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 have never
seen aliens
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our planet has a number of remarkable
qualities that seems 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
will be prone to erratic outbursts and
wouldn't produce enough ultraviolet
light for photosynthesis
if the sun contains 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 plus 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 their 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 moya
mcteer although perhaps you already know
her from her pbs show fate and fable
what you probably don't know is that dr
mcteer is a phd astrophysicist and an
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 humans 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 tama nui terah in one story
tamanuitara travels too quickly around
the earth and humans complain that the
days are too short so his foster son the
hero maui trapped tamanui to ra 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 and
others about the sun paint a nice
picture of its importance but of course
modern science has its own origin story
for our nearest and dearest star the sun
the sun formed around five billion years
ago collapsing from an over-dense lump
of gas inside a much larger nebula
probably in one of the great spiral arms
of the milky way's disc
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 disc 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 five percent 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 no means rocky or terrestrial
planets in the so-called habitable zone
that's the distance from the star where
the intensity of light is just right 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
her star
and guess what moya actually wrote her
phd thesis on the galactic habitable
zone moya 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 with at least some
metallicity metallicity is our measure
of a 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 some 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 racked 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 out
like all galaxies as a slightly over
dense 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 moya mentioned having
excessive exploding stars in one's
neighborhood can be a problem radiation
may lead to excessive mutations and
degradation of the atmosphere for
example a supernova within 150 light
years of us 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 in
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
studies estimate that about 80 of stars
in the center of the galaxy have stellar
encounters that could strip planets away
from their systems or interrupt the
planet formation process altogether 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 disks 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 the metal poor
stars that that gas formed again not the
most likely place 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 30 000 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 backstory 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 that the system has been around so
what'd 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 have not solved the fermi paradox
quite the opposite we've made it worse
this team discovered something
unexpected of the stars in the galaxy
that could currently support life
most of them 75 percent
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 to 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 colonize the galaxy even if
just with robotic probes
but it seems that most earth analogues
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 considering our personal bias that
we orbit the most important yet mundane
star in the apparently uninhabited
breaches
of space time
do you enjoy mythology from around the
world check out fate and 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
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