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.

So 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 the sun contained significantly less heavy elements it could never have formed a planetary system, but   too much heavy elements and it might host only gas giants.

Yep, the 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 Dr. 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 point of view.

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.

This myth paints and others about the sun paint a nice picture of its importance.

But pf course, modern science has  its own origin story for our nearest and dearest star, 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,   that 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 is 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   gues what - Moiya actually wrote her PhD thesis  on the galactic habitable 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 galaxies 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 so 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 possible 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 the galactic   scale.

We’ve 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 into 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.

Do you enjoy mythology from  around the world?

Check out Fate & Fabled,   a new show on PBS's Storied channel.

Co-hosted by me and Dr. Emily Zarka,   we unpack ancient myths and legends to  find out why such tales were crafted,   and investigate mythology’s influence  on humanity.

Be sure to watch our   Solar mythology episode, which features a  visit from Matt to explain nuclear fusion.