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On the Mediterranean shore of Egypt some 2000 years ago,
a scholar was mapping what he thought to be pin pricks in the celestial sphere.
He made careful note of the position of a bright star that hung just above the southern horizon.
But never imagined that he was gazing on a
multi-star system the stellar that hosted a world much like the Earth.
A world that would be humanity’s first destination when we would finally venture to the stars.
Alpha Centauri is the first and brightest of the constellation of the centaur, and the
nearest star in our galaxy of 200 billion.
From Ptolemy of Alexandria's first record in his Almagest,
more than a millennium and a half passed before
we started to realize its incredible significance.
By the end of the 17th century, the star had long since slipped below the horizon of Alexandria
in its galactic wanderings.
But it shone bright from southern India, and there a Jesuit priest peered at Alpha Cen
through a newly invented device - the telescope.
Father Jean Richaud saw two stars instead of one, these days dubbed Rigel Kentaurus
and Tolimar, a binary pair bound in an 80 year waltz.
This telescope thing improved over the next century and a half, allowing astronomers to
watch alpha-cen sway relative to the background stars as Earth orbited the Sun.
That parallax revealed the system to be close - closer than any other - but still more than
four light years distant.
The two stars were so near and so familiar - both within 10% or so of our own Sun’s
mass.
Perhaps they also bore planetary systems - and even planets like the Earth.
Dreams of an interstellar humanity started with the twins at the centaur’s foot.
But our greatest hope of another Earth isn’t with the twins.
Forward another century, and a Scotsman named Robert Innes photographed the southern stars
from Johannesburg to track their galactic wanderings.
The fast-moving alpha-cen system was a prime target, but Innes found something utterly
unexpected - this wasn’t a binary system after all - it was a trinary.
A faint red star crawled on a vast, half-million year orbit around its brighter siblings.
For the entire history of humanity that orbit has placed this red dwarf closer to the earth
than its companions.
So it was named Proxima Centauri, or just Proxima - our solar system’s true nearest
neighbour.
At first Proxima’s proximity was mostly a curiosity.
Red dwarfs are the smallest, coldest, and faintest of the stellar menagerie.
This one was barely a star at all, really, at only 12% of the Sun’s mass and only a
little larger than Jupiter.
Hardly exciting compared to its vibrant, Sun-like siblings.
But as the decades passed, we began to see how special Proxima really was.
The clue to its uniqueness was in its emission lines - sharp spikes in its spectrum resulting
from electron transitions in the atoms and molecules of the star’s atmosphere.
Proxima’s emissions lines seemed to shift back and forth from the wavelengths dictated
by the laws of physics.
And it took until … well, now, to finally understand the mystery of Proxima and the
true magnitude of the importance of the Alpha-Cen system.
In 2016, Spanish astronomer Guillem Anglada-Escudé and the Pale Red Dot team figured it out:
Proxima had a planet.
Or more technically an exoplanet - for extra-solar planet.
Planets don’t really orbit stars.
A planet-star pair mutually orbits its shared center of mass - its barycenter - which is
usually deep inside the star.
Planets make stars wobble, and that motion induces something called Doppler shift in
the star’s emission lines.
Essentially, the wavelengths of all the star’s light are stretched as the star moves away
from us and compressed as it moves towards us, causing emission lines to oscillate in
wavelength very, very slightly - just like with Proxima.
This way of finding exoplanets is called the radial velocity method.
So named because the Doppler effect is only produced by the component of the star’s velocity
that’s either directly towards us or away from us - in the “radial” direction.
We couldn’t see a planetary system that’s “face-on” - fully in the plane of the
sky.
But the more edge-on the orbits, the more motion there is in the radial direction, and
so the more chance of spotting the wiggling spectrum.
Now the most prolific method for finding exoplanets has been the transit method,
in which planets
are identified by their dimming of their parent star as they cross or transit the face of
that star from our perspective.
That restricts the transit method to planetary systems that are almost perfectly edge-on.
Nonetheless the Kepler mission found 2600+ exoplanets this way, and extrapolating from
that revealed that most stars host planetary systems.
Now that we know this, the radial velocity method can step in and potentially catch many
more systems - as long as they’re not perfectly face-on.
The radial velocity method is emerging as a competitor to the transit approach due to
rapid improvements in a class of highly specialized spectrograph.
OK, so, equipped with this cool new tool, our intrepid team of exoplanet hunters realized
that Proxima’s shifting emission lines made sense if the star was wobbling in a tight
circle, moving at about a mile per hour over a period of 11 Days.
That would then be the length of the year of a hypothetical exoplanet responsible for
that motion.
Such a short orbital period, combined with the star’s mass, gave them an orbital radius
for the exoplanet of around 20 times smaller than the Earth’s.
That sounds perilous, until you realize that Proxima’s energy output is nearly 600 times
lower than the Sun’s.
That places the new exoplanet exactly in the habitable zone of the star - just the right
distance for the intensity of the star’s radiation to potentially allow water to exist
in liquid form.
There was one more stunning calculation to come: with the distance between exoplanet
and star combined with the speed of the wobble, astronomers could calculate its mass.
It’s practically the same as the Earth’s.
Maybe 10-50% more massive, but almost certainly a rocky world, of the type sometimes inhabited
by bipedal apes.
Suddenly, after centuries of pondering the alpha-centauri system, it’s full significance
became clear.
The nearest habitable exoplanet orbited the nearest star.
Surely we’d identified the first port of call for our interstellar future.
The discovery of this exoplanet - Proxima Centauri B - or Proxima-B - was just the first
of this little star’s surprises.
In 2019 a new team reported the discovery of a second exoplanet, its signature also
embedded in the slow dance of Proxima’s emission lines.
Proxima C is much bigger than it’s earth-like neighbour, at 7 times earth’s mass.
It’s also way further out, with 5-year orbit at the same distance as Mars.
And then in 2020 a third exoplanet was tentatively identified.
The prospective Proxima D is just a quarter of Earth mass and orbits once every 5 days,
well inside the orbit of Proxima B. That’s interior to the habitable zone, making this
prospective exoplanet literally boiling hot.
Followup observations in January this year somewhat solidified the measurements of Proxima D.
And that brings us to today, approaching 2 millennia since Ptolemy’s observation.
We have a bona fide planetary system in at least one of the stars in the Alpha-Cen system.
And there are admittedly much more tentative detections
of planets around its Sun-like siblings - a
Neptunish body may have been imaged around Rigel Kentaurus and an Earth-sized body may
have transited Toliman on an orbit that would fry it to a crisp.
Those are quite speculative.
But Proxima B is almost certainly real, and so weirdly similar to the Earth.
But is it similar enough to have an atmosphere?
Oceans?
Life?
Let’s take a closer look.
One very important point is that Proxima B is probably tidally locked to its star.
It’s close proximity to the star means strong tidal forces, which will have forced the planet’s
rotation period to be in resonance with its orbital period.
The simplest case is for the length of the planet’s day to be the same as its year
- both 11.2 earth days.
That would keep the same side of the planet facing the star at all times.
One side baked and the other frozen, and perpetual twilight at the boundary.
This situation doesn’t sound conducive to life for many reasons.
For one thing, unless there’s a lot of atmospheric circulation, that permanent night could cause
the atmosphere to collapse, freezing to the surface.
But it may also be that the large temperature
differential drives powerful atmospheric convection
- planet-wide gales of extraordinary strength that could distribute heat from the day-side
to the night.
Or if the planet has significant oceans, these could also distribute heat.
It’s also possible for a planet to be tidally locked without a perpetual day and night.
Other orbital resonances can occur - for example, 2 days per year, 3 days per 2 years, etc.
Proxima B may be in one of these.
However it’s worth noting that any resonance besides permanent day and night would
result in
massive ocean tides, assuming the exoplanet has an ocean to be pulled and squeezed by
the nearby star.
On the other hand, we might actually hope for a 1-to-1 resonance.
In that case, the dark side might be the only survivable part of the planet, as long as
there’s enough atmospheric cycling to keep it warm.
Why?
In order for life to have a chance in this system, it needs to be protected from the
star itself.
Don’t let their size fool you; there’s nothing cute about red dwarfs.
They can be angry, violent little monsters.
Proxima is no exception.
It’s what’s known as a flare star.
Massive convection through the star’s body generates crazy magnetic storms, which can
cause the star to have powerful outbursts - flares - that blast high energy particles
and radiation through the planetary system.
During flares, Proxima B is blasted with X-rays and ultraviolet light and high energy particles.
A sufficiently thick atmosphere and strong planetary magnetic field could in principle
protect any surface dwellers, who would then get to enjoy pretty spectacular auroras.
Auroras which may even be visible from Earth by a near-future telescope.
Despite this violence, Proxima really is faint.
At Proxima B’s location there’s enough light to keep water liquid but most of that
is infrared.
There’s far less visible light, so to our eyes the star would appear very dim.
Photosynthesis in these conditions would be difficult.
Native foliage would probably be black for maximum absorption, and may have to rely on
alternative photosynthetic pathways that can make use of the infrared light.
By the way, unlike the Sun which grows brighter over time, red dwarfs fade.
In order for Proxima B’s hypothetical oceans and atmosphere to have survived this earlier,
hotter period it would have to have formed further out and then migrated inwards.
That’s not crazy, because planetary formation models indicate that there wouldn’t have
been enough material so close to the star for it to have formed in that position.
Nonetheless, we’re focusing in on a relatively stringent set of requirements for this
exoplanet to be able to support life.
But even if it’s a stretch, we need to imagine all the ways that life could have formed there,
because that’ll help us build the instruments needed to detect signatures of life.
We’re now building and planning a new generation of giant telescope - 30 to 100 meters in diameter,
which should be sensitive enough to detect emission lines from molecules in Proxima B’s
atmosphere, if it has one, or even the light reflected from the planet’s surface.
Both of these may bare the characteristic signatures of life, as we’ve discussed previously.
Ultimately we would want to visit our neighbour to pay our respects.
In fact, the mission is already being planned.
It’s the breakthrough Starshot program - something we’ve discussed previously.
The plan is to use a giant laser array to accelerate a train of mylar light sails to
20% the speed of light in the direction of Proxima.
They’ll each carry a tiny computer chip sporting a miniature camera, to take fly-by
pics as they zip through the system 20 years after launch.
There’s no update on when this might launch, but not tomorrow.
Hopefully one day we’ll be sending people instead of computer ships, to study the system
and maybe even settle there.
After all, Proxima, with its many-trillion-year lifespan will far outlive our own Sun.
So let’s fast forward our story another few millenia.
A descendent of humanity stands on the black grassy plains of Proxima B, which sway in
the planet-wide gale.
Four lights are visible through the thick atmosphere - the gleaming white twins Tolimar
and Rigel Kentaurus, the glowering red orb of Proxima herself.
But there’s another white star on the horizon, but it’s slowly slipping away on its own
orbit around the Milky Way.
Perhaps this post-human Proximan will wonder if the stories are true - that people began
there, on a planet around Sol, Alpha-Centauri’s nearest neighbor across spacetime.
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Our supporters on Patreon give us the financial stability we need to keep making this show
- a stability much like the perpetual daytime on Proxima B, and the many-trillion year lifespan
of a red dwarf.
And today I want to make a special shoutout to Daniel Alexiuc, who’s supporting us at
the big bang level.
Daniel, we managed to find a loophole in the international space treaty and have claimed
ownership of Proxima B. As a sign of our enormous appreciation, we’re giving you first pick
of any continents discovered - assuming it has continents.
Offer is null and void if any sentient natives are found.
Travel expenses are not included.
Otherwise, it’s yours to do with it as you wish - name it, claim lordship over it, start
your galactic empire there, or just use it for weekend getaways.
Its just a small token of our gratitude for your support.
Thanks again, Daniel.
Today we’re doing comments on the last two episodes: the one on objective collapse theories,
where wavefunction collapse is explained as a real, physical process.
And the one on cosmic strings - topological defects that may span the length of the universe.
In the objective collapse episode we talked about some field “hitting” the wavefunction
to cause it to collapse, and Kadag asks what exactly is doing the hitting.
The answer is we have no idea.
It has to be a field that has a non-linear influence on the wavefunction.
I’m afraid it’s hard to give a satisfyingly intuitive description of what that means.
But in general it means different branches of the wavefunction are able to influence
each other, which is not the case in standard quantum mechanics.
Check out our episode on the many-worlds or Everett-Wheeler telephone for juicy details
on non-linear quantum mechanics.
And yes, one field that might be able to do this is gravity, in which case the wavefunction
is being “hit” by the non-linearities across the wavefunction introduce by spacetime
curvature.
And on the subject of gravity, Jan Wester asks why the curvature of spacetime can’t
be in a superposition.
Well we don’t know for sure that it can’t - just that if you try to have a superposition
of curvature you get unresolvable infinities at large curvatures that produce hopeless
contradictions between general relativity and quantum mechanics.
Essentially, quantum mechanics and quantum field theory assume a well-defined underlying
framework, upon which all the quantum weirdness can play - those are the dimensions of space
and time.
Even including relativity and the shifting nature of space into time and vice versa,
you still have a consistent if mutable underlying grid.
As soon as that stage itself becomes undefined, everything becomes much more difficult.
Resolving that has been the major work of all of our searches for quantum gravity and
theories of everything.
OK, on to cosmic strings.
A few of you asked how it is that a topological defect can decay.
When those cosmic strings radiate gravitational waves, how is the Higgs field supposed to
smooth itself out?
Two ways: if the loop shrinks itself down to zero size then the Higgs field phase angles
can match up.
Also, the version of the topological defect that I described in that episode - the simple
vortex of phase angles making a loop around the string - is the most simplistic version
of this defect.
In reality, the defects are probably more complicated.
Rather than being a simple knot in the field, imagine something more like a shoelace knot
- it can potentially untangle itself.
This is a bit of a crappy analogy, so maybe I’ll come back to it in another episode.
Brandon Munshaw asks if the Higgs phase angle something that could theoretically be measured?
Or are cosmic strings the only indication of a change in phase angle?
The answer is yes and absolutely no.
The phase angle is fundamentally unmeasurable - just the the phase of the wavefunction - it’s
a symmetry of the Higgs field and doesn’t affect the behavior of the field.
But the relative phase angle can in principle
be measured - that is, we can see the difference in the
phase angle between two points in space.
I don’t know how you’d do this besides observing a cosmic string, but I suspect there
are ways.
Doctor Scoot asks whether cosmic strings could explain the filaments and voids in the large
scale distribution of galaxies in the universe, and whether they could explain the cosmic
microwave background and dark matter.
So the answer is maybe to the first two - but only if cosmic strings influence the CMB.
These strings may have had sufficient density at extremely early times to leave an imprint
on the CMB- on the density fluctuations from the early universe.
Although that hasn’t been detected, at least not yet.
The later development of large scale structure is defined by those density fluctuations,
so in that sense cosmic strings could have influenced it - but not directly in the sense
of cosmic strings causing string-like structure in galaxy distributions.
As for dark matter - extremely unlikely - in our most accepted understanding of cosmic
strings there aren’t anywhere near enough of them to account for dark matter.
But seriously guys one of the weird things we talk about will turn out to be dark matter.
Or Aliens. And that's when our job will be done.
But let's hope it's never either. But wait what if aliens are dark matter!