- 33 years ago, (gentle music)
astronomers knew of precisely zero exoplanets,
planets orbiting stars other than our sun.
The vast majority of astronomers assumed they're out there,
it's just that they were too difficult for us to detect.
Flash forward to today
and over 5,000 exoplanets have been revealed,
and we now know that at least half
of all normal stars have planets.
Exoplanetary science is now one of,
if not the dominant field of astronomical research.
Chewing up large fractions of astronomy research funding,
telescope time, and graduate students
looking to make their mark.
Here's a question though,
why did we do it?
(gentle music continues)
A cynic might say that if you already believed
that these exoplanets are out there
then what was the point in looking for them?
And whenever I talk about the search for exomoons,
moons around these planets,
there are always a few comments like that here too.
So, far we have zero confirmed exomoons
and just two candidates that my team have identified.
But like exoplanets,
we have overwhelming confidence
that they must surely exist out there,
so does that mean that there's no point in looking?
By the same logic, then perhaps there's no point
in looking for extra terrestrial life either
or indeed a multitude of other hypothesized phenomena.
Hopefully, though you've already come up
with some pretty good reasons in your head
as to why this argument doesn't really hold water.
The fruit of the past 33 years of exoplanet discovery
was not really the existential confirmation.
If that was so, then we would've stopped after the first.
No, the fruit we're seeing
what other planetary systems were like,
how do they resemble our own?
How are they different?
How common are other planets?
And are there types of planets out there
which look nothing like the ones in our own solar system?
Put simply, it was and continues to be
a project of sheer curiosity about other worlds.
And how enriching that journey has been,
revealing planets in unexpected places
like dead stars, giant stars, and binary stars,
as well as diverse flavors
like Super Earths, Hot Jupiters, mini-Neptunes,
lava worlds, and ultra eccentric planets.
So, why look for exomoons?
It is not to merely confirm their existence,
but rather it is to reveal the rich treasures
which they will surely reveal to us.
The moons in our own solar system
are already staggeringly diverse,
just imagine what exomoons might behold.
After all, who could have predicted
the transformative impact that exoplanets have had
on the entire field of astronomy,
let alone NASA and even public discourse about science.
The truth is that exomoons would represent
an entirely new type of astronomical object,
and the potential rewards from their discovery
are frankly incalculable at this point.
So, that's my reason number one
as to why we must find exomoons.
But that's not very specific
and it could equally be applied
to many of the potential discoveries too.
So, let's walk through four other reasons
much more specific as to why looking for exomoons
is, in my opinion, essential.
Reason number two, Pandoras.
Anyone who's even remotely aware of science fiction stories
will surely be familiar with the trope of a habitable moon.
Probably the most famous example these days
is Pandora, the habitable exomoon
depicted in James Cameron's "Avatar" series.
So, is this just sci-fi or is it genuinely possible?
Essentially, these are Earth-like planets in every way
except for the fact that they orbit a gas giant.
So, yes, by construction
such a moon would almost certainly be habitable.
One concern though might be if the moons
get too close to the gas giant.
Since we know that Jupiter has intense radiation belts
that bathe its innermost moons
in showers of energetic particles.
For example, the innermost moon Io
receives 3,600 rem,
which is about five times the lethal dose for humans.
But this radiation level drops off
precipitously at wider orbits.
And in fact, Callisto receives less radiation
than the Earth does.
So, I don't think this is really a showstopper,
and in fact, I think that exomoons
might have habitability advantages
over their planetary brethren.
We often assume that habitable worlds
need a magnetosphere to protect them
from harmful charged particles and cosmic radiation,
but moons can actually borrow the field from their planet.
As moons orbit behind their planet,
they pass inside the planetary magnetosphere,
and thus can enjoy shielding for up to half their orbit
despite having no magnetic field themselves.
A second advantage that moons have
is that we do not expect them to get tidily locked
to their stars.
The most common stars in the universe
are diminutive red dwarves,
which the temperate zone is so close in
that any planets residing within it
would surely become tidily locked to their host star.
This means that one side of the planet
is always facing the star,
there's no day or night cycle.
A possible danger of this
is that the night side of the plaque could get so cold
that the atmosphere freezes solid.
Then, the dayside atmosphere rushes around,
it too gets cold and freezes,
soon leading to total atmospheric collapse.
Moons would actually have no such issue here though,
for if that planet had a moon,
the moon would eventually tidily lock
to the planet, not the star.
That means that both sides of the moon
would receive equal amounts of radiation still.
So, sorry Pink Floyd,
there's no such thing as the dark side of a moon,
but there is such a thing
as the dark side to some exoplanets.
So, hopefully it's clear
that moons appear totally viable as habitable worlds,
and we've not even discussed the possibility
of subsurface ocean worlds like Europa or Enceladus,
adding a whole other dimension to this discussion.
So, let me just better articulate
reason two just to summarize.
One of NASA's overarching mission goals
is to determine the frequency of habitable worlds
around other stars.
Indeed, that was explicitly the goal
of NASA's Kepler mission,
which unfortunately it failed to accomplish
for reasons we discussed in a previous video.
But regardless, this remains a central goal for NASA
because it informs,
in fact, I would even say it dictates
the design requirements of their future missions.
Because here's the thing,
if we ever want to take a photo
of a nearby Earth-like world,
then the size of that telescope
is going to depend on how close the nearest Earth is,
and that will obviously depend
on how common Earths are in general.
So, this isn't some cute academic question,
this has fundamental engineering and design consequences.
So, I would claim this,
any effort to calculate the frequency
of Earth-like worlds in the cosmos
cannot ignore the potential contribution of exomoons.
If habitable moons are out there,
they could well make up a large slice of the pie,
and perhaps even be the most common type
of living world in the universe.
By the way, you might notice that my setup
is a little bit different today
thanks to my new beautiful desk
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(gentle music continues)
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Next reason, number three, the Rare Earth hypothesis.
This is a topic which again we have discussed
in detail in a previous video,
but to summarize, Ward and Brownlee
propose that the Earth might be incredibly unusual
in having the right conditions
for a civilization such as ourselves.
Most relevantly, one of the key arguments concerns our moon.
Now, you may not know that our moon is kind of weird.
It's 1.2% the mass of our planet,
which is actually the largest mass ratio
for any planet in the solar system,
unless you count Pluto as a planet.
That freaky large moon is suggested to be vital
to our very existence because of three reasons.
First, the large mass
stabilizes Earth's obliquity against Jupiter's influence,
thus giving us a stable climate.
Second, the Moon is thought to have formed
through a giant impact,
which could have stripped off
much of the Earth's original lithosphere.
Without this impact,
the lithosphere may well have been so thick
that it formed a so-called stagnant lid,
devoid of any plate tectonics, much like Venus.
And of course, no plate tectonics means no carbon cycle,
which would be a very big problem for life as we know it.
Finally, after the impact,
the moon would've coalesced
just a couple of Earth radii away from us,
leading to huge continent covering tides,
entire land masses covered in rock pools,
which may well have been the birthplace for life.
As you can see, these arguments are somewhat speculative,
and that's why the Rare Earth hypothesis
remains a controversial view.
However, I don't think there could be any doubt
that the moon has clearly played an influential role
in the Earth's formation and evolution.
So, on that basis, I think it's fair to say
that when we detect the first Earth twin,
one of the very first questions we will want to ask
is does it have a Moon twin?
And this brings us neatly to reason number four,
why we need to find moons,
how the hell do moons form anyway?
You might think we have some pretty good answers about this,
and indeed, I could give you some,
but remember that 33 years ago
we thought that we had figured out planet formation too,
and just look what happened there.
We looked at the planets in our backyard
and we came up with this detailed intricate model
explaining their origins, evolution and even the exceptions.
Giant planets form beyond the ice line,
Mars gets truncated by Jupiter's migration,
Neptune flings out Kuiper belt objects, and so on.
It was all fairly neatly tied up,
but exoplanets just blew
all of those expectations out of the window.
The Copernican principle was broken.
Hot Jupiter's most famously exemplified this
because back then, essentially no theories
predicted their existence.
And still today,
they remain a puzzling phenomena to explain.
And in the same way, exomoons hold enormous promise
to teach us how satellites form.
As an example, for the moons co-forming around gas giants,
we expect tiny moons,
no bigger than 1/10,000th the mass of the host planet.
But critically, does this rule really hold elsewhere?
And then, we have the weirdos,
most notably our own moon,
which is thought to have formed through a giant impact.
And that raises the question, how often does that happen?
Is that a rare improbable circumstance
or is it the inevitable outcome
of terrestrial planet formation?
With just one data point, there's no way to tell.
So, the truth is that if we want to understand
our uniqueness and our origins
then we have to find other moons.
And finally, reason number five,
a real spicy kicker to finish us off.
In 2014, Hanno Rein and colleagues
published one of the most important exoplanet papers
in the last decade, in my opinion,
but one that has been massively overlooked.
Playing off the title
from Al Gore's infamous Climate Change documentary,
Some Inconvenient Truths about Biosignatures,
Rein and colleagues highlight how exomoons
might be absolutely central to our quest for life
in the universe for a wholly practical reason.
Right now, NASA and many others
are trying to figure out how we could build a telescope
that could take a photo of another Earth
with the current moniker
being the Habitable Worlds Observatory.
The idea is to build a space telescope so powerful
that it could suppress the light of a star so well
that the orbiting planets become visible,
a contrast ratio of a staggering billion to one.
For the second time in human history,
we will see the pale blue glow of a habitable planet,
but this time that light would be of a world
other than our Earth.
From that distant smudge of light,
the telescope will split the light up
into its constituent colors
and look for the chemical fingerprints
of biogenic molecules like oxygen and methane.
Now, that's not the only pair of molecules we can look for,
check out our early video on biosignatures for more on that,
but it is often considered as slam dunk combination.
You see, oxygen by itself could simply be water photolysis,
no life involved.
And methane alone could be non-biological too.
I mean, just ask Titan.
But oxygen and methane don't play nice together
and they quickly react.
I mean, just look at the SpaceX rockets.
And this implies that something or someone
must be actively producing them.
Other pairings are possible too,
but the basic idea is always the same,
two molecules that reside
in a so-called chemical disequilibrium with one another.
So, here's where Rein's paper comes in.
Imagine that we had an Earth-sized ocean planet,
but no life.
Imagine further, this planet had a Titan-like moon
with a methane-rich atmosphere.
So far so good.
Now, when our direct imaging telescope
veers towards this target and snaps that precious photo,
it will not see the planet and moon separately
for they are too close to one another to resolve.
Instead, it would just see
a single pale blue smudge of light,
light that contains photons
from both the planet and the moon.
Splitting that light up into its constituent colors,
the scientists would indeed see both oxygen and methane,
the classic biosignature pair,
and conclude that they had succeeded,
life had finally been discovered.
But they were all of them deceived.
In this story, there is no life
because remember, the oxygen
was merely the product of photolysis,
ultraviolet light splitting water up
in the planet's atmosphere
and the methane was never part of the planet
in the first place, but rather was bound to the moon.
We would've been tricked by the universe, by exomoons.
To me, this underscores just how crucial
the question of exomoons is.
We simply cannot ever hope to conduct this experiment,
one which NASA will likely pour tens of billions of dollars
into one day unless we first figure out
what is it with the moons?
To answer these questions,
we need a programmatic effort to survey
and measure the occurrence rate of moons around exoplanets.
How often are they out there?
How big do they get?
What sorts of planets typically have them, and how many?
These are all questions
that we can, in fact, currently answer today
if we just tried.
As many of you likely know,
exomoons is a career-long passion for me.
And so, some of you might reasonably think
that perhaps I'm being biased in this discussion here.
But in this video,
I just want to explain to you
why it is that I have committed
so much time to this question
and why I genuinely believe
that these questions are paramount
to the goals of the exoplanet field more broadly.
In a recent video,
I shared how we propose an exomoon search with JWST
that was sadly shot down,
which I think is now our fifth proposal in a row declined
to try this work on big space glass.
JWST will not be around forever,
slowly depleting its fuel and cryogen
and risking electrical glitches
or even asteroid impacts every second it's up there,
and yet, it is unquestionably capable
of delivering us these answers,
the first ever telescope which could find analogs
to the moons in our solar system.
If we don't ever try,
if we never even attempt the experiment,
I am convinced that we will regret that decision
for years to come.
So, yes, as strange as it sounds,
exomoons really do matter,
and I suspect in a much more profound way
than many of you would've guessed
before watching this video.
So, hopefully together we can make it happen
and open up a new window into the cosmos.
So, as always, stay thoughtful and stay curious.
(gentle music continues)