What If the Galactic Habitable Zone LIMITS Intelligent Life? (Script) (Patreon)

Our solar system is a tiny bubble of habitability suspended in a vast universe that mostly wants to kill us. In fact, a good fraction of our own galaxy turns out to be utterly uninhabitable, even for sun—like stellar systems. Is this why .. most of us .. haven’t seen aliens?

Our planet has a number of remarkable qualities that seem to make it especially suitable for developing and sustaining life, as we discussed in our episode on the rare earth hypothesis.  But if you want life, one of the only non-negotiable requirements has nothing to do with the planet. Our Sun also seems pretty ideal in a number of ways. A bit more massive and it would have burned out too quickly, much less massive and it would be prone to erratic outbursts and wouldn’t produce enough ultraviolet light for photosynthesis. If it contained significantly less heavy elements it could never have formed a planetary system, but much more and it might host only gas giants. Yep, our Sun seems pretty great.

But is the Sun unique enough to explain one of the most perplexing mysteries of the universe? In a galaxy of 200+ billion stars, why don’t we see any other signs of technological life? This is the Fermi Paradox, which of course we’ve talked about before. But one possible solution is that the Sun and its planetary system are really quite unique in its ability to spawn and nurture complex life. We tend to think of our home star as being pretty important, but how unusual is it really?

To help answer that question, I’d like to introduce Dr. Moiya McTier - although perhaps you already know her from her PBS show Fate and Fabled. What you probably don’t know is that DOCTOR McTier is a PhD astrophysicist, and expert in what we call the galactic habitability zone, as well as being an expert in folklore. Who better to compare the Sun’s uniqueness from the perspectives of science versus our own slightly biased perspective.

From a modern astronomer’s perspective, our Sun is objectively mediocre, average in terms of mass, age, even location. But consider for a moment the ancient human’s perspective, without the help or hindrance of 21st century physics.

To our distant ancestors, the Sun was a wondrous source of warmth, light, food, and stability. It was a powerful, never-tiring giver of life, and in mythology, matters of life and death often get the most attention. This is why you see so many gods across cultures with dominion over water, food, love, and war, but nothing is more important to our survival than the sun. So it’s no surprise that nearly every ancient culture around the world worshipped a deity who represented or personified the Sun.

Many of these cultures cast their solar deities in myths that explained real natural phenomena, like the day/night cycle, eclipses, or the changing of the seasons. To the Maori people of New Zealand, the Sun was personified by the God Tamanuitera. In one story, Tamanuitera travels too quickly around the Earth, and humans complain that the days are too short. His foster son, the hero Maui, trapped Tamanuitera with a rope and didn't let him go until he promised to move more slowly across the sky. Little did the Maori know that Earth's days ARE getting longer, but it's because of the moon, not the sun.

Of course, modern science has its own origin story for the Sun.

The Sun formed around 5 billion years ago, collapsing from an overdense lump of gas inside a much larger nebula, probably in one of the great spiral arms of the Milky Way’s disk. While most of the cloud ended up in the shrinking spheroid that would ultimately become our Sun, a fraction of the material collapsed into a disk around the protostar. Heavier elements clumped together and slowly grew into planets - the smaller ones became terrestrial planets like the Earth, while the larger collected vast atmospheres of hydrogen and helium and became the gas giants. At the same time, the core of the collapsing protostar became hotter and hotter and eventually ignited in nuclear fusion. The outward flow of fusion-generated energy supports the Sun against gravitational collapse. It’s been resisting its own inward crush for 5 billion years now, and has enough fuel for about 5 billion more.

So our home star has a pretty cool backstory, but hardly a unique one. Our Sun is a pretty ordinary G-type main sequence star.  Around 5% of the stars in the Milky Way are G-types, which means there are several tens of billions of them. Having a planetary system is also not unusual. The Kepler mission demonstrated that most stars have planets - at least in the local part of the galaxy. Kepler also revealed that there are around 40 billion Earth-analog planets in the Galaxy, which means rocky or terrestrial planets in the so-called habitable zone. That’s the distance from the star where the intensity of light is in the right range to allow liquid water on the planet’s surface. So it sounds like the galaxy should be full of potential starting points for life, even if we assume that life can only form on Earth-like planets.

But not so fast. There’s another factor to consider. Stars have habitable zones, but so do galaxies. There are huge regions of the Milky Way where life couldn’t possibly have formed, no matter how perfect the host star. And guess what - Moiya actually wrote her PhD thesis on the galactic habitability zone. Moiya, what are some of the factors that make a region of the galaxy habitable or not?

Stars are mostly made out of hydrogen and helium, with trace amounts of heavier elements. But those elements are critical - they’re what planets like the Earth are made out of. So firstly a star needs to form from a nebula at least some metallicity. “Metallicity” is our measure of the heavy element content of a cloud or a star. It comes from the fact that astronomers tend to call any element heavier than helium a metal. On the other hand, if metallicity is too high, it can lead to the formation of too many gas giants like Jupiter - and that can also cause trouble for smaller Earth-sized planets.

Most of these heavy elements are produced in massive stars and then spread through the galaxy in supernova explosions. So a big factor in determining the galactic habitable zone is that enough massive stars have lived and died in that region. But while we’re talking supernovae - it’s generally a bad idea to have too many exploding stars next door when you’re trying to nurture a delicate biosphere. And some parts of the Milky Way were wracked by supernovae for much of the Galaxy’s history.

So it seems that to really understand the Sun’s uniqueness, we need to take our story back even further - beyond the formation of the Sun to the formation of the Galaxy to figure out where habitable planetary systems could have formed in the first place. Our galaxy started like all galaxies - as a slightly overdense spot in the near perfectly smooth cloud of particles that filled the universe after the Big Bang. As it cooled, our local lump started to pull itself together under its own gravity. Fragments broke off to collapse into galaxies. Fragments within those fragments collapsed further, kindling little sparks of fusion reminiscent of an earlier epoch.

This primordial generation of stars were unpolluted by heavy elements, which means they couldn’t possibly have formed planets. No chance for life yet. However these stars were incredible atom factories, rapidly burning their way up the periodic table and spraying heavy elements into the surrounding gas in colossal supernova explosions. The metallicity of the universe began to rise.

The next generation of stars had some metals, and for the first time had the chance to build planets. These stars fell towards the center of the still-collapsing gas cloud like pebbles in a pond, forming a growing cluster that would become the Milky Way’s bulge. As the galactic bulge grew, it was wracked by further waves of supernovae. As Moiya mentioned, having excessive exploding stars in one’s neighborhood can be a problem. Radiation may lead to excessive mutation and degradation of the atmosphere. For example, a supernova within 150 light years would obliterate our ozone layer. On the other hand, some radiation may be essential - because genetic mutation drives evolution. We don’t know where the exact line is in terms of getting blasted by nearby supernovae, but the early days of the galactic core were surely above that line. Those supernova waves have now passed, so you might think that life could take hold on the core. Not so much. The metallicity of that region is now the highest in the galaxy, and high metallicity means too many planets - in particular, too many giant planets.

We know that having a Jupiter-like planet or two in the outer solar system can be useful in protecting the inner system from infalling comets, but gas giants can also disrupt or destroy terrestrial planets. A system with multiple Jupiter-like planets probably wouldn’t stand a chance at forming Earth-analogs.

Other factors make the core the worst place in the galaxy. The extreme density of stars will have led to frequent close encounters between systems. In our solar system, such encounters disturb our Oort cloud, sending comets plummeting towards the inner solar system. A handful of mass extinction events from the resulting giant impacts was probably good for driving evolution, but life needs time to recover. Overly frequent mass extinctions will result in absolute extinction.

As the uninhabitable core was forming, pristine gas continued to pour into the Galaxy’s growing gravitational field. It was swept up into a widening whirlpool where it continued to cool and to fragment. Eventually the disk of gas converted itself into a disk of stars. It took  some time for the emerging spiral disk to seed itself with enough heavy elements to form planetary systems. In fact, some of our galaxy still hasn’t had enough time. The outer rim of the Milky Way formed the most recently. It’s still accreting from the surrounding reservoirs of ancient, Big-Bang gas. Towards the rim we see metal-poor gas and metal-poor stars that they formed - again, not the most likely places to find planets or life.

So the inner and outer parts of the Milky Way don’t look promising. But right in between we find the Galactic habitable zone. It emerged around 8 billion years ago, starting out as a band between around 20 and 30 thousand light years from the center, it expanded inwards as the supernova rates dropped, and outwards as metallicity increased. It now covers around half of the galactic disk.

Based on the Milky Way’s formation history, our starting intuition seems right: the Sun and solar system are NOT special as far as our galaxy goes. But now that we have so much back-story, perhaps we can say a little more about the emergence of life-friendly planetary systems. And by “we”, I mean Charlie Lineweaver, Yeshe Fenner and Brad Gibson from Swinburne University in Australia. This team of astrophysicists estimated the historical emergence of life-friendly planetary systems accounting for all the stuff we talked about. They started with the Milky Way’s history of star formation and folded in estimates for the probability of the  emergence of planets  based on heavy element abundance; the likelihood of surviving obliteration by supernovae; and the probability that life could emerge given the amount of time the system has been around.

So what did they find? As we suspected, the Sun is not unique, but it’s also not the most typical. Fewer than 10% of stars formed in the Milky Way have optimal conditions for the development of life, and that would drop down to a percent or two if we ruled out the erratic red dwarf stars, the most common star type. That still leaves billions of potential origins for life in the Milky Way, so we haven’t solved the Fermi Paradox. Quite the opposite - we’ve made it worse. This team discovered something unexpected: of all the stars in the Galaxy that could currently support life, most of them - 75% - have been around longer than the Sun - by an average of a billion years. So if our analysis of the galactic habitable zone was supposed help explain the Fermi Paradox by reducing the potential origins for life, it’s done the opposite.

One factor that may explain the apparent absence of technological life is that other such civilizations just haven’t had time to make their presence known on a galactic scale. We’re talked about how it should only take a million years or less for one species to colonise the galaxy - even if just with robotic probes. But it seems that most earth-analogs have a head start of a billion years - more than enough time to establish galactic empires. So it sounds like we haven’t made progress solving the Fermi Paradox. But actually we have. There is a roadblock in the chain from forming a habitable planetary system to sparking simple life to complexifying to a galactically-visible civilization. We’ve solidly ruled out the first as a so-called “great filter”, so now we can think harder about the others. There is SOMETHING special about this planetary system, even with our personal bias that we orbit most important and yet mundane star in the apparently uninhabited reaches of space time.