The galactic recipe for a living planet

So I'm pretty sure that I'm not
the only one in this room

who at some point have found myself,
you know, looking up towards the stars,

and wondered, you know, "Are we it,

or are there other living planets
out there such as our own?"

I guess it is possible
that I'm then the only person

who has obsessed enough
about that question

to make it my career.

But moving on.

How do we get to this question?

Well, I would argue the first thing to do

is to turn our eyes back down from the sky
to our own planet, the Earth.

And think about just how lucky
did the Earth have to be

to be the living planet it is.

Well, it had to be
at least somewhat lucky.

Had we been sitting closer to the Sun

or a bit further away,

any water that we have had
would have boiled off or frozen over.

And I mean, it's not a given
that a planet has water on it.

So had we been a dry planet,

there would not have been
a lot of life on it.

And even if we had had all the water
that we have today,

if that water had not been accompanied

by the right kind of chemicals
to get life going,

we would have a wet planet,
but just as dead.

So it's so many things that can go wrong,

what are the chances that they go right?

What are the chances that the planet forms

with at least the basic ingredients needed

to have an [unclear] of life happening?

Well, let's explore that together.

So if you're going to have
a living planet,

the first thing you're going to need

is a planet.

(Laughter)

But not any planet will do.

You're probably going to need
a rather specific and earthlike planet.

A planet that is rocky,

so you can have both oceans and land,

and it's sitting neither too close
nor too far away from its star,

but at the just-right temperature.

And it's just right
for liquid water, that is.

So how many of these planets
do we have in our galaxy?

Well, one of the great discoveries
of the past decades

is that planets are incredibly common.

Almost every star
has a planet around them.

Some have many.

And among these planets,

on the order of a few percent
are earthlike enough

that we would consider them
potentially living planets.

So having the right kind of planet
is actually not that difficult

when we consider that there's
about 100 billion stars in our galaxy.

So that gives you about a billion
potential living planets.

But it's not enough to just be
at the right temperature

or have the right overall composition.

You also need the right chemicals.

And what the second and important
ingredient to make a living planet is --

I think it's pretty intuitive --

it's water.

After all, we did define our planet
as being potentially living

if it had the right temperature
to keep water liquid.

And I mean, here on Earth,
life is water-based.

But more generally,

water is just really good
as a meeting place for chemicals.

It is a very special liquid.

So this is our second basic ingredient.

Now the third ingredient, I think,

is probably a little bit more surprising.

I mean, we are going to need
some organics in there,

since we are thinking about organic life.

But the organic molecule

that seems to be at the center
of the chemical networks

that can produce biomolecules
is hydrogen cyanide.

So for those of you who know
what this molecule is like,

you know it's something
that it's a good idea to stay away from.

But it turns out

that what's really, really bad
for advanced life forms,

such as yourselves,

is really, really good
to get the chemistry started,

the right kind of chemistry
that can lead to origins of life.

So now we have our three
ingredients that we need,

you know, the temperate planet,

water and hydrogen cyanide.

So how often do these three come together?

How many temperate planets
are there out there

that have water and hydrogen cyanide?

Well, in an ideal world,

we would now turn one of our telescopes
towards one of these temperate planets

and check for ourselves.

Just, "Do these planets have water
and cyanides on them?"

Unfortunately, we don't yet
have large enough telescopes to do this.

We can detect molecules
in the atmospheres of some planets.

But these are large planets

sitting often pretty close to their star,

nothing like these, you know,
just-right planets

that we're talking about here,

which are much smaller and further away.

So we have to come up with another way.

And the other way that we have
conceived of and then followed

is to instead of looking
for these molecules

in the planets when they exist,

is to look for them in the material
that's forming new planets.

So planets form in discs
of dust and gas around young stars.

And these discs get their material
from the interstellar medium.

Turns out that the empty space
you see between stars

when you are looking up towards them,
asking existential questions,

is not as empty as it seems,

but actually full of gas and dust,

which can, you know,
come together in clouds,

then collapses to form these discs,
stars and planets.

And one of the things we always see
when we do look at these clouds

is water.

You know, I think we have a tendency
to think about water

as something that's,
you know, special to us.

Water is one of the most abundant
molecules in the universe,

including in these clouds,

these star- and planet-forming clouds.

And not only that --

water is also a pretty robust molecule:

it's actually not that easy to destroy.

So a lot of this water
that is in interstellar medium

will survive the rather dangerous,
collapsed journey from clouds

to disc, to planet.

So water is alright.

That second ingredient
is not going to be a problem.

Most planets are going to form
with some access to water.

So what about hydrogen cyanide?

Well, we also see cyanides
and other similar organic molecules

in these interstellar clouds.

But here, we're less certain
about the molecules surviving,

going from the cloud to the disc.

They're just a bit more delicate,
a bit more fragile.

So if we're going to know
that this hydrogen cyanide

is sitting in the vicinity
of new planets forming,

we'd really need to see it
in the disc itself,

in these planet-forming discs.

So about a decade ago,

I started a program
to look for this hydrogen cyanide

and other molecules
in these planet-forming discs.

And this is what we found.

So good news, in these six images,

those bright pixels represent emissions
originating from hydrogen cyanide

in planet-forming discs
hundreds of light-years away

that have made it to our telescope,

onto the detector,

allowing us to see it like this.

So the very good news

is that these discs do indeed have
hydrogen cyanide in them.

That last, more elusive ingredient.

Now the bad news is that we don't know
where in the disc it is.

If we look at these,

I mean, no one can say
they are beautiful images,

even at the time when we got them.

You see the pixel size is pretty big

and it's actually bigger
than these discs themselves.

So each pixel here

represents something that's much bigger
than our solar system.

And that means

that we don't know where in the disc
the hydrogen cyanide is coming from.

And that's a problem,

because these temperate planets,

they can't access
hydrogen cyanide just anywhere,

but it must be fairly close
to where they assemble

for them to have access to it.

So to bring this home,
let's think about an analogous example,

that is, of cypress growing
in the United States.

So let's say, hypothetically,

that you've returned from Europe

where you have seen
beautiful Italian cypresses,

and you want to understand, you know,

does it make sense to import them
to the United States.

Could you grow them here?

So you talk to the cypress experts,

they tell you that there is indeed

a band of not-too-hot, not-too-cold
across the United States

where you could grow them.

And if you have a nice,
high-resolution map or image like this,

it's quite easy to see
that this cypress strip

overlaps with a lot of green
fertile land pixels.

Even if I start degrading
this map quite a bit,

making it lower and lower resolution,

it's still possible to tell

that there's going to be some fertile land
overlapping with this strip.

But what about if the whole United States

is incorporated into a single pixel?

If the resolution is that low.

What do you do now,

how do you now tell whether you can grow
cypresses in the United States?

Well the answer is you can't.

I mean, there's definitely
some fertile land there,

or you wouldn't have
that green tint to the pixel,

but there's just no way of telling

whether any of that green
is in the right place.

And that is exactly the problem
we were facing

with our single-pixel
images of these discs

with hydrogen cyanide.

So what we need is something analogous,

at least those low-resolution maps
that I just showed you,

to be able to tell whether there's overlap
between where the hydrogen cyanide is

and where these planets
can access it as they are forming.

So coming to the rescue, a few years ago,

is this new, amazing,
beautiful telescope ALMA,

the Atacama Large Millimeter
and submillimeter Array

in northern Chile.

So, ALMA is amazing
in many different ways,

but the one that I'm going to focus on

is that, as you can see,
I call this one telescope,

but you can there are actually
many dishes in this image.

And this is a telescope
that consists of 66 individual dishes

that all work in unison.

And that means that you have a telescope

that is the size of the largest distance
that you can put these dishes

away from one another.

Which in ALMA's case are a few miles.

So you have a more
than mile-sized telescope.

And when you have such a big telescope,

you can zoom in on really small things,

including making maps of hydrogen cyanide
in these planet-forming discs.

So when ALMA came online a few years ago,

that was one of the first things
that I proposed that we use it for.

And what does a map of hydrogen cyanide
look like in a disc?

Is the hydrogen cyanide
at the right place?

And the answer is that it is.

So this is the map.

You see the hydrogen cyanide emission
being spread out across the disc.

First of all, it's almost everywhere,

which is very good news.

But you have a lot
of extra bright emission

coming from close to the star
towards the center of the disc.

And this is exactly
where we want to see it.

This is close to where
these planets are forming.

And this is not what we see
just towards one disc --

here are three more examples.

You can see they all show
the same thing --

lots of bright hydrogen cyanide emission

coming from close
to the center of the star.

For full disclosure,
we don't always see this.

There are discs where we see the opposite,

where there's actually a hole
in the emission towards the center.

So this is the opposite
of what we want to see, right?

This is not places where we could research

if there is any hydrogen cyanide around
where these planets are forming.

But in most cases,

we just don't detect hydrogen cyanide,

but we detect it in the right place.

So what does all this mean?

Well, I told you in the beginning

that we have lots
of these temperate planets,

maybe a billion or so of them,

that could have life develop on them

if they have the right ingredients.

And I've also shown

that we think a lot of the time,
the right ingredients are there --

we have water, we have hydrogen cyanide,

there will be other
organic molecules as well

coming with the cyanides.

This means that planets
with the most basic ingredients for life

are likely to be incredibly
common in our galaxy.

And if all it takes for life to develop

is to have these basic
ingredients available,

there should be a lot
of living planets out there.

But that is of course a big if.

And I would say the challenge
of the next decades,

for both astronomy and chemistry,

is to figure out just how often

we go from having
a potentially living planet

to having an actually living one.

Thank you.

(Applause)