Why do we live around a yellow (*technically white but appears to us yellow!) star, and
not a red one?
It’s the kind of question you’ve probably not given much thought to before, but in fact
it’s a challenging one to explain.
Count up the stars, and one finds that red dwarfs greatly outnumber Sun-like stars, but
then why don’t we live around one?
Join me today as we explore a new research paper from the Cool Worlds Lab in which we
outline the Red Sky Paradox.
Paradoxes hold a special fascination to us, because they represent riddles to be solved,
brain teasers that tickle the intellect.
The term paradox is rather broad, with one example being self-contradictory statements,
for example saying “This statement is false”, the famous Liar’s Paradox.
But more generally paradoxes simply mean statements that run contrary to expectation.
For example, the Tea Leaf Paradox describes the phenomenon where the tea leaves in a stirred
cup of tea migrate to the center of the bottom of the cup, but physical intuition tells us
that the centrifugal force should push them out to the edges.
In 1857, physicist James Thomson suggested the solution may be related to secondary flows
caused by friction with the container, which was rigorously proved by Albert Einstein in
1926.
Was there anything Einstein couldn’t solve?
One of the most famous paradoxes in science is the so-called Fermi paradox.
Whilst working at Los Alamos National Laboratory in 1950, physicist Enrico Fermi was discussing
the probability of alien life in the universe when he famously asked, “so where is everybody?”.
Fermi pointed out a seemingly logical contradiction, if there are billions of stars and life occasionally
spawns, surely we should see evidence for any aliens?
The Fermi paradox is the theme of dozens of books, hundreds of lectures and countless
after dinner conversations, trust me.
Of course, it’s only really a paradox if one accepts the premise, that alien life is
common and should be evident to us.
And so efforts to resolve the paradox focus on trying to refute some aspect of that premise,
for example that life is perhaps rare, or that aliens are shy.
There is literally a hundred different speculative resolutions to the Fermi paradox, each of
which in some way challenge that premise in different ways.
This really isn’t any different from the Tea Leaf paradox where the premise that centrifugal
forces dominates was ultimately wrong, and perhaps one day someone will prove one of
the resolutions to the Fermi paradox.
So now we have this new Red Sky Paradox that I’m introducing in my paper, which I’m
happy to share was just published in the prestigious Proceedings of the National Academy of Sciences.
To understand this paradox, let’s begin by considering three simple facts established
by modern astronomy.
First, M-dwarf stars, that is stars with masses between one twelfth and one half that of the
Sun, are the most abundant type of star in the Universe.
Indeed, they make up about three-quarters of all stars.
Second, these M-dwarfs live far longer lives than Sun-like stars, in fact they’re so
long lived that some will last ten trillion years, whereas the Universe is only 13.8 billion
years old.
And third, M-dwarfs apparently have no shortage of planets, especially rocky planets.
Indeed they seem to have just as many habitable-zone Earth-sized as Sun-like stars do, and possibly
even more.
So as we sit here, thinking about these three facts under the light of our yellow star (*technically
white but appears to us yellow!), one is faced with an apparent logical contradiction.
If these red M-dwarf stars are far more common, far longed lived and have just as many rocky
planets around them, surely we would expect to be born around one of them, under a red
sky?
For years, I’ve laid awake at night wondering this, why is why recently I felt compelled
to finally write it up.
Now immediately, you might wonder, as I did, perhaps the surprisingness here isn’t so
extreme.
If the odds of being born around a yellow star (*technically white but appears to us
yellow!) versus a red star are say 1:5, we could perhaps write this off as a mere coincidence,
an outcome which is somewhat uncommon but not highly unlikely.
So what are the odds of this being mere coincidence?
We know that M-dwarfs stars are, altogether, about 5 times more common than Sun-like stars.
For the astronomers out there, I’m being generous here by defining Sun-like to include
F, G and K type dwarfs.
Ok so 5:1, not so extreme.
Now let’s factor in the time aspect.
M-dwarfs live for, on average, about 20 times longer than Sun-like stars.
Yes some live for a thousand times longer, but when we average over the entire population
the factor drops down to 20.
If we simply combine this factor of 20 with the earlier factor of 5, that gives us a factor
of 100.
So now things start to become rather uncomfortable.
In fact, we are really violating the famous cosmological principle here, which assumes
that our astronomical view of the universe is typical.
Now before we start thinking about resolutions to this, and certainly we can think of some,
let’s just step back to that factor of 100 for a second.
In that calculation, I’m really assuming that a star lasting 20 times longer gives
you a 20 times greater chance of spawning an intelligent civilisation, all things beings
equal.
This makes sense if life and intelligence evolution is a slow, gradual process that
infrequently occurs.
It’s a lottery and M-dwarfs are simply buying more lottery tickets.
But what if planets typically spawn civilisations within a billion years or so, a fast and inevitable
process.
This clearly erodes the longevity advantage that M-dwarfs enjoy.
If so, it doesn’t matter as much whether you live for a trillion years or a billion,
either is generally sufficient time for a civilisation to pop up.
This discussion of timescales might remind some subscribers of earlier work we published
which calculated the statistical probability distribution of this timescale using our evolutionary
record as a guide.
Using that approach, we can ask, how likely is it we would live around a Sun-like star
versus a red dwarf, depending upon this evolutionary timescale?
As shown here, if the timescale is fast, a hundred million years for example, the odds
are about 10%.
Unlikely but not incredibly so.
Following the curve to longer timescales though, the temporal advantage of red dwarf stars
kicks in and we end up with the 1% odds as discussed earlier.
So which one is right?
10% or 1%?
Well with current data it’s impossible to solve this completely, but what we can say
is that everything in this red region is actually inconsistent with Earth’s evolutionary history.
That’s because it would entail complex multicellular life emerging on the Earth far earlier than
the geological record shows it.
Incidentally, life everywhere also exacerbates the Fermi paradox.
The long timescales, shown in blue, are actually all consistent with Earth’s history.
This might surprise you because if the typical timescale is a trillion years, surely that’s
inconsistent with our emergence within a few billion years?
The answer to this is selection bias, we’re only here because success occurred and so
we can’t actually say that a trillion-year emergence timescale is any less likely than
billion-year timescale.
If that sounds confusing, don’t worry, this is highly unintuitive.
We did a video about this a while back using prisoners picking locks as an analogy, so
check that out for a deeper explanation.
Now coming back to our graph, there is a much larger region of this plot consistent with
us being a 1% fluke than a 10% fluke, in fact you really have to fine tune the evolutionary
timescale quite a bit to end with the 10% number.
Yet more, if we consider that planets can spawn multiple civilizations, not just one,
then the longevity factor of M-dwarfs gives an unequivocal advantage.
So the 1% fluke value really looks to be the correct value.
OK, 1%.
Well, maybe that’s the answer.
We really are just a fluke.
It’s certainly possible and indeed my paper formally states this as a Resolution 1 to
the Red Sky Paradox.
But frankly it’s a not a very satisfying resolution.
We’re basically just saying it’s luck, no explanation whatsoever, and a statement
that flies in the face of the cosmological principle.
So I’m leaving this on the table, but it’s not my preferred resolution.
Perhaps like the Fermi paradox, and indeed the Tea Leaf paradox, one of the assumptions
built into the Red Sky Paradox might be wrong, in such a way that it resolves the paradox
altogether.
Aside from the fluke possibility, my paper suggests three additional possibilities, resolutions
2, 3 and 4.
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Now back to the Red Sky Paradox, in the second half here I want to talk about these 3 other
resolutions.
They’re pretty straight-forward so let’s go through them swiftly.
Resolution 2 is that life or by extension intelligent life just rarely emerges on M-dwarf
planets.
In fact, to resolve the paradox, it occurs about 100 times less often, or even less.
There is some theoretical ideas out there motivating this.
For example, M-dwarfs spit out high energy flares, perhaps that impinges life.
As another example, they take far longer than Sun-like stars to settle down into a mode
of stable output, this could actually desiccate nearby planets of their precious water leading
to Dune-like desert worlds.
As a final example, M-dwarfs rarely have Jupiter-sized planets, perhaps those are crucial for life,
by protecting us from asteroid impacts.
We don’t know which or indeed if any of these are true, but we have good reasons to
be suspicious that M-dwarfs are just as good for life as yellow dwarfs.
Resolution 3 is somewhat similar to 2, except that it looks at the temporal aspect more
closely.
Perhaps M-dwarfs and Sun-like stars are equally likely to start life, it’s just that Sun-like
stars are the ones who enjoy far longer windows of opportunity.
This seems counter-intuitive because M-dwarfs live so long, but maybe the actual temporal
window when life is possible is very short.
In fact, if it were 5 times shorter than Sun-like stars, that would be enough to resolve things.
One possibility is that Mars-like planets on the outer edge of the habitable-zone become
habitable during that early, violent M-dwarf phase, where the extra energy could actually
be useful.
Indeed, if we studied young M-dwarf planets, we could potentially test that hypothesis
directly.
Now finally resolution 4 is that habitable-zone rocky planets are in fact rare around M-dwarfs.
A paucity of pale-red dots.
Now you’re probably thinking, hold on, you said earlier that they seem to have just a
many habitable rocky planets as Sun-like stars.
Yes, *seem* to.
But appearances can be deceptive.
You see, M-dwarfs come in a diverse range of masses, and frankly we’ve only really
surveyed the heaviest examples for planets.
That’s because they’re the brightest and easiest to look at.
But what if the smallest red dwarfs are different beasts and rarely have habitable-zone planets?
If planets here are a 100 times less common we’d have our solution.
This works out because the smallest red dwarfs are in fact the most numerous.
So if they’re different it’s going to be dominant.
Again, the good news here is that this is testable, we are now looking at these smaller
stars, and already have detected some systems like Proxima Centauri b and the TRAPPIST-1
system.
It’s still early days, but I’m little skeptical of this resolution given these early
examples.
So there you have it, four resolutions, luck, inhibited evolution, truncated habitable windows,
or a paucity a pale-red dots.
Right now, we really don’t know which one is right.
Just like how we don’t know which resolution dissolves the Fermi paradox.
Now, you might be thinking this Red Sky Paradox is purely academic but it actually has major
practical consequences.
Figuring this out is crucial because it would reveal where we should be pointing our future
life hunting telescopes.
In resolution 2, M-dwarfs are perhaps best avoided altogether.
In resolution 3, they could be worthwhile but only if we target young systems.
And in resolution 4, we should focus on the heaviest M-dwarfs, skipping systems like Proxima
Centauri.
Who knows, one day, we might plan a interstellar mission to a distant star.
If life is off the cards for 3/4 of all stars in the universe, that seems like something
we should probably figure out before we leave.
So let me know down below, what is your favourite resolution?
Do you have others in mind not discussed here?
I want to finish by just saying a special thank-you to our supporters.
This research was supported by many of you, donors to our research team at Columbia University.
The idea is that we conduct original research, make videos about them, which then lead to
many of your supporting us, which then leads to more research and more videos.
A cycle of research and outreach.
It’s an pretty unique way to help fund research and I’m so grateful to all who have believed
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So if you want to support a research team, not just a YouTube channel, then please do
consider helping us out.
So until next time, stay thoughtful and stay curious.