Welcome. My name is Tom Nolan.

I'm the Associate Dean in the School of Science and Engineering, and I'm here to welcome you to our second Harrington STEM lecture this year.

I'm going to have Amy Bartholomew do our introduction of our speaker, but I just want to point out that we have one other.

Let me push the buttons here. Nothing's happening.

Our next talk is on November 15th.

Pollen Analysis as a tool for forensic analysis of trace evidence.

I think it's going to be another extremely nice topic that's appropriate for everybody.

If you're not coming to all of our talks, don't miss the next two.

Then we'll have information available on our website about the spring seminars.

There's three of those. I'm not going to go through all of that right now.

Right now, I'm going to introduce Amy Bartholomew from the Astronomy and Physics department.

[APPLAUSE].

We need a little of adjustments here. Do you need anything else?

No. That's it here.

No. You don't. Awesome.

Hi, everybody. Thank you for coming.

I'm just going to to introduce our speaker today.

Caleb Scharf.

He is the Director of the Columbia Institute.

Sorry. Columbia Astrobiology Center.

He received his bachelor's degree from

Durham University in physics and then went on to get his PhD in astronomy from the University of Cambridge.

His original background is in x-ray, astronomy and observational cosmology, but he has more recently turned to studying life in the universe, extrasolar planets and astrobiology.

He has quite an extensive record of peer-reviewed literature and articles and he also has written a textbook for undergraduate astrobiology courses.

He additionally has written two popular fiction books that you might like to look at called Gravity's Engines: The Other Side of Black Holes.

[inaudible 00:02:10].

[LAUGHTER]. Yes. Thank you. Yeah. Fiction books, nonfiction [LAUGHTER] and like a friend of his, [inaudible 00:02:16] sorry, yes, they're popular nonfiction books [LAUGHTER] and in addition to that, he does a lot of popular publications and he does write a blog and it's published on the Scientific American website.

You can follow him there for more information.

Without further ado, we are very pleased to welcome Caleb Scharf.

Thank you very much. I had to correct you on the fiction, nonfiction.

[LAUGHTER] I mean, it'd be lovely to be a fiction writer, but thank you.

Well, it's a great pleasure to be here this afternoon.

Thank you very much for having me.

Today, I'm going to spend some time taking you through a bit of an overview of this field we call astrobiology, which I termed the science of life in the universe.

I'll also plunge into a few details on some of the work that I'm involved in and some of the ideas that I think are bubbling up for the near future.

Astrobiology is an interesting field, and it's interesting because the questions beneath it, the questions driving it, the questions that are deep questions for our species and having deep questions for a long time, even in a non-scientific context.

All of astrobiology, I think can be summed up,

Let's see if my remote works.

Yes. No, there's a delay, in these two questions,

"Are we alone and where do we come from?"

Simple questions, but they are the root of astrobiology.

To start with, I'll spend a little bit of time on the, "Are we alone?" question, and later on I'll drift somewhat into the "Where do we come from?" question. Again, these are questions that now can be tackled with the rigor of science.

They're also questions that I think even at an emotional level, matter to our species.

Let me plunge right in and try to deal with question number one with a little bit of rigor.

To do that, I'm going to talk for a moment about this idea of Bayesian Analysis.

This question, [LAUGHTER] all right, I'm not going to use that.

I'm just going to because it will keep revealing all my secrets.

[LAUGHTER] [NOISE] This question of so, okay, life has happened here. What does that tell us about the odds of life happening elsewhere?

If you stop someone on the street and ask them, do you think we're the only life in the universe?

I wouldn't necessarily recommend it, especially in Manhattan, you might get some very strange looks.

But if you do that very often the answer is, for people thinking about it is, well, look it's a huge universe and we're here, so yeah, sure, there's got be something else out there.

The chances of us being the only example of life, when I say us, it's the sort of royal we, it's all life on earth.

But is that true?

Does our existence really tell us very much of anything?

In science you can ask this sort of question in a Bayesian framework.

Bayesian's probability with Bayesian analysis is a way of weighing evidence against models and gauging your confidence in the correctness of a particular model.

You can ask the question in that way and the data that you have, it's really pretty simple.

The data is that life has occurred here on the Earth, and it actually seems to have started up pretty early on in the Earth's history, not that long after the last final formation epoch of the planet.

We need a few hundred million years of the planet settling down into something close to its present physical state.

Is it that early start to life on Earth and the existence of life here in the big universe, is that evidence for what I've called a high abiogenesis probability?

Is life going to happen pretty easily?

Well, if you do this properly, mathematically, the answer, perhaps not too surprisingly, is well, not really, doesn't tell you anything.

It doesn't tell you very much.

It's one data point.

Of course that should have told you all along that it wasn't going to tell you very much.

The answer you get out is acutely dependent on the assumptions you make in the first-place, the prior that you put into this analysis.

In fact, life could be very numerous throughout the universe.

We could also still be the first example of life anywhere in the observable universe.

There is no constraint on the onset. But what we do learn from this analysis and this is the crux behind everything

I'm going to talk about this afternoon, is If you could just find one example of independent abiogenesis, one independent origin of life event somewhere, anywhere.

It would change the statistics significantly.

In fact, it would tell you that in a suitable environment, life is going to crop up once, at least once every billion years or so, which means that the universe would be full of life.

I would characterize modern astrobiology as being that quest, that search for that independent, abiogenesis event, the independent origin of life event.

Where can we look? Well, I'm going to very quickly summarize a number of things and I'm going to talk about some of them in greater detail.

The earth is actually our first port of call to look for an independent abiogenesis event because a couple of possibilities on the earth, first is this idea of shadow life.

I'm not going to go into that in any detail, but the idea of shadow life is what if there's some type of life here on the Earth, probably microscopic that just plays by different rules.

It would actually be quite easy for that to exist and for us to not noticed it yet.

I don't know what it is, but it could exist and that would be an independent example of life.

The other possibility is that life started many times on

Earth and in truly independent ways.

Maybe it started over there and it started over there, completely independent of each other.

That would also help us evaluate this probability, this universal probability of life occurring.

Mars is also a great option.

Mars is similar to the Earth in some ways and other ways it's completely radically different, but it is accessible to us and we could go and look for signs of past life on Mars.

We could even go and look for signs of life on Mars today, and I'll circle back around to that.

But one problem with the Earth and Mars [NOISE] is proving that you've really found something truly independent of our version of life, of our origin event, and that's in part because Earth and Mars have exchange material constantly over the last four billion years. Every time an asteroid hits Earth or Mars, it spews off chunks of matter into space that sit on orbits that eventually may cross the orbit of the other planet.

We know there are bits of Mars on the Earth.

There are almost certainly bits of the Earth on Mars.

They may have been able to carry organisms back and forth.

If you find life on Mars, it might be related to us.

In fact, some people even claim that

Mars is probably a better place to incubate life four billion years ago and that we're all Martians.

Quite serious people, I'm not kidding around.

It's going through this quickly.

There are also these icy moons in our solar system, which have really changed our picture of potential habitats for life.

Certainly in our solar system, presumably further afield in the universe, these icy moons contain, we think, interior oceans.

Oceans beneath thick crusts of water ice, liquid oceans and liquid water is one of those ingredients, at least for our type of life we feel is absolutely essential in some form.

We can go to these icy moons and again,

I'll circle back around to these and talk about it in more detail.

We can look at what's on their surfaces.

Some of these moons are active, they're actually spewing some of that internal ocean out into space, so we could sample that.

Maybe one day we could even go into the interior of these moons, and the nice thing about the icy moons is they're covered in tens of kilometers of thick ice.

If life could be incubated in one of these objects in our own solar system, the odds, I think, are better that it will be truly independent of us.

Again, this works well for this question of the universal abiogenesis probability.

There are other things in our solar system, other bodies, other places that we could look at.

Places like Titan or Venus, very strange places that

I'll say a little bit about Titan, Venus, the interesting thing about Venus is that if you go high up enough in Venus's atmosphere, you probably know that the surface of Venus is extremely hot and high pressure.

I keep knocking this mic, let me move it down a little bit and see if that improves things a little bit. [NOISE]

In the upper atmosphere of Venus, where actually if you go to a sufficient altitude, you can hit pressure and temperature pretty much the same as the pressure and temperature in this room, which is interesting.

Finally, the exoplanets.

I'll spend quite a bit of time talking about exoplanets, those planets around other stars that we have discovered in the last 20 years has been a revolution in science.

These planets and other stars really, if they have life, are definitely independent of us.

Exoplanets can be studied through atmospheric signatures remotely, this is all remote sensing, looking at the pigmentation or surface characteristics, or possibly techno signatures.

I will come to that towards the end of the talk.

I'm going to try to say something about everything in red, which is probably a little bit ambitious, but we'll see how we go. Mars, this is a picture of Mars.

The wonderful thing about Mars is you can go there, we're there right now.

This was taken not too many weeks ago by NASA's Curiosity Rover on Mars.

This is an outcrop.

The rover is currently making its way up this place called Mount Sharp, in the middle of a giant crater on Mars.

It's a mountain, and as it goes, it's looking at different geological eras and different geological material.

I still find these images astonishing.

This is another planet, it's another world, it's a whole world, it's a complicated world.

Mars has long been the target of our imaginations for life in the universe.

It's actually really interesting if you go back to the 1960s and 1970s and read up about what scientists were thinking before any spacecraft went to Mars.

They were pretty optimistic.

I can find a paper where even Carl Sagan is writing about, well, if there's vegetation on the surface of Mars, there will be some sort of herbivore consuming the vegetation, and so on. The first flyby quashed all of those hopes.

Because the first flyby mission showed that Mars had a very thin, tenuous atmosphere.

It's extremely dry.

There is nothing obviously alive on the planet.

So the focus in recent years, there's been an enormous amount of work done on Mars,

I can't possibly do it justice.

I'm just going to pick a couple of things to talk about.

The focus on Mars has been to look for the water and to look for the history of the water and chemistry of Mars, the deep history.

Because what Mars is today is almost certainly not representative of what Mars was in the past.

We have strong suspicions and now good evidence that Mars has had periods where it was much more amenable to life than it is today, had surface water.

Again, water is this magic ingredient that we look for.

Part of Mars exploration at the moment is in reconstructing the climate history of

Mars and the chemical history of Mars and where and when the water existed close to the surface of Mars.

Ingredients that we think go together to making it a potentially habitable place. I would say there's now clear evidence that there were periods of substantial water flowing or sitting on the surface of Mars at least a billion years ago or further back.

For example, on the upper right here is a picture.

You can't really see too much detail there.

Again, taken by the Curiosity Rover showing what is in effect a fossilized stream bed.

In that crusty looking material, lots of little pebbles, the size and shape distribution of those pebbles is entirely compatible with what happens to rocks being tumbled in moving water here on the Earth.

The chemistry of that material is also consistent with it having being put together in a wet environment.

Those little pebbles have now being cemented together in what is essentially silt, that was in this stream that used to flow on Mars a long time ago and there's more evidence like that.

Today, there's actually evidence of flowing water right now on the surface of Mars, except it's not exactly a river or a stream.

It's water that appears to be seeping down the sides of some crater wall.

This other image up here shows these features and are animated.

I think I have an animation here.

You'll see this is a time-lapse set of images across many months that there are these features.

What you're looking at, this is taken from orbit by the Mars reconnaissance orbiter looking down on what is really quite a steep crater wall, probably with an angle of about anywhere between 20 and

40-degree gradient of a crater wall.

During the late spring and early summer, these features appear and they seep down the crater wall.

They're several dozens of meters long, and they go away again during the winter. Now, what are they?

They happen all over the surface of Mars, so we've seen them where these black dots are.

This is superimposed on a map of Mars.

Those are the places where these recurring slope lineae are found.

It's not the easiest thing to roll off the tongue.

Those streaks they tend to be clustered towards the southern part of Mars.

It's not clear whether that means anything very much, it's hard to see them.

There's a lot of observational bias in this data.

But recently, again using the orbiter and being able to do optical spectroscopy looking at the reflected light from these features, what they find is that the light that's being reflected from these features shows evidence of hydrated salts.

Wet, salty water.

That salt is stuff like magnesium perchlorate and magnesium chloride.

It's incontrovertible, I think, that there is water there.

Perhaps it's just below the surface or it's seeping along.

Where is it coming from?

It's coming from somewhere up the side of the crater walls.

We don't know whether it's coming from aquifers, large bodies of water, or whether it's some seasonally varying ice deposit.

But at certain points today on Mars, there is salty liquid water close or at the surface, which is pretty extraordinary.

These are the kind of places that we'd love to go to with instruments capable of looking for life.

I'll just throw this in as well.

In addition to looking for water, of course, people are interested in finding more complex chemistry, signs of organic chemistry that could have been produced by past life. I think there's increasing evidence of organic deposits, meaning just carbon chemistry that could be consistent with material laid down by living organisms a long time ago.

This evidence comes out of looking at fossilized lake beds on Mars.

The problem on Mars is it's constantly getting hit by meteorites.

Unlike the Earth, which has a thick atmosphere and basically burns up lots of meteorites, on Mars, more material gets down to the surface and it's carbon-rich.

So there's a problem of deciding whether what you're finding in organic matter is truly indigenous to Mars or whether it's just rained out of primitive Solar System material.

So Mars is an ongoing expedition.

In the future, in the relatively near future, for example the NASA 2020 rovers are now talking about sending a microscope capable of looking for fossils.

Specifically looking for fossils in Martian samples, we haven't really done that properly before.

That's a little bit of Mars.

Let me get to why I think it's actually perhaps even more exciting.

Actually, it's definitely more exciting than Mars. Mars is wonderful and maybe we'll find stuff there.

But these icy moons are really interesting and really strange.

They really turn around our ideas of where you might look for life in the universe.

So a couple of pictures up here.

On the left is Europa, icy moon around Jupiter.

For a long time there's been evidence that it has an interior ocean.

That evidence comes from the surface morphology, it comes from induced magnetic fields sensed as spacecraft have gone by the moon.

Enceladus, the two images of it on the right is a moon around Saturn.

What's extraordinary about Enceladus is, first of all, it's spouting geysers of water and you can see those in the image at the bottom, and I'll show you another image in a moment.

There's geysers appear to come from these crack like features, the stress like features called tiger stripes on the southern part of Enceladus.

Enceladus is only about 300 miles across.

So for this small object to be this active and to have an interior ocean is still,

I think, a real puzzle. At least it's really surprising because small objects, as you know, the surface area to volume ratio increases as you get smaller, so you cool down quicker.

An object like this should have cooled down long ago, should have frozen solid, but it clearly hasn't.

And even Europa shows signs of active release of water into the environment.

The data is pretty crummy compared to the data from Enceladus.

But the advantage of Enceladus is, right now we have a spacecraft, the Cassini mission, there, taking these images.

These other images are taken from the earth.

So on the left for Europa, there's an image as Europa passes in front of Jupiter.

You can see it has a dark smudge to its southern polar region, we think that maybe water geysers is coming out of Europa.

And then on the right, the blue image is actually ultraviolet data taken by the Hubble Space Telescope.

Stuff is going on these moods.

So they're an increasingly interesting subject for astrobiology because they've got water and we find that water is salty.

The water has minerals in it.

That means it's been in contact with rock.

Water and rock is one of those things in our list of magic ingredients that we think is going to be important for starting life up.

It's also where you get chemical energy from the inorganic compounds in rock.

And when you dissolve those into water you can do all sorts of good stuff.

You can extract chemical energy, and that's what life does.

The water is salty.

We see that on the surface of Europa, we see it indirectly in the plume, water coming out from Enceladus.

Those plumes are a couple of 100 miles high.

They jet up because the surface gravity is so low.

I put some organics,

I think the evidence for that is still a little dicey.

There are some more complicated molecules in that water and there's probably ammonia too.

For a while, it wasn't clear whether little Enceladus really had a global ocean inside it, because it's so small.

It was thought that it probably just had some pocket ocean or a lake, subsurface lake in southern polar region.

But some really exquisite data from the Cassini mission, the space mission that's currently in orbit around Saturn, seems to lay rest to that question and suggest that

Enceladus really does have a full-blown ocean and its interior.

The way this works,

I'm just telling you this because it's cool aside and how you do these things.

Basically, you watch the surface of Enceladus in relation to the rest of the universe, you see how it moves during its orbit.

The orbit of Enceladus is slightly elliptical around Saturn.

What that means is that the moon will wobble a little bit as it goes around the side, it's in a state of tidal lock where it's always facing sat in the same way, but it shifts a little bit about that facing position.

Now, if it was a solid object, that movement, that liberation would be really tiny.

But what they were able to measure was a plus or minus 0.12 degree liberation , which also sounds really tiny, but it's not, it's huge. What it means is that the surface of Enceladus is somewhat decoupled from its interior, it's slipping around.

The easiest way to explain this, and actually the only way that really works is that it's got a global ocean.

The icy crust of Enceladus is sitting on top of that global ocean, wobbling as Enceladus moves around, so it has a full global ocean.

This is further evidence that the water in

Enceladus has been in contact not just with rock, but hot rock, the kind of situation we find that the bottom of the earth's oceans, these systems called hydrothermal vents, hydrothermal systems where ocean water is being circulated through warm, chemically rich rock in a volcanically active region, because Enceladus has a rocky core.

In the plumes, or rather, the plumes from Enceladus actually create the E ring around Saturn.

Its enormous ring that spans, goes all the way around Saturn, the E ring is distinct from the inner rings, in as much as it's made up of microscopic particles, not little chunks of ice but truly microscopic particles of water, salt and these silica nanoparticles.

Silica nanoparticles like these get made by hydrothermal vent systems on the earth.

We're not sure of any other mechanism that would make them for Enceladus.

And they fill out the E ring.

So the beautiful thing about Enceladus is it's probably got an ocean that's being siphoned through hydrothermal vent systems or at least has been in the past, and that ocean isn't just locked away inside Enceladus, it's actually out there spread around Saturn in the E ring.

So we can sample it.

Not only that, scientists working on this data have identified almost exactly where the geysers are coming out from the surface of Enceladus.

This is a 3D reconstruction, zoom in on one of those [inaudible 00:26:52].

The area where some of the geysers come out is just a couple of meters across.

There's evidence that as the water comes out, it's not just as a gas, that there are droplets that are literally droplets of water coming out, which means there's a direct connection to the ocean.

This isn't water that's just filtering up and then sublimating or evaporating out into space.

There is a direct pipeline to the ocean, or well, I say direct, it's probably a little wiggly, but you're getting real ocean water pumped up here.

It's such a tempting target.

We don't yet have any plans to go back and do anything about it.

I think a lot of people in the community are hoping that we will do something about it.

On a broader scale, the extraordinary thing about these ocean moons is it really flips around our concept of where the habitable parts of our solar system are, if there is life in places like this.

Because if you just do some crude calculation, you look at, well, how much water is there, liquid water is there in all the icy bodies in the outer solar system compared to the amount of water on the earth?

Well, you have about 15 times, at least 15 times the volume of

Earth's oceans in these outer solar system, icy bodies, in these dark interior oceans.

That's pretty extraordinary because if there's life in these places, it means that we've got it all backwards in the solar system.

The majority of the,

I don't know what the term would be, the solar system biosphere, if that were true, would not be on the Earth, it would be out there beyond the orbit of

Mars and beyond the orbit of Jupiter, which is pretty extraordinary, and it's food for thought about our parochial approach to things.

Look at the Earth and go, well it's got to be like that.

That's going to be a tropical island or somewhere in New York.

But it may not have to be.

Of course we don't know yet, but these are such tempting targets, especially for looking for truly independent abiogenesis.

I had to say something about Titan just because it's freaky and cool and interesting, and this is a little bit of a quirky thing to put up, but this is based on published research by respectable people. So Titan, another moon of Saturn, is the only moon in our solar system that has a dense atmosphere.

Dense atmosphere of mainly nitrogen, with a bunch of methane and other organics in it.

The surface pressure on Titan is about 1.5 of atmospheres, so it's not too bad.

The problem is, the surface temperature on Titan is 94 Kelvin, so it's pretty nippy, even by New York State standards during the winter.

This is a snapshot taken about 10 years ago.

Part of the Cassini mission dropped a probe into Titan's atmosphere called Huygens.

Huygens floated down on a parachute, landed on the surface, it's really extraordinary.

I encourage you to go and look it up.

There are even movies showing the images reconstructed as it floated down.

That's an image taken from altitude inside Titan's atmosphere.

What you're looking at, the textured crusty stuff is frozen water that makes the continents on Titan.

The darker gray stuff is a sea of liquid ethane and methane, liquid hydrocarbons at these temperatures. It seems very alien.

But the interesting thing about places like Titan is there's all sorts of chemistry you could do that could provide energy for living things, however you make those living things.

For example, the reaction of molecular hydrogen with acetylene can produce methane and does produce methane.

In these low temperatures, you've got to catalyze that reaction somehow.

They're not living temperatures, that would be a more explosive reaction.

These very low temperatures, you have to catalyze that kind of reaction.

Now the interesting thing is the data on

Titan suggests that there's a flux of molecular hydrogen in the atmosphere that's being formed by reactions in the upper atmosphere due to photochemistry.

So the molecular hydrogen is drifting down in the atmosphere.

There's also acetylene forming high in the atmosphere, drifting down.

But there's no sign of accumulation of either molecular hydrogen or acetylene at the surface of Titan, something is removing it.

It's reacting, it's doing something with it.

And here's the rather speculative and wacky, though I shouldn't say wacky, speculative idea.

Which is that this would make a really great metabolic process for life.

Because life could catalyze that reaction, producing methane, which would then go back into the Titan atmosphere.

When people have looked at how this might work, it all locks together in terms of the rate at which you would have to convert things to make the methane and so on.

Maybe there's stuff living on

Titan that is consuming all the hydrogen and acetylene.

We just haven't really looked at it, or looked for it.

It's something really funky, really weird, it's not going to be like us, but it's there.

Who knows? It's interesting though.

It's provocative and it's interesting.

Again, I think it's important that people think this way because it pushes us away from this parochial view of life in the universe.

Okay, I'm going to change gears to the exoplanets.

I thought a lot about whether I should put up fancy movies or animations about exoplanets because that's what everybody does.

Look, here's a spacecraft going to look at exoplanets, here a fantastic world's orbit.

I think, no I'm just going to put up a graph.

[LAUGHTER]

Because actually the graph is pretty impressive.

Since the mid 1990s, before then we didn't really know whether there were planets around any other stars in the universe though we expected there had to be some, but since the early mid 1990s, there's been this revolution in astronomy.

It's been brought about by persistence, but also improved technology where we now have detected not just a few planets around other stars, but thousands.

And in particular, one of the most successful surveys for planets is a NASA mission called Kepler.

It was a space telescope, it's still up there but it's doing something a little bit different at the moment, that stared at about 140,000,

150,000 stars for a number of years looking for planetary transits.

If a planet just by sheer geometric chance passes between you and its parent star, just because the orbits are arranged that way, it would block a little bit of light from the parents star.

You can measure that, and it's a very characteristic blocking of light.

It's a little miniature Eclipse.

You can use that to detect the planet.

It will tell you the rough size of the planet, the actual physical dimension of the planet, and if you wait for it to happen again, you'll get the orbital period of the planet, which are two important pieces of information to extract.

Kepler has been incredibly successful.

This plot even needs updating.

Just only about a year ago, between Kepler and other techniques, we now have over 3,500 confirmed planets around other stars.

Many of those are in multiple planets around single stars.

This is a plot of the size of those planets up the y-axis, in terms of Earth radii, versus the orbital period in days along the x-axis. I don't expect you to absorb all of that is just messy plot.

The main thing is shock and awe that there's an awful lot of points on this graph.

Two other comments, one is that you can see that at the low-size side on the lower left-hand part of this plot, there's all these planets that are smaller than the Earth, so we're detecting worlds between the size of the Earth and Mars.

Then there's still a gap of small things with these long orbital periods.

That's not a real gap, that is a systematic bias due to what this survey can and can't do.

It's missing data as far as we know, that bid of the graph is also really filled with planets.

We've detected all these planets, where does this get us to?

Well, I'll just say a couple of things about what you can begin to do with this data.

I think one of the most extraordinary things is just how diverse planets are.

We've been stuck in this rut of looking at our eight major planets. I will say, I like Pluto,

I'd like to include Pluto as a ninth planet, but now people are talking about this other planet in the outer

Solar System as planet ninth, so I just have to say eight major planets in our Solar System.

We've been stuck in this rut of staring at them and assuming they're the archetype for everything.

And it turns out they're not.

Just one sampling of a hugely diverse population of astrophysical objects.

I won't go through this in any detail, but there's all stuff out there but there are hot Jupiters, which are gas giants orbiting their star very closely.

There are these things called super-Earths, planets somewhere between the size of the Earth and the size of Neptune, but lots of those.

We don't have one in our own Solar System.

It's a very frequently occurring type of planet, except it doesn't exist in our Solar System.

There are rocky planets with huge hydrogen atmospheres, there are planets we think are largely water, and so on.

How do we get at some of this?

Well, I'll just show you. This is a little bit technical, but it's actually pretty easy.

Some of the data we get from planets, is on the planetary size.

You can get that from the transit data, the radius of the planet, that's the y-axis here.

And the planet mass we can get through a variety of other techniques.

When you have radius and mass, you can infer things about the mean density of a planet.

The mean density turns out to be a pretty good gauge of overall bulk composition of planets.

It's a little bit of a messy graph, but what you can see there the solid curves.

The solid curves are based on particular composition.

The line that says Earth at the bottom is of rocky planet.

The one up that says 50 percent water, that's a rocky planet with a lot of water, and so on, all the way up to the top where you've got planets that have larger and larger envelopes of hydrogen and helium.

Things that approach plants like

Neptune and Jupiter and so on.

The real point of this is that the dots, the colored circles are exoplanet observations, and they're all over the place.

Just to bring focus to that, if I draw a box here, the planets with masses between roughly Earth-mass and maybe three times the mass of the Earth, what I can see is in that relatively narrow range of masses.

You have this incredible range in composition, everything from a rocky planet to I didn't know what at the top.

Perhaps it's a rocky planet with an enormous envelope of hydrogen and helium, but it's a low mass planet with an enormous envelope of hydrogen helium.

We don't know exactly how that forms.

Diversity is extraordinary.

The numbers are extraordinary too and the statistics, because what we can do with all these planets is some goods statistics.

Going through this quite quickly, something like 50 percent of cool stars, these are stars slightly less massive than the Sun and those are the most numerous type of stars in our galaxy, at least 50 percent of those have roughly Earth-sized planets orbiting them.

Because orbits, in this case, are constrained to be quite small because of the data, chances are there planets further out as well but we don't know exactly what that distribution is.

These are conservative statistics.

I decided to pick the low end of the estimates.

Some of these estimates go up by a factor of two.

Per cool star or per star, its easier to say that, we expect at least 15 percent of those stars to have a roughly Earth-sized planet orbiting the star in the so called habitable zone.

That means they're orbiting the star,not too close to be too hot, not too far away to be too cold.

They're orbiting so that liquid water could conceivably exist on the surface of those planets, if it has an atmosphere and so on.

Which is pretty extraordinary.

That means that with high confidence there's at least one of these habitable worlds within a mere 16 light years of us, which in cosmic terms is just next door.

It means that there should be many potential analogs for the earth out there. Billions of them. Basically tens of billions of them.

This is at the forefront of the [inaudible 00:40:20] end of astrobiology, is trying to find and characterize these potential Earth analog planets.

There's a lot of stuff I could go into here,

I'm going to focus on something that

I've been involved in in recent years, that I think is interesting. [NOISE]

All these planets are out there.

Detecting them in the first instance is step number one.

But once you've said, "Well, there is a planet there that might be interesting," how do you go to the next step?

How do you decide whether or not it's a really good candidate for being a habitable planet, a candidate for being a true Earth analog and a place where life could have gotten going?

Well, becomes quite a tricky problem.

The problem is that planets are complicated and earth-type planets are extremely complicated.

It's because of things like climate, because that evaluation of the habitability of a planet is talking about the surface temperature of the planet. The Earth is only,

I was saying this to a number of people earlier, the Earth is only about 68 percent habitable on its surface area, average over time because we have ice caps and stuff like that.

If you don't understand the climate condition of a planet, how its atmosphere interacts with any ocean and how the chemistry of the planet works, and what the temperature distribution on the planet is, you can't pick the good candidates.

Even when you find them, interpreting any data that you get on those planets, data that might reveal their temperature or something about their chemical composition is going to be really hard to peel apart, unless you have a working physical model of the climate state of the planet.

So myself and a number of colleagues, people at the Goddard Institute for

Space Studies in Manhattan, that's an old NASA outpost in Manhattan.

We're building this thing called Rocky

3-D. Rocky 3-D is a state-of-the-art climate model.

It's a fully three-dimensional model of a planet that has an atmosphere that circulates like a fluid.

It has oceans, it has chemistry, it makes clouds.

It does radiative or radiation transport in the atmosphere in great detail.

It does a bit of photochemistry for light hitting the top of the atmosphere and so on.

The idea is to do for exoplanets what we can now more or less do for the Earth, which is run a climate model and ask what the current state of the planet is given the amount of energy going in from the nearby star, given what we know about the planet itself in terms of mass, rotation and so on.

It's really difficult.

It's difficult because it's a lot of software.

It's a million lines of code.

When we started this a while back, we decide to take a state of the art Earth's climate model and tweak it so that we could just on a pull-down menu say,

"I want to look at this type of planet,

" and it will pop out the result.

Well, it turns out it's not that easy, unfortunately.

When we started, we realized that this code had certain things hardwired into it that prevented us from doing it.

Stuff like 365 and 24 were hardwired into the code because why would you ever want to ask questions about planets other than the Earth?

I shouldn't do that. Climate scientists have a hard time these days.

One of the things we've been doing is saying, well, we have some idea about the climate history of our solar system over the last 4 billion years.

We can use that to help calibrate these models.

I should probably speed up a little bit and I hate it when people say I should speed up a little bit, but I probably should.

I'll show you two examples of what we've been doing.

We have a partly functioning climate model which is flexible enough that we can ask questions about alien worlds, about imagined worlds, and then see what they might look like.

We're using our solar system as a jumping place, a springboard to get to those really alien worlds.

Let's think about this habitable zone, where you can either be too far from the star or too close to the star.

If the planet gets too far from the star, it doesn't get enough energy and if it's got water, it's going to start freezing.

Water and climate have an interesting relationship because water tends to create a positive feedback loop.

Water, when it's liquid, has a certain reflectivity to radiation.

When water freezes that reflectivity goes way up.

It goes from about, in terms of fractional reflectivity or a thing called albedo for about 0.3 to 0.8.

So you get cooler, the water starts freezing, you start reflecting even more radiation, you get even colder, and you do this thing called a snowball.

Planets like the Earth tend to snowball.

We think the Earth snowballed at least a few times in it's deep history.

But we can use that as a proxy to ask questions about planets like the Earth that sit at the edge of their habitable zone, just far enough away from the star where they're in danger of freezing up.

What we did was say, well, let's take the Earth as it may have been half a billion to a billion years ago and that's why you can see the little outlines of continents on here.

Those are the assumed positions of Earth's continents that time ago.

Crank down the amount of carbon dioxide in the atmosphere, so there's 40 parts per million CO_2 rather than 400 parts per million CO_2, which is the current state thanks to us and also make the sun a little bit fainter, because the sun has gotten brighter over time.

In the past it was fainter because of the interior processes of fusion in stars, that's how it goes.

So actually four billion years ago, the sun was 30 percent fainter than it is now.

We have this model and we can run it and see if it agrees with geological data on what happened in these freeze ups in Earth's history and so on.

I'll just do that. It cranks through a few centuries.

What you can see is it gets pretty icy.

But the number to really pay attention to is that number on the bottom right-hand corner.

That's actually the mean atmospheric temperature at the surface of the planet, so it's essentially the surface temperature of the planet. By the time you reach an equilibrium state, what has happened is the whole planet has not frozen over.

That was surprising. That's actually different than what a lot of geologists were thinking.

The whole planet doesn't necessarily freeze over when you put it in this perilous cold place.

It freezes down to quite low latitudes, but not all the way.

But the average temperature, if you were an astronomer looking at this weird alien planet from a long way away and you manage to make an estimate of the temperature of the surface of this planet, you would find it was minus 12 Celsius.

You'd go away and say, "It's not habitable."

But clearly, a lot of it is and not only of that, from the case of the Earth, we went through periods like this and life, it may have had a hard time, but it didn't go away during those periods.

So we've got to think very carefully about interpreting the data that we get on exoplanets.

Let me scoot to the next thing here, which is the other extreme, making planets too hot. This was inspired by thinking about Venus in our own solar system; And in particular, the fact that Venus is a very slowly rotating planet, it actually rotates in the other sense that it orbits, but that's another question.

It's a very slowly rotating planet, and we were curious about what happens to climate when you just change the rotation rate of a planet.

So we can do that experiment.

I'll fill this graph out in a minute.

What's plotted here is the surface temperature on our simulated planet, it's an Earth that we take and we do things to it, versus the amount of stellar flux; the amount of solar radiation coming in.

The point on the lower left there is pretty much Earth today.

If you moved Earth today closer into the sun, we get more and more energy and the surface temperature would go up as this plot shows.

The increase in that kind of rate as you crank up the amount of solar input, it gets hot pretty quickly.

What happens if we slow the Earth down?

Well, here's a whole bunch of plots doing just that.

You take this planet and you slow it down.

It's nice to be God-like occasionally, just going to slow the planet down and see what happens.

These are the same kinds of plots where you take the planet and you're moving it closer to the star, but we're also, in each curve, we've slowed the planet by a certain amount.

You slowed it by 16 times,

64 times, and so on.

The bottom curve is what's called, tidally locked.

I mentioned this before, it's when the day length of the planet is equal to its year length.

In other words, it's like the Moon and the Earth, we always see the same side of the Moon because the Moon is tidally locked to us.

This is a state that we think is going to happen around many, many stars, especially these lower-mass, fainter stars where you've got to get closer to get enough energy to stay warm.

As you get closer, gravitational tides are going to have a better chance of putting you into this tidal lock state.

So, the same side of the planet always faces the star.

The remarkable thing is, even when that happens, you can still have a stable climate. But what we see is that, as you slow the planet down, the temperatures will diminish, compared to what they were for a faster rotating planet.

Some of that has to do with energy transport being more efficient as you slow down, but also the curves are slightly shallower. What is going on?

We did not know what was going on, but the beauty of these simulations, you go and look and that'll tell you what's going on.

If you have a planet with water and you do this, as it slows down, it starts to form a colossal cap of high altitude clouds on the side facing the star.

The high altitude clouds, condensed water, are very reflective.

It builds its own umbrella, and it just does it.

When we talk to other groups working on these kinds of problems, they also were seeing this phenomenon.

It's very, very interesting.

We haven't really found a planet that's doing this evenly on the solar system,

Venus is just too different.

But we think not only does this change where you think a planet can be and still be habitable, this is a potentially observable signature.

A big cap of cloud of one side of a planet is something you might be able to spot, even if indirectly using astronomical techniques.

I should really go through this a bit quicker. I'll just throw this out.

This is just because Proxima Centauri B, maybe the closest exoplanet to us.

There was a lot of talk about this recently.

The claim of detection of a planet around the nearest star to us, just 4.2 light-years away.

This is an artist's impression.

Proxima B, if it exists, would probably be one of these tidally locked planets.

These models are slowly rotating.

Planets could help us disentangle what's going on in a place like this.

This is a little technical, but I'll just throw it out, and then I'll get towards the end.

Biosignatures.

I've talked a lot about, without really specifying, how we're going to extract information about life on exoplanets that are so far away.

One option is, when they transit their parent star, when they pass between us and the parent star, the light of the star gets filtered through any atmosphere surrounding the planet.

In effect, that atmosphere will imprint its chemical composition spectroscopically on the light from the background star.

In principle, when planets transit, if you can get a spectrum of the star, you might be able to figure out what the composition of the atmosphere is.

The atmospheric composition is applied potential biosignature, the Earth's atmosphere, which has a lot of oxygen and stuff like methane in it.

Strongly suggestive of something like biological process.

It needn't always be that case, but it's strongly suggested, it's an out of equilibrium situation.

All I'm going to say, I mean, this plot is complicated.

The point of this plot, because I'm using up my time to it, is that even now when we have early examples where you can do this with super-Earth planets, in this case, a 6.5 Earth-mass planet.

What you find, the point of this plot is that the black dots do not line up with the three models plotted in solid lines.

All the models are hopeless. It just means that it's really difficult to interpret what's going on.

This planet may have lots of clouds in its atmosphere.

Clouds are opaque, they actually block different parts of the atmosphere, which is problematic because that's removing the transmission spectroscopy signal you want to get at.

We don't know how well this is really going to work.

I'd go so far as to say we may find some planets out there where we can do these sorts of experiments and it all shines through, and you see the oxygen, and the methane, and so on.

It's just wonderful.

But we still may have a hard time convincing ourselves.

We may have to do this statistically, we may need to use cruder data and a lot of examples and say, well, you know what, this center planets just look different than all these other planets."

What could be that difference?

That difference could be life.

How much longer do I really have? [LAUGHTER]

Well, I'll skip through some of this because some of this is getting into the fluffier, flakies aside of things, but it's fun. So it's fun at the end.

Technosignatures.

If finding chemical signatures of life can tough, and perhaps it's going to be tough to convince ourselves that really is life that is doing that.

Another option is to go the whole hog.

For you to find technological life, intelligent life.

But not only have we found life, but we've also learned something about how complex it's become.

People taught about looking for signs of pollution in planetary atmospheres.

In addition to biological pollution, you can have technological pollution.

We know all about that.

Artificial structures, there was a bit to do last year over a star which had some very strange properties showing very strained variations in light.

Someone made the mistake in talking to a news reporter and saying, well, one of the options is it's an alien megastructure.

Well, yeah, one of the options is that it's an alien mega structure.

But you shouldn't really say that to a reporter.

[LAUGHTER] We still don't know actually what's going on, so it's fascinating. But from the possibility that these transit signals include other stuff.

If there is an alien megastructure around a planet, when it transits, it's going to look really weird.

Then thermal output from energy conversion.

Life and technology take in low entropy energy and spits out high-entropy useless energy.

In other words, waste heat, thermal radiation.

If you have a big enough civilization, using your iPhones, or running your electric cars, or calling your Uber's or whatever.

You're going to produce increasing amounts of infrared radiation that you really can't do anything with, and that radiation could get to the point to astronomical signal.

Some people have even recently done the first thing which is to ask whether or not entire galaxies out there have been taken over by technology and have excess infrared radiation?

The answer is probably not.

But it's interesting that people sort of taking these things much more seriously and we have the capacity now to answer those questions fairly definitively.

It's fairly definitive that there is no galaxy-spanning civilizations within a few million light-years of us, probably, unless they're very eco-friendly.

[LAUGHTER] This is a little philosophical, but I think it's interesting.

It's full circle back to Mars, and it's the other side of the coin.

There's a very gloomy assessment that you can make about this quest to find another example of life.

It has to do with a thing called the Great Filter and the Fermi paradox.

Here's the argument.

Let's say we found an example of independent abiogenesis on Mars.

Another origin event happened on Mars either in the past or there's life there today, we would immediately know that the probability of abiogenesis is high.

There should be life happening all over the place.

But then where is everyone?

This is the Fermi paradox part of it.

We haven't seen any signs of life at all, and we certainly haven't really seen signs of technological life.

The argument goes that if life happens often, it will have happened a long time ago and by now that life, if even just some of it gets complicated and crazy enough, it will have populated the galaxy.

It doesn't need to travel faster than light, it just travels along and it fills up the galaxy.

That's the Fermi paradox.

If life is easy, then where is it?

Why hasn't done that?

Well, the answer that has been proposed is that one way of preventing life from doing that is if something goes wrong always, and it's called The Great Filter.

Life never gets past a certain point, it never really becomes an interstellar species or interstellar organism.

[LAUGHTER] The take on this, and this is a philosopher,

Nick Bostrom, is that finding life on Mars would be the worst news you could possibly get.

[LAUGHTER] Because we clearly haven't yet gone through that Great Filter. But it's coming.

I think it's interesting.

That's a really gloomy assessment of this.

More optimistic assessment is that,

I think we're much closer than we've ever been to finding some answers, either answers about technological life or probably more likely answers about microbial-type life or organisms altering the chemistry of their environments across the universe, either in our solar system or further afield.

This question of this Fermi Paradox here, why haven't we seen technological life?

I'll be optimistic.

I'll say actually we're a lot worse at seeing what's going on around us than we perhaps think we are.

The fidelity of our data has reached a point where it becomes reasonable to actually ask these questions, but it's still not good enough.

We don't know what scooted around in our solar system.

We don't even know if we've got a ninth giant planet in the outer solar system.

How could we miss that?

Well, quite easily, in fact.

And we only been around for a very short amount of time in cosmic terms.

It's possible there's a lot going on that we just haven't seen.

Last two slides. Again, coming full circle.

What about going at this from entirely the other direction?

If we want to understand the likelihood of life arising in any given direction, we should probably try to understand the "Where do we come from?" question. What is it that sets life in motion?

How does life originate?

How did it originate on the earth?

If we understood that from first principles, we could then probably make these predictions about the cosmic rate of life, how much life there is out in the universe.

So we can study the origins here. It's tricky.

I'll just throw this up because I'm proud of it and it's a silly little bit of research, and it was published in the proceedings of the National

Academy of Sciences recently.

I did a paper with a colleague, a chemist where we said,

"Well, you know what?

We could probably do something really stupid and begin to build a mathematical framework for how to connect what we understand about the mechanics of life."

We don't understand that much yet, we really don't.

Especially the mechanics of how life got going.

But can we not connect that to questions of the composition and environment of the entire planets? We think there's this connection; planet A is going to work for life, planet B is not going to work for life.

Can we connect that to what we think we know about biology and about the possible mechanisms of early life?

We started trying to make an equation and I won't go through this in detail.

It's a really simple, stupid, heuristic equation.

There's nothing profound in this, but it turns out that you actually can start to do this already by saying life is built out of stuff, building blocks, maybe they're molecules, maybe they're even just atoms.

And those building blocks, those molecules assemble somehow.

They have to end up in a certain form, in a certain structure in order for us to consider it life.

We can put some constraints on how many of those building blocks you need to make a single living organism.

You can put constraints on how much of a planet is available to do that.

The crust of a planet, yes, but what bits of the crust?

It's molecules dissolved in water, and so on. You can begin to reduce the numbers, you can begin to home in on just how much raw material you've got to experiment with, or for nature to experiment with.

What we do is, we hide all of the nasty, dirty details about everything we don't know in a probability, which is what physicists love to do.

[LAUGHTER] I don't know how this works, but I'm just going to assign a probability too.

That's what we did here.

The interesting thing is already, we can use that to say something about the probability of what

I called here an "assembly event."

Where you take some unspecified chemical building blocks and manage to make life or nature manages to start something that we consider to be a living system.

You can evaluate that probability per unit time, per unit set of building blocks, and you can use what you know about the earth and life on earth, to actually constrain that.

We think that for the earth, that probability is really small. Probability that somewhere on this barren planet, four billion years ago, the right set of bits got together and went,

"[NOISE] I'm alive" and scurried off.

The probability of that happening per your time is tiny, but it's offset by the fact that the planet is pretty big.

The planet is like a big chemistry search engine, so we think that offset may be a critical piece of this.

Anyway, astrobiology is very interesting, it's a really interesting time to be involved in it.

Watch this space. Thank you.

[APPLAUSE].

Let's take some questions and then you guys can [inaudible 01:04:45]

I have two questions, and I will make it quick.

[inaudible 01:04:56] I was wondering about Titan.

The formation of methane there.

[inaudible 01:05:02] hydrogenation [inaudible 01:05:06] is there enough energy at

94k to break that carbon-carbon bond?

I'm not specialized enough to answer that question, but I think that that is related to my mentioning the idea of a catalyst, you need something to help this reaction take place.

[inaudible 01:05:27] My second question and it's just speculation really, is if you were a betting man, where would you think we were, if we find life in our solar system

[inaudible 01:05:38] [NOISE] on Mars, on Europa [inaudible 01:05:43]

Yeah, it's a good question.

If I was putting money on it, where would I say we'd find life?

I'd have to say it's a toss up between Mars and Enceladus.

Europa is lovely, but Enceladus is the place that we know we have access to what's going on in its ocean, because that ocean is coming out and we could grab it, we could sample it, we could analyze it.

Mars, we've literally only scraped the surface on Mars.

If life exists on Mars today, it's going to be in the subsurface.

You don't have to go down very far in the subsurface to get more temperate conditions.

Just a meter or two, and the Martian regolith will get you to an environment where you're not being blasted by ultraviolet radiation, where there may be water, where the temperature is much steadier and warmer.

The immediate surface goes through great fluctuations of temperature.

If I had to choose between those, that would be a horrible choice.

I guess Mars is easier to get to

[LAUGHTER] and it's very interesting.

Why haven't we sent more of the experiments that were sent with the Viking mission in the 1970s?

Actually, it did wet lab experiments on the spot.

Part of it is, those experiments were very confusing because we didn't understand what the chemistry of Martian soil was like.

It's very reactive through these perchlorates.

But right now, I don't know why we're holding back so much on except that everyone wants to do it carefully and not get false positives in finding life on Mars.

But what you really want, you want something, a rover with a deep drill, a Bioassay Lab on it and a really good microscope, and then I think you would probably be able to say one way or the other.

[inaudible 01:07:53]

You say physicists?

[LAUGHTER] Physicist is a great filter.

This is like when they get to switch on the Large Hadron Collider, there were these rumors, "They're going to make black holes that are going to destroy the world."

Of course, if that happens, it's been happening all the time when the cosmic rays coming in from the galaxy.

What is the great filter?

[NOISE] Trump.

All kidding aside, that's got to be one of the options.

I mean, that's so interesting, so fascinating about our species as we can be so swayed by single individuals who can drive the direction of our culture and civilization, which is really weird.

It's horribly unstable situation.

What's the great filter?

It could be biological.

It could be horrible diseases crop up and collapse.

People talk about how civilizations can undergo collapse, and we've seen this in the past.

Civilizations have undergone collapse for a set of complicated reasons.

Climate changes, some cultural changes, maybe there's disease thrown in.

But to really eradicate that totally, I'm not sure.

I mean, it might have to be an asteroid.

That's going to happen one day, and maybe that has to happen everywhere, maybe habitable planets are always pounded by asteroids.

But if you're just not quick enough to get off world.

More questions?

I've exhausted everyone.

[LAUGHTER]

Thank you very much. [APPLAUSE]