," and would "establish quantitative upper bounds on certain types of technologies or developmental stages of civilizations in the solar neighborhood."
"The search for TS deals with questions that have profound implications on the future of humanity," the researchers concluded. "Perhaps one the most important is whether technological civilizations are ephemeral or, on the contrary, can be long lasting. A closely related question is whether space faring civilizations are common, and if humankind will eventually become one of them."
"We do not yet have any answers for these and other important questions but if we can start to explore the search parameter space, even in the absence of any detection we may be able to gain some valuable insights."
Imagine if we could cut paths through the vastness of space to make a network of tunnels linking distant stars somewhat like subway stations here on Earth? The tunnels are what physicists call wormholes, strange funnel-like folds in the very fabric of spacetime that would be—if they exist—major shortcuts for interstellar travel. You can visualize it in two dimensions like this: Take a piece of paper and bend it in the middle so that it makes a U shape. If an imaginary flat little bug wants to go from one side to the other, it needs to slide along the paper. Or, if there were a bridge between the two sides of the paper the bug could go straight between them, a much shorter path. Since we live in three dimensions, the entrances to the wormholes would be more like spheres than holes, connected by a four-dimensional "tube." It's much easier to write the equations than to visualize this! Amazingly, because the theory of general relativity links space and time into a four-dimensional spacetime, wormholes could, in principle, connect distant points in space, or in time, or both.
A wormhole connecting two points in space.
Credit: TDHster via Adobe Stock
The idea of wormholes is not new. Its origins reach back to 1935 (and even earlier), when Albert Einstein and Nathan Rosen published a paper constructing what became known as an Einstein-Rosen bridge. (The name 'wormhole' came up later, in a 1957 paper by Charles Misner and John Wheeler, Wheeler also being the one who coined the term 'black hole.') Basically, an Einstein-Rosen bridge is a connection between two distant points of the universe or possibly even different universes through a tunnel that goes into a black hole. Exciting as the possibility is, the throats of such bridges are notoriously unstable and any object with mass that ventures through it would cause it to collapse upon itself almost immediately, closing the connection. To force the wormholes to stay open, one would need to add a kind of exotic matter that has both negative energy density and pressure—not something that is known in the universe. (Interestingly, negative pressure is not as crazy as it seems; dark energy, the fuel that is currently accelerating the cosmic expansion, does it exactly because it has negative pressure. But negative energy density is a whole other story.)
If wormholes exist, if they have wide mouths, and if they can be kept open (three big but not impossible ifs) then it's conceivable that we could travel through them to faraway spots in the universe. Arthur C. Clarke used them in "2001: A Space Odyssey", where the alien intelligences had constructed a network of intersecting tunnels they used as we use the subway. Carl Sagan used them in "Contact" so that humans could confirm the existence of intelligent ETs. "Interstellar" uses them so that we can try to find another home for our species.
If wormholes exist, if they have wide mouths, and if they can be kept open (three big but not impossible ifs) then it's conceivable that we could travel through them to faraway spots in the universe.
Two recent papers try to get around some of these issues. Jose Luis Blázquez-Salcedo, Christian Knoll, and Eugen Radu use normal matter with electric charge to stabilize the wormhole, but the resulting throat is still of submicroscopic width, so not useful for human travel. It is also hard to justify net electric charges in black hole solutions as they tend to get neutralized by surrounding matter, similar to how we get shocked with static electricity in dry weather. Juan Maldacena and Alexey Milekhin's paper is titled 'Humanly Traversable Wormholes', thus raising the stakes right off the bat. However, they are open to admitting that "in this paper, we revisit the question [of humanly traversable wormholes] and we engage in some 'science fiction.'" The first ingredient is the existence of some kind of matter (the "dark sector") that only interacts with normal matter (stars, us, frogs) through gravity. Another point is that to support the passage of human-size travelers, the model needs to exist in five dimensions, thus one extra space dimension. When all is set up, the wormhole connects two black holes with a magnetic field running through it. And the whole thing needs to spin to keep it stable, and completely isolated from particles that may fall into it compromising its design. Oh yes, and extremely low temperature as well, even better at absolute zero, an unattainable limit in practice.
Maldacena and Milekhins' paper is an amazing tour through the power of speculative theoretical physics. They are the first to admit that the object they construct is very implausible and have no idea how it could be formed in nature. In their defense, pushing the limits (or beyond the limits) of understanding is what we need to expand the frontiers of knowledge. For those who dream of humanly traversable wormholes, let's hope that more realistic solutions would become viable in the future, even if not the near future. Or maybe aliens that have built them will tell us how.
"Do you think we are alone?" That question is, without fail, one of the first things people ask me when they learn I'm an astronomer. And I get why. It's also the question I most want an answer for. But that answer may depend a lot on what kind of life the universe favors (if it favors any at all). So, the question I want to briefly touch on today is how common will it be for any life that appears on any planet in the universe to start climbing up the evolutionary ladder of complexity?
On Earth, the history of life is mainly a story of single cells. Earth's origin lies some 4.5 billion years ago, and the best fossil records put the emergence of life as single-celled creatures about a billion years later. After life's first appearance, almost two billion years go by during which all evolutionary activity was on those single-celled organisms. There was some really amazing biochemical machinery evolving within those little cells but if you are interested in multicellular creatures, they don't appear until sometime around 700 million years ago.... if there is one thing we know is true, it's that nature is more clever than we are. That means it may know lots of ways to produce animals without oxygen around or even in the presence of buckets of CO2.
What are we to make of this incredibly long run of Earth as Planet Bacteria? (Note, there were actually other kinds of single-celled creatures too). Well, it certainly tells us that evolutionary success does not demand multicellularity. During these long eons, life invented the most amazing array of nano-machines for a jaw-dropping variety of purposes. For example, single-celled critters invented photosynthesis for turning sunlight into sugars, metabolisms for turning sugars into energy, and complex intracellular transport mechanisms to move stuff where it was needed and get rid of waste. Earth before plants and animals was already a fertile place full of life that had, in its way, become spectacularly complex at least on the level of biochemistry.
Given the long run of this version of Earth, it may be that there is no reason that more complex life should be expected to form in all or even most cases on other planets.
Protozoa—a term for a group of single-celled eukaryotes—and green algae in wastewater, viewed under the microscope.
Credit: sinhyu via Adobe Stock
Another way the story of life on Earth might not get repeated elsewhere in the cosmos relates to the composition of planetary atmospheres. Our world did not begin with its oxygen-rich air. Instead, oxygen didn't show up until almost two billion years after the planet formed and one billion years after life appeared. Earth's original atmosphere was, most likely, a mix of nitrogen and CO2. Remarkably it was life that pumped the oxygen into the air as a byproduct of a novel form of photosynthesis invented by a novel kind of single-celled organism, the nucleus-bearing eukaryotes. The appearance of oxygen in Earth's air was not just a curiosity for evolution. Life soon figured out how to use the newly abundant element and, it turns out, oxygen-based biochemistry was supercharged compared to what came before. With more energy available, evolution could build ever larger and more complex critters.
Oxygen may also be unique in allowing the kinds of metabolisms in multicellular life (especially ours) needed for making fast and fast-thinking animals. Astrobiologist David Catling has argued that only oxygen has the right kind of chemistry that would allow for animals to form on any world.
Atmospheres may play another role in what can and can't happen in the evolution of life. In 1959, Su-Shu Huang proposed that each star would be surrounded by a "habitable zone" of orbits where a planet would have temperatures neither too hot nor too cold to keep life from forming (i.e. liquid water could exist on the planet's surface). Since then, the habitable zone has become a staple of astrobiological studies. Astronomers now know that the outer part of the habitable zone will be dominated by worlds with lots of greenhouse gases like CO2. A planet in a location like Mars, for example, would require a thick CO2 blanket to keep its surface above freezing. But all that CO2 could present its own problems for life. Almost all forms of animal life on Earth, including sea creatures, die when placed in CO2-rich environments. This has led astronomer Eddie Schwieterman and colleagues to propose a habitable zone for complex life: A band of orbits where planets can stay warm without requiring heavy CO2 atmospheres. According to Schwieterman, animal life of the kind we know would only be able to form in this much thinner band of orbits.
So, we have three lines of evidence that may suggest multicellular life (including thinking animals) may not be the road most taken across the universe. If this were true, then the galaxy might be awash with life but be sparse in terms of tentacles, paws, or boots on the ground.
Now, before your shoulders sag in sadness, it's important to note some facts. First, there are likely 400 billion planets in our galaxy alone. This provides a lot of leeway for experimentation. Second, if there is one thing we know is true, it's that nature is more clever than we are. That means it may know lots of ways to produce animals without oxygen around or even in the presence of buckets of CO2.
We just won't know until we start looking. And here is the good news. We finally are ready to start looking.
