Good afternoon, ladies and gentlemen. Welcome. You're very welcome to this Kinsey lecture.
This is the latest in the series of lectures which have been funded by the Hindu, the Family Charitable Foundation for some time.
I am particularly pleased today that we have Sir Michael and Lady himself with us.
It's very, very welcome. It's wonderful to see you again.
And they've sponsored these lectures now for a few years, and they've they've sponsored some other things in astrophysics as well.
But today, I'm particularly pleased to be able to announce a further gift from the charitable foundation.
So they have donated one and a half million pounds to establish the University of Oxford Centre for Astrophysical Surveys.
The centre will fund a team of researchers and graduate students.
That will be the focus of physicists at Oxford working on surveys to probe how galaxies evolve.
The nature of the dark matter and the dark energy that we think makes up about 95% of the energy of the universe.
And exploring the last frontier of astrophysics, the time domain, the so called transient universe.
Michael and Dorothy, we are extremely excited about what your gift will enable us to do, and we're extremely grateful.
Thank you very much. So.
So now we move on to the business of the day. So it's a we are privileged to have with us David Charbonneau, who is a professor at Harvard.
He did his undergraduate degree in Toronto and got his Ph.D. at Harvard, and he was awarded the Robert J.
Trump awarded the Astronomical Society of the Pacific for that work, which was on the trends,
the work on extrasolar planets focussed on transit transits of those planets.
So he characterises exoplanets using these distant using transits as the tool.
And in fact during his Ph.D. in 1999, he actually used a four inch telescope to make the first detection of an exoplanet, eclipsing its parent star.
This yielded the first ever constraint on the density of a planet outside the solar system,
finally confirming that things that had been seen in the wobbles of radial velocity were actually planets.
So this was a fantastic first.
Now, first, some things you have to be very careful about claiming, and this is one first, which David is universally acknowledged for.
But I you know, the amazing thing about David is that there is yet another first, which is that in a pioneering study,
he's used space based telescopes to undertake the first studies of the atmospheres of exoplanets,
using Hubble Space Telescope to study directly the chemical makeup of the atmospheres of exoplanets and finding the.
So this is obviously on the route to discovering habitable worlds.
Each of the projects of the many projects that David is currently engaged in
is aimed at looking for suitable places where there may be complex activity,
chemical activity and biological activity on exoplanets.
So it's my great pleasure to introduce the fast track to finding an inhabited planet.
David Charbonneau. Are you ready?
Okay. I want to begin by thanking the Hennessy family and the the University of Oxford for this for this wonderful opportunity.
I have a great visit here. I'm staying for most of the week.
And just the opportunity to talk about this exciting field to my colleagues has been wonderful.
I want to spend an hour and share it with you. I'd like to make the case that that we are alive at a very special moment in human history.
There is this incredibly exciting field which really was born in astrophysics,
but it is just about to move outside of astrophysics into the much broader scientific domain.
I'd like to tell you what we know and what we can hope to learn.
And I'd like to answer your questions. And we're going to begin at the end of the title.
This. This is not actually a period for the title. That's actually an image.
And then the image was taken about a year and a half ago. Did any of you see this with your own eyes?
Yes. What is that black dot in front of the sun? It's Venus, right?
So that's a real picture. And of course, Venus passed in front of the sun in 2004.
It did it again in 2012. If you missed it, I have terrible news for your unless you're very young, you're not going to catch the next one.
It's in about 103 years from now. But the transit of Venus really, I hope,
puts in your mind the geometry that I'm going to talk about today when we get to see planets pass in front of their stars,
both in the solar system and outside of it, we get unprecedented access to their physical properties.
And moreover, a very special opportunity opens up.
So in the case of Venus, it told us about the relative people, the actually the absolute sizes in the solar system, the size of Venus.
And then in 1761, something very interesting happened, which is that a Russian scientist by the name of Lomonosov was observing the transit of Venus,
and he noticed that just before Venus was entirely within the disk of the sun.
Here, we've zoomed in on Venus. Venus is the big black circle, and you're only seeing a tiny portion of the sun.
And what he noticed, if I just dropped the lights down a little bit here.
Is right here. That he could see illuminated the outer crescent of the planet, even though it wasn't in front of the star.
And what he concluded was that Venus must have an atmosphere.
We were seeing refraction in the atmosphere, and it's really one of the most remarkable,
first astronomical inferences based on this sort of observation.
So what I would like to do in the hour that we have is the following.
I'd like to first of all, describe to you the methods by which we can detect and really characterise planets.
I think that's important so that you can, in your own mind,
play with ideas about how we might do things differently and how we might improve in the future,
and what the fundamental limitations are to the way that we currently do things. I'd like to motivate why not just astronomers, but physicists,
and I would say broadly physical scientists might find a lot of this subject very intriguing.
I'd like to tell you about our current state of knowledge of small planets and in particular Earth like planets around other stars.
And then, as advertised in the title, I really want to present an opportunity for for very rapid progress,
particularly on the topic of upsetting Earth like planets and possibly even searching for life in the universe.
I want to I want to begin by acknowledging my research group.
These are the recent alums as well as the current group members.
I don't have time to share all of their results today, but I want to highlight a few individuals.
So recent graduates are Sarah Ballard and Zachary Berta, and then a long term member and now a permanent scientist.
On the Smithsonian side is Jonathan Irwin, as well as contributions from Francois first and Courtney dressing.
And then importantly, I want to acknowledge the funding, which is from the National Science Foundation,
from NASA, from the Packard Foundation, and from the John Templeton Foundation.
Okay. So how do we go and find planets?
Well, I know that you're not all astronomers, but perhaps you've met a few astronomers,
and you might find them to be, well, let's say, clever, but sometimes indirect people.
And so our methods are clever, but indirect. The first method really came about successfully in 1995, and that's the Doppler method.
So the idea is we don't see the light from the planet directly, but we can measure stars very accurately.
So we measure the apparent speed with which the star is moving towards us or away from us.
And we monitor that over time because the star is orbiting with the planet about the centre of mass.
Then the idea is, even though we don't see the planet, we can see that the star seems to be dancing with the companion.
We can deduce properties of the companion. In particular, we can deduce its mass and we can deduce their separation.
So measuring things with the Doppler method, measuring the stars allows us to deduce the planet mass.
The other method, of course, is the transit method, while the transit method is even simpler.
The transit method is simply that the planet passed in front of the star.
It blocks some of the light by measuring the relative amount of light that is blocked.
We can reduce the size of the planet compared to the star.
Now, why? I hope I hope you agree. These methods are straightforward and that's a really good thing.
Often in astrophysics,
we have to make very big assumptions about the interpretation of data because of course we can't play with it in the laboratory here.
Using nothing more than Newtonian gravity and geometry, we can deduce fundamental things like the density of a planet around another star.
And of course the density is really your first clue as to the competition. That's what the composition of the planet.
If the planet is massive and the density is low, then you might think you have a ball of gas.
So this would be a Jupiter if the planet is low mass, but high density, perhaps the same density as rock,
then you might think the planet is made of rock, for example, like the earth. Okay, so with only measured masses and radii.
Then already some very interesting questions arise.
And so here I'm showing you our state of knowledge as of oh, not too long ago, about three or four years ago.
And what I've done is I've taken all the planets that were known at that time.
So these are all the planets that have been discovered after roughly ten years of work,
but leaving out the most recent results, which of course, will be the real topic of discussion today.
And I plotted every planet as a red dot, and I'm showing you the radius of the planet, and I'm showing you the mass of the planet.
So just to orient you, this is five times the size of the earth.
Ten times the size of the earth. Jupiter is 11 times the size of the earth.
Okay, so there's Jupiter. This is the mass of the planet, this logarithmic scale.
One times the mass of the earth. Jupiter is about 300 times the mass of the earth.
So it sits right up there. Now many of you are physicists and you probably think you understand hydrogen.
So, for example, a planet like Jupiter is really a ball of hydrogen and helium.
And so based on the mass of that ball, you should be able to predict the radius that simply uses the equation of state of hydrogen and helium.
It's a basic physics problem. Well, unfortunately, exoplanets don't seem to to agree with us.
So one of the first mysteries that arose when we started finding large planets around other stars was that they were too big.
And this is a very major problem that remains unsolved to the current day.
These are the positions of the planets that we did that we do know about.
This is the size of an object as a function of its mass.
Something up here is somehow inflated. It's much puffier than basic gravity and the creation stage should predict.
So at large masses. The planets are larger than the equation.
State of a hydrogen helium mixture would wouldn't really physically permit.
So perhaps you might find that intriguing. Really? At lower masses.
We turn things around a little bit.
So as we start moving down in our discovery space, and that's what we've been up to over the last few years to find these lower mass planets.
What we're going to do now is we're going to use our knowledge of the equation of state,
by which I mean the relationship for a substance between its its pressure and its temperature and its density.
We're going to use our understanding of rocky materials and of ice so that when we make measurements about the size and the mass of a planet,
we can deduce its composition, figure out what it's actually made of. Based on our physical understanding of those materials.
So at low masses, when we measure the masses and sizes of planets, we can figure out what the planets are made of.
And that suddenly informs a very venerable physics problem, which is that of planet formation.
Right. We'd like to know how the planets of the solar system formed and how the planets around other systems formed,
whether the solar system is a common place or whether whether it is a rarity.
And then finally and I won't talk much about this today, but we can take it up later if you're curious.
The architectures of planetary systems have been astounding in their variety.
So I think it's fair to say that most astronomers thought when we when we headed out with our telescopes to find plants from other stars,
we would find copies of the solar system.
And the solar system has the big gassy planets and circular orbits far from the star or the small rocky planets and circular orbits close to the star.
And everything orbits in a plane. None of those things are true.
Around other stars, we often find the gas giants in close.
We often find the rocks farther away. And we find planetary systems which are orbiting roughly pole on compared to their stars.
So all sorts of interesting conundrums in terms of understanding how those systems could have formed.
But then to go even further.
The benefit of transiting planets is not only do learn about the physical properties of the planet, but you also learn about the atmospheres.
So you might imagine, well, why do we bother? Why don't we just take a photograph?
Let's go take a nice high resolution image of a star next to it.
We'll see a little point of light. That'll be the planet.
We'll take a spectrum of that planet and we'll start learning about the chemical composition.
We'd like to do that, but we can't.
The technology to go and image earth like planets around sun like stars that would require enormous mirrors in space.
It's far beyond our current technological capability. It might be something that we do, but it probably is at least 30 years away.
This talk is about the future, but it is about the immediate future.
And so what I'm going to outline today is is a is a fast track approach that uses the inspiration of the transit of Venus.
The idea is we don't have to wait for an image of the planet next to the star, but instead we can wait for the planet to pass in front of the star.
So when the planet goes in front of the star, some of the light passes through the atmosphere of the planet and then imprinted on that light.
What we can do is you can take a spectrum of the star before the planets in view.
You wait for the planet to come into view. You take the same spectrum and you and you subtract them.
And the new features, the new absorption features that appear in your spectrum tell you about the chemical elements,
the molecules and atoms in the atmosphere of the planet. Okay?
And that's the method that has been so productive, at least for larger planets for for the past decade.
And so what I won't talk about today are the enormous work that's been done to try to detect atoms and molecules,
even infer the presence of clouds and even winds. We can even learn about the weather patterns on planets orbiting other stars.
Okay. But I think I think we can also draw some additional inspiration from thinking about going after these sorts of ideas.
The first one is I've been I've been amazed at the really innovative optical experiments that have been designed by my colleagues in
laser physics and advanced high contrast ratio imaging and really pushing a lot of our understanding of light and how to control them,
manipulate light to to really go after some of these exciting questions.
So perhaps you find that intriguing. And of course, perhaps it's obvious, but I think I see in this geometry,
I see a way for us to go and make some progress on the question of life in the universe,
and I think would be thrilling to to make even some modest progress in that regard.
The implications of finding life in the universe really extend beyond astrophysics.
So, you know, you can I find often some astronomers say, well, that's that's not really our thing.
We you know, that's a little outside of our comfort zone. So I'm going to I'm going to stick with what's more comfortable.
I'd like to advocate instead that we are, as astronomers, really in a special position, because it falls to us to do the work,
because we know about building telescopes and gathering light and interpreting the data in the presence of stars.
But the implications of what we find are going to go much farther.
And I think ultimately I see a field perhaps ten or 20 years down the road where there are very active and exciting collaborations between
initially astronomers and planetary scientists and physicists to design and interpret the data that comes out of these experiments.
And ultimately with with biochemists and with origins of life, biologists and people have really studied the history of life on the earth.
So I would even predict that 20 years from now we'd have departments devoted to that topic.
Okay. Right. The field is moving very quickly.
How quickly? Here. Here is how quickly?
So if we look back just about a decade, there was one transiting planet and we had just learned how to study its atmosphere.
Zooming ahead to last year. The count is now that there are 350 such plants.
Well, actually, there's 3500. But if you're stingy about it, I'll give you the 350.
Those are the ones that appear as individual published planets in the literature.
But really, we're talking of at least four that 3000 or 4000 planets.
And the number is growing at an incredible rate. And we've studied the atmospheres now for almost a hundred of them.
The way that we've done those atmospheric studies is using the not not really building purpose instruments,
but actually using the most powerful telescopes ever constructed for for this for the science case.
And so, for example, the European very large telescope is an array of four telescopes,
each eight and a half metres, and it is arguably the most powerful observatory ever created.
It has done groundbreaking work, really getting at the atmospheres of these planets.
Similarly in space, we have the Hubble Space Telescope and the NASA's Spitzer Space Telescope.
Those because we're out in space and because you don't have to worry about looking up through the Earth's atmosphere,
you get a very clear view of the atmospheres of these other planets.
And those are really allowed us to make penetrating insights into the chemical compositions of these worlds.
Okay. So the story so far is that over the past decade,
we've gone from knowing nothing about planets around other stars to knowing a great deal about large planets,
both their densities and their atmospheric compositions.
Hmm. Let's think ahead. Well, many of us are currently engaged in building the next generation of large telescopes.
So, for example, the European community is investing €1 billion into building the European extremely large telescope.
This will be the largest and most powerful ground based telescope ever constructed.
It'll be an enormous collaboration between a community of perhaps what, a thousand scientists?
All told, it will be located in Chile. And just for scale, there's the telescope.
That's a truck. And we're really going to do this.
Okay, so what about space? Well, in space, €1 billion doesn't get you very far.
So the that the community, particularly NASA, is spending 8 billion USD to build the James Webb Space Telescope.
Now, I'm not I'm not bragging about these sums of money. We do a very good job, I think, communicating to the the community.
And by which I mean the non-scientist community, the broader community. But why these why these telescopes are so important for everything that we do?
What I'm going to do in this talk is I'm going to look at these observatories and think,
is there an opportunity to do for earth like planets what the current telescopes did for gas giants?
Right. Because we're going to have more capability. These telescopes can can gather more light.
They can see more deeply into the atmospheres. Maybe maybe we can really go after something truly revolutionary.
Okay. So what's the challenge? Here's the challenge. Do you see it?
This is a picture of the sun and the earth in front of it.
Hi there. Okay. So compared to the sun, the earth is very, very small.
Well, okay, it's not a real picture, right? That would be really hard to take. I made it, but it's to scale.
So although actually actually, you know, we do have robots that can out in space that actually can take these observations now.
So this is just a mock-up, but it is a scale.
Okay. So when you were looking at Venus, Venus looked big because we're actually closer to Venus than the sun.
Right. The first image. But here I'm saying, if we if you backed away from the vista of another solar system and you gazed,
then you would just see the geometric radii and you would learn that the sun is very big compared to the earth.
And not only that, but in fact, the sun has these other spots which are themselves about the size of the earth.
So it's a really difficult problem. So that's why until recently, we haven't been able to even find these planets by the transit method.
You have to be very good at measuring the light of stars to even see the very,
very small amount of light that's blocked by the passage of such a planet,
especially in the presence of other spots and things that are that are messing up your data.
So the revolution came with NASA's Kepler mission.
It was a four year mission. It launched in 2009.
It was the vision of a scientist named Bill Baruch who lobbied for it for 20 years.
It was rejected five times before being selected. I think it's fair to say it.
So if you're a graduate student. Draw inspiration from this.
He really fought for this. And I think it's singlehandedly revolutionised our understanding of rocky planets around other stars.
So just to just to show you a little bit about that's the that's the spacecraft there.
This is the camera, a very impressive array of CCD, of digital cameras.
You can even see that it's curved, which is very interesting. Again, I was telling you about some of the innovative optics.
It's not flat. And here it was on its launch on March 6th of 2009.
Okay. So what was the idea behind Kepler? The idea was if you put a large telescope in space and you stare at one patch of the sky uninterrupted for,
say, four years, then you would be able to actually see the passage of Earth like planets around their stars.
Right. The Earth takes a year to go around the sun.
You have to look for four years of the same star so that you would see three or four of these transits and you would have to have a big enough
telescope and be out in space so that the quality of the data was sufficient so that you could actually believe it when you saw this little blip.
The Earth would block for a sun like star that that diminishment star starlight would be less than one part in 10,000.
So it's a very, very small change. I, I certainly don't have time to review all the Kepler results.
So what I'm going to do is show you one result on one planet,
and then I'm going to talk about the statistics, the really important thing that came out of Kepler.
But I want to certainly convey to you that the Kepler photometry is exquisite.
So here we're measuring the brightness of a star as a function of time.
This is over, let's say 8 hours. So you can see the data bounces around a little bit.
And then I think you would all agree that something happens here and then it goes back.
But to put this in perspective, this is the relative change in brightness.
This is one part in 10,000, right? That's 0.9999.
So if you've ever tried to measure the brightness of stars, you would realise that this is very impressive data.
This is data being presented in an upcoming paper by a former student mind.
Sarah Ballard. What was so exciting about these data was not only could you get exquisite
measurements of the transit of the passage of the planet in front of the star, but actually we could see the astral seismic signal.
And so what that means is stars like the sun, we know, have sound waves that propagate through them.
And so essentially the star rings with a very distinct set of frequencies.
Now it's very, very hard to measure. We can measure it for the sun. It's very hard to measure it for other stars.
But what's exciting is that those sound waves essentially are propagating through the interior of the star.
So they're telling you about the actual interior physics of the star.
And in particular, as stars age, they convert hydrogen to helium, they become denser in their cores.
So because the density changes with age, then if you can measure the spectrum, you can really learn about the ages of stars.
Now, why does the age of star matter? Well, think about it.
If you if you were studying the earth, the earth, that 1 billion years and 4 billion years and maybe 8 billion years is a very different place.
And so we actually have, through Kepler, a really exciting way to pin down to perhaps an accuracy of 10% the ages of stars.
Many people have been asked for astro seismology and there's other projects to do it.
But I think Kepler really this is what you never hear about from Kepler because everybody's trying to tell you about the planets.
Just a really exquisite data.
So, for example, for this one system, which is known as Kepler object of interest, number 69, there were thousands of these.
We are able to study a planet which itself is only a little bit bigger than the earth.
It's about 48% larger than the Earth.
Keep in mind, Jupiter is 11 times the size of the earth, but we can measure the size of the planet to an accuracy of 100 kilometres.
So think about how remarkable that is. Think about the scale of 100 kilometres compared to the Earth.
And importantly, we can figure out the age of the system. In this case,
it is 6.6 billion years and these are five times more accurate than than have been done for
other systems and many times more accurate than than what have been done without Kepler.
Okay. So we have a way now to very precisely get the the the measurements of the sizes of planets.
How do we go and actually figure out their densities? We need an accurate way to go and measure their masses.
So actually, this year I'm on sabbatical at the Geneva Observatory.
The reason I picked this year is because for the last six years,
we've been building a spectrograph together with our UK partners and with the Italian partners as well,
who own the telescope where we host this instrument. The Harps North Spectrograph is a very stable spectrograph.
It's you can't even really see the details because it's all locked inside of this giant door here.
It's been pulled back to expose some of the optics.
What it is able to do is make extremely precise measurements of the speed of stars as a function of time.
And in particular, we want to be able to measure the wobble of stars due to an orbiting Earth like planet.
So when I was a graduate student, not not that long ago, we were able to measure the wobbles of stars to about ten metres a second.
Currently the cutting edge is about one metre a second. If we can do another factor of ten better, then we can get to true earth like signals.
Right? Earth are the earth tugs on the sun at the level of about ten centimetres a second throughout the year as it orbits around.
And so harps north is now just started operations and I want to show you a first result from it which we published just a few weeks ago.
So again, if we really zoom in on the mass radius plot here, I'm showing the radii of planets and the masses of planets.
The plot I showed you before would have extended up through three stories in this building.
Okay, here, we're just really zooming in on the lower part. And here we have the Earth at one earth radii, one earth mass.
We can see that people have been working their way down to find true earth like planets.
But just a few weeks ago, we published this guy, which is really getting close.
It's only 10% larger than the size of the earth.
And that is published in two papers in nature.
That was Kepler 78. Okay.
So what do we learn statistically from Kepler? Okay, so I showed you one particular system.
I said the data was exquisite. Well, really, what Kepler did was it broke open the piggy bank and just revealed thousands of small planets to us,
which now we can go and characterise and understand their properties as a population in particular.
What Kepler showed us was if you show the number of planets per star,
once you correct for all the biases of your survey, I'm not going to go into that.
Then as you march down in size from 11 times the size of the earth to four times the size of Earth to two times the size of the Earth,
the number of planets per star goes up and up and up.
So we have learned that small planets are much more common than big planets, at least in these relatively short orbits in the initial Kepler data.
And importantly, the galaxy is full of planets that don't exist in the solar system.
Okay, so this plot is telling us that the most common kind of planet in the galaxy is about two and a half times the size of the earth.
Now, in the solar system, you have the Earth, which is one times the size of the earth,
and then you have Uranus and Neptune, which are roughly four times the size of the earth.
So this is very strange. We really don't know what these planets are.
But the great thing is we can go and figure it out through our instruments.
We can go and figure out what they're actually made of,
perhaps even study their atmospheres and understand how they came to be and perhaps why we don't have them in the solar system.
Okay.
So now the big question you might be asking is, okay, what did Kepler tell us not just about earth size planets, but about Earth temperature planets?
So what did we actually learn about planets that are the same size and temperature as the Earth and therefore might actually,
for example, have liquid water on the surfaces and might host life?
Well, this is a study that was published just a month and a half ago by Eric Pettigrew and colleagues at Berkeley.
And what they did is they took the data for the 42,000 best Kepler stars and they searched for all the planets in those data.
And then they're showing you the properties of the planets.
So they're showing you the size of the planet and they're showing you the amount of light that the planet receives.
So if the planet receives the same amount of light as the Earth receives from the sun, it would fall on this line if it gets 100 times as much light,
i.e. it's closer to its star or its stars, more luminous than the sun it would fall over here.
Planets are easier to find if they're close into their stars.
So the number of red dots each of these red dots is the planet.
The number of red dots obviously goes up this way, but that's just due to the geometry and we can correct for that.
And then what's, I guess not shown here, but does show up over here.
So I'll direct you to the left screen is they drawn a green box showing the earth like zone in this plot.
So if a planet falls in this green box,
then it really is more similar to the earth in terms of receiving roughly the same amount of energy as the Earth.
And it's roughly the same size, perhaps at least smaller than twice the Earth's radius.
So we don't know for sure whether those plants are rocky or at least they look, as far as we can tell, to be Earth like.
Okay. What's a little scary, though, is if you look carefully at this green box, there's no true earth analogues.
So even though we put in this enormous effort for the Kepler mission,
it's not clear yet that we actually have found any, or at least not a significant number of real earth analogues.
There are planets that are bigger than the Earth. These planets are twice the size of the Earth or those planets that are the same size of the earth.
But they're hotter. But a real earth analogue would be right here.
And there's really nothing in the immediate vicinity.
Now, there there's a very active field in planetary science where we try to predict the habitable zone.
We try to figure out for detailed understanding of the atmospheres of planets,
how light is able to pass through the atmosphere, what the greenhouse effect would be.
That all goes into really figuring out what the range of impinging energy is that a planet can receive and still have liquid water on its surface.
Here, this green box goes out to four times the earth, the radiant.
So planets here are getting four times as much energy per unit time than the Earth receives.
We don't think that such planets would be habitable.
And so what they have to do is they have to take the population of larger and hotter planets and extrapolate.
Extrapolation in astronomy is usually disastrous, but they did it anyway.
They're very honest about it. And they concluded that about 5 to 10% of of sun like stars have an earth like planet.
But they didn't really measure them directly. They had a guess by projecting forward the data from larger planets.
Okay. Well, that being said, there have been some planets that we think are squarely in the habitable zone.
So I didn't want to leave you with the idea that Kepler didn't find them. Kepler has found them.
They just tend to be a little bit bigger because they're easier to see.
Right. They block more of their light or their stars are a little bit smaller.
And I'm showing you one such system. This is Kepler 62. So Kepler 62.
Here are the one, two, three, four or five planets in the Kepler 62 system.
And here drawn to scale is the solar system and in particular the outer two planets.
In Kepler 62, 62 after 62 e probably do have temperatures very, very similar to the earth.
The trick is they're about 50% larger. And we really don't know if that's a big ball of rock or really a mini Neptune,
which would have lots of gas and ice and really wouldn't produce an ocean of liquid water as we know it.
Okay. So so let's look ahead then at the Kepler planets.
So so I hope this news is exciting.
Kepler has has found thousands of small planets. Presumably many of them are rocky.
We can figure it out using our our spectrographs.
What are the prospects for really going and studying the atmospheres of these planets in the way that we'd study the atmospheres of gas giants?
Okay. Well, I would say in terms of transit detection, that works, right?
We know how to find them. That's what the Kepler mission did, measuring their masses.
Yes, we can do it. That was the HARP spectrograph that I showed you. It's going to be tough, but I'm pretty sure we can pull it off.
But I'm sad to say, atmospheric characterisation is going to be a no go.
So for the actual Kepler planets, we will never know about their atmospheres.
We will never know whether the atmospheres have oxygen or carbon dioxide or whether there are clouds.
We'll never really get into that, that interesting understanding.
And the reason is the design of the Kepler mission was meant to deliver really robust statistics, which meant it had to look at a lot of stars.
And so almost all the stars are very far away. Kepler had a fixed and a field of view,
and so the typical stars that we're studying in that field of view were much farther away than the than the nearby stars to us.
And so even though we have powerful telescopes that will allow us to study planets orbiting nearby stars,
Kepler didn't find those that found a population of distant planets.
Its study, 150,000 stars. So it was great for statistics, but the individual planets are really beyond the reach of any foreseeable telescopes.
So we need to do a little bit more.
So what is the way we follow on to the Kepler mission to really go and pursue this idea of actually characterising Earth like exoplanets?
Okay. So a big part of my time over the last few years has been what we hoped would be the answer to that question, which was the TESS mission.
Tess is a smaller mission. It is more rapid construction.
It's cheaper than Kepler. And what it's meant to do is to take the legacy of Kepler that Kepler told us about
the rates of occurrences of planets and actually go and look at all the nearby stars.
So look over the whole sky and actually find the planets that we know must be there based on statistics,
but actually find them around the closest stars that we can go and characterise them in detail,
really measure their masses, really go and study their atmospheres after much work.
Tess was selected through a very stiff competition.
It will construction is underway. It's a very it's it's called the Explorer program.
And so the idea is keep it cheap and launch it quickly. Make sure it works.
Launch in 2017.
It's a two year mission, so it spends one year looking at the northern sky, one year looking at the southern sky and can possibly be extended.
And it looks at 500,000 of the closest and brightest stars.
So these are as good as it's going to get. These are the very best just stars for us to study.
If we take the results from Kepler, we predict that Tess will discover 1000 small exoplanets.
But they will be, unlike the typical Kepler stars, very close to us and amenable to study,
really go and learn about both their physical properties of the of the bulk planet and of the atmosphere.
Okay. And so just to to show what that opportunity is going to be a few years.
Oh, and very importantly for Tess, the data are not proprietary in any way.
So if this is intriguing to you.
Tess gathers the data, finds the planets, and immediately makes them public so the entire world community can go and study these planets in detail.
Okay. So just to just to put the impact of tests in perspective.
So this is the current state of knowledge. So these are all of the known transiting planets that orbit bright stars.
If you're an astronomer. Bright here means 10th magnitude.
If you're not an astronomer, bright means bright. These are just the really close, bright stars.
I won't get into the screwy systems that we got left. The Greeks left us anyway.
Okay, so here I'm showing the radius of the planet. So all of the planets we know about, with a few exceptions, are big, right?
That these were the gas giants. Remember, all the Kepler ones don't show up here because they all orbit faint stars.
These are the ones that have been found with those fringe telescopes and so on.
And I'm showing their orbital period. And the idea is they're also tucked in very close because we found them from the ground and you only get
to look for a few nights and then there's clouds and it's hard to do after we fly the test mission.
This will be the planets that we these will be the planets that we have to study.
So the red dot, as are the simulation of the population of planets that Tess will provide.
Of course, we don't actually know which stars they will orbit yet, but this is the right number,
and I hope you're left with the understanding that there'll be a very large amount of planets for us to go and train all of our telescopes on.
Some of these will even be in the habitable zone.
So typically, Tess will find planets that are very close to their stars, but it will find some that are even a little bit farther out.
And those, for some kinds of stars will actually correspond to temperatures that allow liquid water, as I'll show in a moment.
Okay. So very good.
Now we've got we've got the Kepler results. We're now we've got the next big thing, which is in 2017 that's not too far away.
What will test to do in terms of the question of earth like planets will test really provide the targets that we need.
And here I want to use the one other trick that I have. So.
So the problem with Kepler was that the first problem is that the Kepler stars were too far away.
The other problem with the individual Kepler stars was that they were all like the sun.
Okay. The sun is really big, and the sun and the earth compared to the sun is really, really small.
So if you actually want to study, not just find this planet, which is what Kepler had to do with,
you know, a $600 million, ten year investment, but actually study its atmosphere.
You really have to figure out a way of making the problem easier for you.
And so here is the trick that we can play. I don't know if it's time to you.
When I was in high school, my science teacher lied to me.
Did that ever happened to you? It's a lie. Was a lie. My science teacher.
Well, my science teacher meant well. Science teacher said was the sun is an average star.
Right. So how many of you actually don't show your hands? But probably many of you think that the sun is an average star.
The sun is not an average star. The sun is a very, very strange star.
It is much more massive. It is much more luminous than most stars in the galaxy.
If I draw a bubble around the sun corresponding to ten parsecs, that's very small in astronomical terms.
It's about 30 light years and I count up all the stars in that bubble.
I find that there are roughly 20 sun like stars.
Astronomers call them g type stars. Those are shown in yellow and they all have names in the astronomers.
I'm sure if I pointed out, they would be able to tell me that the names for all 20 of them, they're stars.
You can see with your eyes. There's a small number of stars that are bigger.
But that's really the sample of stars. In the last decade,
one thing that really we sort of knew and now we've really locked down is that there is an enormous population of stars that are not like the sun.
So in the same volume of space, there's these stars which until about ten or 15 years ago, really went unappreciated.
These are called dwarfs. These stars typically have about 10 to 20% the mass of the sun, and they emit only 1/1000 the energy.
So think of the sun as your big thousand white light bulb. These are your little Christmas tree lights.
And that really changes the problem that we're after. I see a huge opportunity there that I want to now describe to you.
Okay.
So keep in mind the thing that we're now now that we're really focusing on earth like planets, we really want the planets to be in the habitable zone.
So the simple idea is the planet obviously can't be too close, can't be too far away.
It has to land at just the right distance from its star so that there might be liquid water on the surface.
Well, because these M dwarfs put out so much less energy than that habitable zone is now tucked in very close.
Right. So if this is the sun and this is the habitable zone around the sun, the earth, well,
it's not in the habitable zone until you take into account the greenhouse effect. But we know how to do that.
It would be to scale. This is how the situation would look for an M star.
And keep in mind, this isn't just some quirky little star.
This is the most common kind of star in the galaxy. These stars outnumber us 10 to 1.
Okay, so this these are the habitable zones that are out there in space.
You might have a few of these, but this is where the action is. Okay.
So the habitable zone is tucked in close and that has all sorts of implications.
So let me let me plop down a planet in the habitable zone of the sun and let me plop down the same planet,
same temperature, same size, same mass in the habitable zone for one of these M stars.
And let's if you're think about experimental design, let's think about how much easier or harder it might be to study those relative planets.
Okay. So first of all, the transits are deeper.
So what I mean is when that same planet now passes in front of a much smaller star, it blocks proportionally a lot more of the light.
Right. So for the sun, even a big planet,
what I might call a big earth like planet called a super earth blocks such a tiny fraction of the light of the sun
that you have to go to space and you build a $600 million spacecraft and you you do your very best for the M dwarfs.
The planet that same size planet would block almost 1% of the light.
Many people in this room have been measuring the brightnesses of stars to 1% with small ground based telescopes for decades.
So this is duck soup. The next point is that transits are more frequent, right?
So since the habitable zone is tucked in close, then instead of taking a year for that planet to go around, it goes around.
Oh I can say fortnight and a fortnight. Normally say that in America people like, all right, okay.
And transits are more likely. So the trends are more like, what do I mean by that?
Well, this is important. What I mean is that here's the star and here's the planet.
If I want to figure out what are my chances of seeing the planet pass in front of the star?
Keep in mind, the planet could be orbiting like this. It could be orbiting like this.
And you only see the small fraction where the range of angles is sufficient to bring it in front of the star from your point of view.
So for example, if the planet was aligned like so you would see it,
but the people at the back wouldn't just because of your different places in the galaxy.
Okay. Now for M dwarfs, the planets are tucked in closer.
Okay. So there's a wider range of angles and I'll bring it in front of the star.
And so that means that of the planets that are out there, actually a much higher fraction of that population transits.
So that's a very important effect, as it turns out.
And then, of course, there's the Doppler wobble.
So you really want to know the mass after you find these planets because you're closer to the star and you're kicking around this low mass star.
There's a much bigger wobble of the star. That means with the given precision that we have,
we can measure the acceleration of star due to earth like planets in the habitable zones for M dwarfs with current technology.
We cannot do that for sun like stars. Okay.
So with that motivation in mind, we set out four years ago to build a project which was dedicated to this idea.
We were going to look for Earth like planets around M stars.
So we called it the Merge Project and we called it mergers because it made us happy.
That was a snappy name. But the idea behind Mirth is the following.
So what we're going to do is we took an old shed in Arizona at our observatory.
It had been built actually following the launch of Sputnik for satellite tracking and then had been largely abandoned.
We filled it full of these relatively humble telescopes.
So just to show you, I mean, these are small compared to the some of the other pictures I showed you.
That's my graduate student, Philip Gottesman. That's the telescope to scale.
And the idea is picked a list of really nearby dwarf stars, these nearby small stars that are so common.
We know about them now. So let's make a list and let's just go and follow them.
We don't care about what's going on in the distant reaches of the galaxy.
We just want to look at this very local neighbourhood of stars. So Mirth is very different.
Most transit surveys, what you do is you take your telescope, whether it's the Kepler telescope or the small four inch telescope,
and you look at one patch of the sky and you just deal with all the stars in that patch.
Here we want to look at the very brightest stars over the whole sky. And so the telescopes have to move around.
And that's why we need eight of them, right? Because they're all over the sky.
And we have to make sure to observe them as much as we can, certainly every night.
And oh, and I think I wanted to actually show you what that operation looks like.
So I will turn down the lights here.
And what I'm showing here is a time lapse movie of the Earth Project in action.
So there are indeed eight telescopes.
There are eight telescopes. Once once this opens up. One, two, three, four, five, six, seven, eight.
You're looking at a fisheye lens. So you're seeing the whole sky. And what you saw was the roof move out of the way.
The telescopes have gone to work. And what they're doing is they know where all of these nearby dwarfs are.
And so they're quickly moving from one to the next to the next. But they have to repeat their observations of any one star every 20 minutes.
These planets take about an hour to go in front of their star. You don't want to miss it.
So you need to look at every star, every 20 minutes. And so hopefully this can be as true as the sky rotates overhead.
The sky is moving overhead. What these telescopes are up to, they operate every night.
They will operate tonight. They're completely autonomous and really under the supervision of a immensely capable
Smithsonian scientist named Jonathan Erwin has really made all this possible.
Okay. So what has.
Has the idea work? Yes, the idea worked. Maybe that's why I get to talk about it.
I don't know. The the idea is we did find one.
We've only found one. But I'd like to actually it's fair to say it is it is the most studied planet exoplanet ever.
It is been enormous.
Attention has been lavished on it. Recently, the Hubble Space Telescope stared at it for four days.
Four days. It's a deep field for those astronomers when you on this one planet because it's so interesting.
Why is it interesting? Because it is just a little bit bigger than the earth.
And yet because it's only because it's so close to us, it's 13 parsecs away and because it orbits a small star,
we can study its atmosphere, we can learn about the properties in detail.
We can actually go and make meaningful comparisons.
Just to put it in contrast, these were the planets we knew about before, and here is this planet that we had found.
Since then, Kepler has found thousands of planets in here, but we can't study them.
But at the time that with this one new found, it really was opened up this field.
And I won't go into this in detail.
But just to say, the first thing we did right after discovering this planet was to go and measure its mass so we could figure out the density.
Then we knew the density was higher than a gas giant. It was it was not clear it was rocky or whether it was icy.
But there were competing ideas about what the planet was. Some people said it basically is a Waterworld.
It's a ball of water that would work out in terms of what we knew about the density.
That would be excuse me, very interesting. The people said, no, no, no, it's not a Waterworld.
It's a it's a it's a Neptune. So it has a thick envelope of hydrogen, helium like our own Neptune does, very different formations,
certainly very different physical understanding of what the surface would be like for that planet.
The point is that debate didn't have to end there.
We went and figured it out because we could go and observe the atmosphere because it orbited nearby M Dauphine, because it was close to us.
Okay. So we've carried this idea forward.
And so we just gosh, two weeks ago turned on an observatory in the Southern Hemisphere, which we've given the innovative title of Earth South.
But here it is. These the telescopes look much shinier and newer, of course.
And if you're ever in the mountains in Chile, come and visit us.
I was very excited, actually.
These are the first seven nights of data and I'm showing that we did not discover something on the first or third nights.
Okay. Actually, so that so that's a real signal, which just shows you how these things pop up.
We don't we don't think it's a planet. We think it's a little too big for that. But but it is very intriguing.
Okay. So then we come towards the end.
So now the question is how far away, you know, take an idea like Merce or take similar ideas.
How far away is that closest real earth like planet going to be?
By which I mean how many light years away from us is it okay,
so can we actually figure it out and can we figure out whether we actually could ever go and study its atmosphere?
So there's two parts of this question. The first is how far away is the nearest transmitting habitable planet?
And the second is, well, given that distance, if we know that distance,
is it close enough that we actually could look for the chemical signatures of life?
So working with a student, Courtney dressing what Courtney did is she was looking at the Kepler data and she realised Kepler did almost by accident,
well, not by accident, but at a very small fraction.
The stars really were these small M dwarfs, but it was the data were so good that actually you could do statistics.
And in the case of the armed forces, I've explained to you,
Kepler was able to find true earth sized Earth temperature things, unlike the case for the sun like stars.
So what Courtney was able to do was look carefully at stellar properties.
We had to make a number of important corrections and then go and actually say,
Okay, how many planets did Kepler find in the habitable zones of M dwarfs?
And therefore, what's the rate of occurrence? And so I'm showing that plot here.
So these are cool stars. So the start that the sun is about 5800 Kelvin.
So it would be way up here. These are the cooler M dwarfs are typically about half the temperature of the sun
and the green band shows their orbital period if you're in the habitable zone.
So for the coolest stars, you would take 11 or 12 days to go around for the hotter stars.
Of course, this moves outward because you have to balance the energy.
These were the planets that Kepler found, and we found that there were a number of planets in the habitable zone.
And based on that, Courtney deduced that Earth like planets around M doors are really common.
Okay.
It looks like roughly 60% of M dwarfs have an earth like planet, and by which I mean not just in the habitable zone, but is also really earth sized.
Now, if you think back 5 minutes when I said M dwarfs outnumber us 10 to 1,
it seems virtually a lock that the closest habitable planets are not going to orbit sun like stars.
They're going to orbit these m dwarfs. Well, that's good for us, because I just explained to you that they're the ones that we could study anyway.
Okay. So very important that we say this clearly.
The transiting earth like planets are very common. The closest one is probably eight parsecs away.
If eight parsecs doesn't mean anything to you, then here's what I want you to tell your kids tonight.
If you take England and you say, okay, I'm going to take the galaxy and I'm going to shrink it down to the size of England for a scale model.
So imagine I took the Milky Way and I, I distributed it to roughly the same scale as England.
Then the closest transiting, the closest one that actually goes in front of a star,
their closest transiting humble planet would be inside this building.
Likely inside this lecture hall, but certainly inside this building.
So these these stars are so common and they host planets with such high frequency, they are very nearby examples of earth like planets.
We have to go and find them. Second part of the question, is it close enough that we could search for the chemical signatures of life?
Well, let's think a little bit about this question. This is new.
I'm learning a lot about what we're calling atmospheric biosignatures.
What does it really mean to go for an astronomer to go and start talking about finding life on other planets?
Right. So we have a severe limitation.
We can never go to these planets. We are doing it for remote sensing.
So what are the things that we would measure that really would allow us to conclude that a planet has life or likely has life,
or at least compare it in a meaningful way to the geo biology, to the history of life on the earth?
Well, certainly looking for evidence of oxygen would be one straightforward idea.
We know that the oxygen in the Earth's atmosphere is all produced by biologically.
We don't know of any abiotic process that really would allow a large concentration of oxygen and other planets really does seem to need life,
but that idea really does need to be fleshed out. Certainly looking for direct evidence of liquid water,
analysing the emitted light from the planet to make sure it does have an atmosphere, and looking for other signs of biological activity.
Such as? Well, yes, methane produced by various processes on on the earth, but other what we call broadly speaking,
biosignatures so atmospheric biosignatures refer to the fact if you could take a spectrum of these planets,
what are the molecules that you would detect in such and such ratios that
really would drive you to the conclusion that there was life on those planets?
And of course, rule out other explanations. So what's so exciting and this is really happening just in the last couple of
months is people are really working that problem as as experimental designers,
as as as very careful,
careful thinking about the statistics and about the optics of the telescopes that we have to really figure out if we could do this experiment.
So what what is the experiment? In a nutshell, it means the following. You're trying to detect this?
What is this? This is the signature of molecular oxygen.
So what I'm showing here is the wavelength of light. This is 760 nanometres or 770 nanometres.
That means red light. Okay, this is red optical light.
And what we're showing here are bands due to oxygen.
So these are various transitions in the oxygen molecule. They are a fingerprint for oxygen.
If we got the spectrum to be no doubt you're looking at oxygen. The challenge is, of course, that that's the signature in their atmosphere.
But if you're doing it from the ground, well, we have the same thing in our atmosphere, and that's shown here as the blue curve.
So the first trick was, well, how would you hope to see the same thing that's in our own atmosphere?
If you take your telescope, you look up to the Earth's atmosphere. Aren't the very places in wavelength space you want to look at obscured?
The beauty is that our telescope is on the earth. The Earth is going around the sun at a changing velocity.
And so what that means is that these lines change in Doppler shift relative to the Earth's lines.
Okay. And so what that means is, if you were doing this experiment over time, the alien earth signal would actually be moving back and forth.
So there would be moments of the year when you couldn't observe it, but there would be plenty of moments when you could.
And then the other trick, of course, is the the additional component is this is what the spectrum of the star looks like.
So if you're an astronomer and you're used to looking at the spectre of stars, this is not your typical sun like star.
These stars are so cool that they themselves have enormous quantities of molecules and they have very interesting spectrum.
But, you know, this is this is a well posed problem.
So the question is, if we take a telescope, will we imagine how much light we could gather?
Does the experiment work out? We gather enough light. Is the noise sufficiently low?
Can we account for all the confounding factors? And this is this is preliminary work, but it builds on very nice work.
And I do want to give credit here to Ignaz Celan's group in Leyden.
Ignaz really threw down the gauntlet and said, I think we could actually do this with telescopes we are building.
Later, Florian Rosler and Mercedes Lopez Morales picked it up and worked the detail, worked the problem in more detail.
And again, I'm showing you the spectrum here of oxygen.
And I then went and asked my colleagues, well, what if we really thought about boosting the resolutions?
What that means is build an even more powerful spectrograph, which is on the same telescope,
but actually go and see if we can really pull out those oxygen lines by separating the wavelengths of light more carefully.
So what we considered was we said, well, let's let's use the investment that European and the American communities are already putting in.
Right? So we're already building this €1 billion telescope, the Europe that's the European telescope in in the US.
There are two American telescopes that also have a similar budget.
Those serve a very large image.
But given that we're building those telescopes, is there an experiment we can do to really go after this ground breaking science?
And so this is the experiment. Let's take the telescopes.
Let's put a spectrograph on the back of them that would actually allow us to go and detect this feature.
You might be able to use the spectrographs that are being designed, or you might have to go and build a special spectrograph.
But that cost would be very humble compared to the investment that's already been been signed for it.
These telescopes are being constructed. Okay, so does it work?
So what we learn is if we take the Kepler results, we figure out that the very closest planet is roughly 6 to 8 parsecs away.
Then we can say, okay, let's put that planet in front of the likely dwarf.
Let's take imagine, let's do a mock observing campaign with our telescope and we find out that it just barely works,
that you would need to observe 10 to 15 passages of that planet in front of its star.
But keep in mind, these happen every two weeks.
The difficulty from the ground is you can't look at any one star for a few months before it's up during the daytime.
So you have to wait. But you could imagine, and I view this almost as like the experiment to measure the Higgs.
You know, it's it's a it's a very specific question.
It requires an enormous community to invest in it.
It it sort of has a yes or no answer, not in terms of whether it's life, but in terms of there's oxygen.
That's that's a really idea detector. You don't and you learn something about the abundance, just like you learn essentially the mass.
It's sort of in my in my view, it's a grand experiment. It might inspire an enormous amount of work or we might find something truly unexpected.
The takeaway message is it it looks like it might work and it certainly merits more.
More investigation. Okay. Well, just to wrap up, I wanted to mention one idea, which is, of course,
that the amount of oxygen and the chemical signature in in Earth like planets,
atmospheres has changed in the Earth has changed throughout the Earth's history.
Keep in mind, as I said at the beginning of time, that we have a method for very accurately determining the ages of stars.
So you can imagine a very, really exciting set of experiments where you start detecting these biosignatures and then
asking as a function of the age of the planet how those chemical signatures change.
In a sense, astronomy gives us an ability at a time machine where we can go and find Earth like planets
that are 1 billion years old and actually look at what we might have looked like back then.
But also we can find Earth like planets that are 8 billion years old and view that almost as a window into the future.
So I don't know where those investigations would lead, but it certainly sounds like something that that could really inspire enormous amount of work.
Okay. This is the summary. I'd like to take your questions.
I am I do want to I do want to mention a few key points here.
I was thinking she writes summary. You write summary at the top. That always seems so dull, I think.
Talk to your colleagues about these ideas. Talk to your children about these ideas.
I think there there are some interesting conversations.
Okay. So the takeaway points, we haven't measured the rate of earth like planets around sun like stars,
but extrapolation from the Kepler data puts that at about 5 to 10%.
We have measured the rate of occurrence of Earth like planets around these smaller stars.
The number is very high. It ranges between 15 to 60%.
But that's not a statistical uncertainty. That's really just how you define a habitable zone.
What how wide you think that that range of distances can be.
But it's it's keep in mind, astronomy, the number could have been one in a million.
Right. So the number is high or very high. But in any event, it's, it's it's it's almost of order unity.
We do need this complete census of medium dwarfs within 20 parsecs.
I don't think we have to go outside 20 parsecs.
But in that volume, based on the Kepler results, we expect 63 planets, of which about six would be truly Earth like.
Okay, so that's really astounding. And then finally, I think that a high resolution spectrograph on an extremely large telescope.
And again, these are things being built. This is not a proposed telescope might be able to detect the analogue of the earth oxygen bands.
And intriguingly, the sounds noise estimates really put it just at the level of feasibility.
Okay. And that's the end.