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Next in Science | Astronomy and Astrophysics | Part 2 || Radcliffe Institute


-Well, welcome back
to the second half. We’ve gone from a cosmic
scale to gigalight year scale, and
massive black holes, and now we’re sort of
zeroing in on things that are closer
to life on Earth, and things like star formation. So the next speaker
is Blakesley Burkhart, who got her PhD from the
University of Wisconsin in 2014 and does work on– she’s a
Fellow in the Submillimeter Array Collaboration, and also
a fellow, an Einstein Fellow, in the Institute for
Theory and Computation at the Center for Astrophysics. And she’s going to talk
about star formation, which is actually something
that, as a dilettante, am very interested to hear. So her talk is– title
of her talk is “Galaxies as Star Forming
Engines: Simulating the Turbulent Birth of Stars.” And I remind you to
hold your questions until after the
second speaker goes, and then we’ll have
a panel up here where you can ask your questions. OK, Blakesley. -So, yeah. I’m extremely happy to
be talking a little bit about my research to you
today, and to give you some impressions on
what I’ve figured out. In this four panel series,
I must represent purgatory. So Cora and Salvatore are
representing the heavens, now we’re moving
down, and so clearly, the scales of the galaxies,
we’ve now reached purgatory. So Sarah, you have a
big task ahead of you. And I want to leave
you with the feeling that the 21st century physics
that we’re getting into, galaxies represent
a huge enigma still. We still don’t know things
like how a star is formed, or how galaxies
evolve in cosmic time. And so today I’ll
be telling you more on the lines of how
stars are forming and some new progress
that we’ve made in terms of simulations
and comparisons with state of the
art observations. So I’d like to take a step
back and start with a classic. Right, we gotta go
back to the classics, and that’s Carl Sagan. Probably many of you
at least watched Cosmos or heard of the Cosmos
program, the original one. Carl Sagan was the one to
say, first, that we’re all made of star stuff. What did he mean by that? Well, going back to what Cora
was talking about with the Big Bang, the Big Bang produced
hydrogen, helium, lithium, and where did all the other
heavy elements come from? Where did the carbon, nitrogen,
oxygen, all the heavy elements that make up most of the
world that we interact with and indeed our own bodies? All of this was
synthesized within stars. And so without
generations of stars that came before our own star,
before our own solar system, we would not be here today. And so then it’s a big
question about the nature of our existence to ask
how do galaxies make stars. We know stars are
predominately made in galaxies, how does this work? It’s important for
us to find this out. And so we can take a
picture of a typical galaxy. This is one of many billions of
galaxies that we’ve observed. And we can ask
about how they get gas, how they get gas in order
to make this gas and the stars. So first, we know that galaxies
don’t exist in isolation. This galaxy here is
actually part of a network of galaxies in the cosmos. Gas accretes onto this
galaxy through a cosmic flow. And then what
happens to this gas? How does galaxy turn
this gas into stars? There’s a whole
cycle that this gas, once it falls onto the
galaxy, goes through in order to become a star. So we can start with this
gas being very warm and hot. It sits sort of
outside the galaxy. And it cools via number of
different cooling processes into colder and denser gas. Eventually, this cold
gas becomes dense enough to start to collapse under
its own gravity, which we call self gravity. And then forms these
very dense clouds, sort of sitting in the
disc of the galaxy, that we call molecular clouds. They’re dense enough,
they’re cold enough that they’re able to start
sustaining molecules. And then at some point, we have
this process of star formation. So you know, if you just think
of gravitational collapse, if you continue to
collapse your cloud, poof. Eventually you should collapse
it so much that a star forms. We don’t actually know so
much about that process. But OK, once you
have a star, then you can have solar systems. You can have stellar evolution. And eventually the star will
either disrupt its parent cloud through its own winds
or feedback processes, or eventually it will
explode as a supernova. And then this
cycle then repeats. OK, easy, right? We also have in
the galaxy, again, it’s not an isolated system. There is still outflows that
are returning this hot material back to the
intergalactic medium. This part here you could
say is one big bottleneck. We still really
have no idea how we go from the formation
of clouds, how this cloud forms into a star. This is still a big mystery. But we do have some
evidence, from observations, in particular, that
clouds are forming in what we call dense molecular clouds. These are just a few
images, very famous images from the Hubble Space Telescope. A lot of these
images were produced in the 1990s, when
I was growing up, and I would say images
like these really got me excited to be an astronomer. These sort of images just made
me want to study astronomy. Because we just– there’s so
much variety of structure, and that you can see here,
there’s so much physics going on. For example, in this one
here, you can see a bow shock. This is a triangle
shape here is a bow shock that’s being ejected
from a young forming star. All of this diffused
light material here is actually
ionizing radiation from young, forming stars. And then these dense
filamentary structures are still the dense,
colder gas where stars can continue to form. So there’s a lot
of physics going on here that we can already
see in the observations. We still don’t
understand it very well. But we can start to
speculate, how would you take these big clouds and
then collapse them down to form a star? Well, gravity’s probably
a good start, right? So you can come up
with a cartoon picture of star formation, starting
with your dense cold molecular cloud. Then you have some sort of
gravitational collapse, that’s radial, the cloud collapses in
on itself towards the center. At some point you
reach densities that are high enough in order
to ignite nuclear fusion. That’s what a star is, right,
so we now have fusion going on. There’s some outflows, there’s
still an accretion disc from this original material. And eventually you start to
form planets, and a disc, and so forth. So this looks
pretty good, right? I mean, this cartoon, I
feel pretty good about this. But unfortunately,
the theorists had to come and ruin it for everyone
with just a simple calculation. So I’ll walk you through this. So we can write
down the time scale it would take for a cloud
of some density to collapse. This is called the
free fall time. The free fall time is
really just a function of the cloud density,
assuming that the cloud is a spherical cloud, like
I showed in my cartoon. So for some typical
densities of these clouds, we can calculate a time
scale for collapse. This is something
like 8 million years. Now then I can say,
well, I know roughly how much gas is in the galaxy. If I look out into the galaxy,
I look with the Hubble Space Telescope, with
other telescopes, I can calculate the
gas in the galaxy, and in particular, the dense
gas mass in the galaxy. Because we know stars form
in dense molecular clouds. And I can estimate a mass in
our own Milky Way galaxy of star forming gas. So that I can just say,
well, I want to find out the star formation rate, right. I can just divide the mass of
the molecular gas in the galaxy by the collapse time, in order
to get a rate of conversion of gas into stars. So if I do that, I
get a expected rate of around 250 solar
masses per year. Now, what do we
actually observe? This is what my theoretical
calculation would predict. The actual observed star
formation rate in the Milky Way, is significantly lower. Something like three
solar masses per year. This is a big problem, right? We have a big,
theoretical mismatch between what the theorists
that are studying star formation and the observers. And so something seems to be
slowing star formation down. So we really have an issue here. So how are we going
to solve this? So really, the problem
of star formation is not how we form stars, right. We know gravity eventually
wants to bring everything together and form
a star, but really the problem of star formation
is how do you not form stars? How do you get such an
inefficient star formation in our galaxy? Well, it turns out the galaxy
is actually a complicated place to live in, right. There’s a lot of
physics going on here. This is why galaxies are
so fascinating to study, is because there’s so
much going on here. In this galaxy here in this
picture, you’ve got dust. You can see these
dark dust slings that are obscuring
the stellar light. Stars that are producing all
this bright blobs of light. You have supernovae
that are exploding, that are creating extremely
bright areas within the galaxy, brighter than the
rest of the stars all combined, of
course planets that are around the stars
and then other, more exotic physical phenomena
that you’ve maybe heard about, like cosmic rays. Right, so, high energy
particles that are produced by things like black holes and
supernova, or magnetic fields of turbulence, which I’m going
to touch on in this talk. So there’s a lot of different
physics going on here. It’s very complicated. So we can easily see how
in my cartoon picture, maybe we missed something. Maybe we weren’t
thinking about something. Another complication,
unfortunately, is that a lot of the time scales
of galaxy evolution of star formation are much, much longer
than our own human lifetimes. So I showed you
the free fall time, that’s the fastest
that you’re ever going to get one of
these clouds to collapse. It’s still millions
of year time scales. And so human life times,
unfortunately, are not so long. So how are we going
to make progress here? Well, I’d like to propose
to you that we really should make progress in part
with numerical simulations. So these are simulations that
are done on a computer, where we take some known
physics like gravity, like the way fluid
motions work, et cetera, and then evolve them in space
and time using computers in order to study
star formation. I’m showing you a movie here,
and I want to start it over. So we start with some
initial conditions. So in this case it’s
a spherical cloud, very similar to my
cartoon picture. And then we allow this
cloud to collapse, and we also include
other physics that we think are important
in star forming regions, like turbulence and gravity
and magnetic fields. We then evolve this is time. Now you’ll notice that
time is running here. This, you’re getting
a density image here as it zooms in to the
collapsing region in the cloud, because there’s so many orders
of magnitude and spatial scale and also in density,
this color bar here, which is a long scale. So these are factors of
10, has to also zoom in. So another part
of the difficulty of the star formation
problem is not just the time scales involved. It’s also the extreme range
of scales, ranges of density. So finally, we get down into
the kind of densities where the star formation
can actually happen, and then you can see
in the simulation these star particles
are starting to form in the simulation. So this is a very
difficult problem. But simulations can
help us understand it. OK, so if you’re going
to make a simulation, right, you need a cookbook. You need a recipe. It’s up to you,
you can decide what you include in your simulation. Well, what kind of recipe
would you come up with? So obviously, we
need gravity, right. So gravity’s going to
pull everything together into collapse. And the movie I’m showing you
here is one such simulation that I’ve been working with
with the Enzo Collaboration. So you can see that
these filaments form at the start of the
simulation, and then begin to collapse
in on themselves. So pick your favorite
part of the filament, and you can follow
it in and see how it’s sort of pancaking together
as the simulation proceeds. So we need gravity. What other things would you
include in your simulation of star formation? How about turbulence? So if you look at this initial
gas density distribution, it’s very fractal. It’s very non smooth. There’s a lot of
discontinuities there. This is what we
mean by turbulence. And so, for the most part,
when we talk about turbulence in our everyday lives, it’s
usually an uncomfortable moment on the airplane when the captain
comes on the PA, and says, you know, we’re going to
experience turbulence. And you start bumping all around
and you wish it would stop. So this is random fluid motions. When we encounter
this in the airplane, this makes the airplane, of
course, bump up and down. This is a very nice
picture by da Vinci showing a turbulent flow. And a very similar
sort of thing is happening in a molecular cloud. When we measure velocities
in these star forming clouds, we see that the velocities
are characteristic of a turbulent flow, not
a flow that’s very smooth. And so you wouldn’t want to
fly an airplane through one of these molecular
clouds, I can tell you. Now what does
turbulence do for us? Why do I want to put it in my
simulation of star formation? Well turbulence adds a
pressure term, right. So does anyone drink coffee? Does anyone like
French press coffee? OK, when you stir the coffee
around in your French press, you don’t immediately
push it down, right. You need to wait, you need
to let it settle down. Try it. Next time you want to
drink a French press, stir the grinds around a lot,
and then try to push it down. It’s very, very difficult.
You’ve induced turbulence into the fluid,
and that turbulence is providing an
additional pressure term that’s making
it harder for you to push your French press down. So in some sense,
that’s a similar thing to what’s happening here. The turbulent
motions in the cloud are resisting the
gravitational collapse, and it’s not able to
push the cloud in. That’s the main thing turbulence
is doing in the cloud. It’s also doing some
other things, especially on the small scales. It’s actually allowing small
scales to collapse even easier. But for the most part, you’re
adding a pressure term, so that’s going to slow
star formation down. Right, so ultimately
we want to figure out how can we slow
star formation down in order to match that three
solar mass per year number that we’ve sort of
observed in the Milky Way. So turbulence will slow us down. What else? Magnetic fields. I bet you wouldn’t’ve
thought that. But these clouds are
actually magnetized, and that means that we
have an additional pressure term from the
magnetic field that can also resist the collapse
and slow down the clouds. Now the funny thing is that
when you talk to astrophysicists and you mention the
words turbulence, and magnetic
fields, usually they run away from you very quickly. And that’s because these
things are very complicated. There’s no full
theory of turbulence from a fluid dynamics
standpoint of view, this is one of the
unsolved problems. And magnetic fields in a moving,
fluid plasma are also ugly. But we can also think of
some intuitive pictures to understand why
magnetic fields might be important for star formation. So for example, another
real life scenario. Think about a
rubber band, right. So if you stretch
out a rubber band, a rubber band provides some
kind of tension, right. It wants to stretch back down. But with your rubber band,
which is like the magnetic field lines, you can’t really
move perpendicular to them. You can only move things
parallel to your rubber band, right. So if I take something like
this, something stretchy and rubbery, I can’t
move my hand this way. I can only move
my hand this way. And so this is very similar
to what the magnetic field is doing in a star forming cloud. So here’s a picture of that. You have your spherical cloud,
and you have the red lines here, being representing
the magnetic field lines. So the collapse is going to
only be able to proceed parallel to the field lines. The field lines
are going to resist the collapse perpendicular,
such that you will eventually arrive at a situation
where you have the collapse parallel to the field, and the
field being slightly pinched along the
perpendicular direction due to gravitational
contraction. This is observed, actually. So here’s an observation with
the SMA telescope in Hawaii, looking at field lines
in a star forming object. And you can also,
even just by eye, see this pinching effect of
the field line, perpendicular to the collapse. This is called a so-called
hourglass morphology, or an hourglass shape
in the magnetic field. And this indicates that the
magnetic field is strong. OK, what does strong mean? Well, your fridge
magnet has a strength of roughly 50 gauss or so. These fields are at the
microgauss scale, so 10 to the minus sixth. And that’s a pretty
big difference, but for a molecular cloud,
this is actually very strong. So in this case, we might expect
that the magnetic forces are very strong, stronger than
things like turbulence or maybe even the
gravity itself. So things look very ordered. However, in other
observations– so this is observations of
dust polarization from the Planck satellite,
so similar to what Cora was showing. But instead, we’re actually
now interested in what’s happening in the galaxy. In this case, the fields
look very random, right. So you can see the field lines
here looking very chaotic. And this might be a situation
where things like turbulence, which wants to
randomize the field, is stronger than
the magnetic forces. So how can we further study the
effects of the magnetic field in star formations? It’s very complicated,
we need to go back to our numerical simulations. And so I’m going to show you
some very, very recent state of the art numerical
simulations that we’ve recently performed in order to study the
effect of the magnetic field in a collapsing
stellar environment. And we can do this sort
of in this typical way where we observe some sort
of real world phenomenon. So as an example, I’ve shown
you this instability that can actually form
in clouds, it’s called the Kelvin-Hulmholtz
Instability, where you have two shearing flows. You say, I want to
try to simulate this. I want to include fluid dynamics
equations, mass conservation, momentum conservation,
energy conservation, and I want to try to simulate
this on a parallel computing resource. We have a very nice one here
at Harvard called Odyssey. And then ultimately– I’ll
restart that movie for you. We use a particular numerical
simulation, so in this case we’ve used the AREPO code,
which is a code where the grid points
in the simulation actually move with the fluid
and change with the fluid. So they’re not static,
and you can actually model very precisely
instabilities, turbulence, et cetera. So this is the code we used. This code is unique, in
that we are able then to, following the fluid flow
as it evolves with gravity, we are able to follow it
down over many, many orders of magnitude and scale. So going from 16 light years
down to 100 astronomical units. An astronomical unit is about
93 million miles, the distance of the sun and the Earth. So this is a huge
range of scales. We’re also able to go over
a huge range of densities, many orders of magnitude
of density going from the diffuse cloud scales,
down to the very dense cores, where the stars are forming. And the original
thing we’ve done here and additional to that, is we
have four of the simulations. These simulations are very
computationally expensive. Four different simulations, each
with a different magnetic field strength, so we
can really study, in depth, the interaction
of the magnetic field and the collapse of the cloud. So I’m going to show
you these simulations. I’m going to show
you all four of them, and just keep your eye
on the top of the screen. This is the magnetic field
strength in microgauss. Here’s the first simulation. And I’m going to show you all
four in a very similar way. Just to give you some
idea of the scale here, so this is about 15,
16 light years across. This is the cloud
scale, you can see by eye the density structure
is very fractal, very turbulent looking. And this panel here,
this inset here, is one pixel, zoomed in one
pixel from this image here. So we’re talking about
huge ranges of scales. And you can already see that
there’s a filamentary structure here, and then this panel
here is even further zoomed in, a region where
a star would be forming. Now what does the
magnetic field look like? This case has a very
weak magnetic field. And if I over plot the
orientation of the field lines, it looks pretty random. So the field is somewhat
ordered on the largest scales, but as you go to the
smaller and smaller scales, it’s pretty random. It’s changing its orientation
at every sort of step that we go down. We expect that
because in this case, we have a very weak field. Now if we increase the
field strength– so again, here’s the density structure
that you’re seeing, and here’s the magnetic field. It’s already slightly a little
bit more ordered at each scale, because we’ve increased
the field strength. And we can continue to
increase the field strength, so this is our second
most strong field case. You can see now
the large scales, you have a very
uniform, ordered field, and this somewhat persists
down to the smaller fields. However, in this case it
still looks fairly random. Finally, this is the
highest field strength. Things look actually quite
different from the other cases. So if you compare the
density fields, for example, especially at the smaller scales
where stars are able to form, now there’s this very
nice filamentary structure that’s formed. In the case where
the magnetic field is higher in terms of its
energy and then the turbulence and the gravity. And if we look at the
corresponding field structure, we said it’s extremely ordered,
all the way down through all these different scales
of star formation. So once you get to
this very smallest scales of star
formation, you actually start to also see this very
nice hourglass morphology that we predicted when gravity
was very strong, interacting with a magnetic field. So this is what the
simulations tell us. They tell us that there are sort
of two modes of star formation. One where there’s random,
turbulent energy that’s dominant over magnetic
energy, and another where the magnetic field sort
of dominates the collapse. This is from simulations,
what can observations tell us? How can we better compare
these two observations and simulations? Well, I’ve been harping on
the advance of the simulations that we can go through
all these scales, include all these
physics, but actually the observations have
also been advancing. So meanwhile, we’ve
been able to go from larger and larger
scales with the observations to smaller scales. And so ALMA, the
ALMA telescope, which is a millimeter wave
telescope in Chile has really opened up a
very high sensitivity, high resolution observations
for star forming regions. And so we can combine previous
generation instruments like the CARMA telescope
and the JCMT telescope to get different scales of the
star formation process as well. So the JCMT data is
giving large scales, so sort of a tenth of a
parsec type scales. CARMA’s scales and
ALMA’s scales are giving this medium and smaller scales. And so I’ll show you
one particular object from a recent study that
my collaborators at Harvard and myself have performed
using these different data sets of a particular
object called Serpens-8. Here’s the JCMT data,
so this is something like a tenth of a parsec,
down to the CARMA data, down to this finest resolution,
which ALMA has just now opened up for us. And all of these data
also have polarization, so they all have magnetic field
orientation measurements, which I’m showing you down here. And you can see that the
field looks very random. And it looks very random all
the way down towards the scales of the ALMA data. So this is actually
very similar to what we found in the simulations
where the field is very weak. And so it seems that this ALMA
scale magnetic field is not following the field from
the very largest scales. That’s in contrast to this
nice hourglass picture. So in some cases we
see an hourglass type of morphology, that indicates
the field is very strong. In other cases,
like in Serpens-8, we found that
turbulence is perhaps more important in shaping
the field morphology. And this is in
contrast to decades of theoretical and
observational works, which say that you should always
form this hourglass. So in this case we
did not see that. So I would like to conclude,
going back to our quote from Carl, “We are all
made of starstuff.” If we really want to
understand how we got here, how our star was
formed, we really have to take into all
the complicated galaxy physics, including turbulence,
including magnetic fields. And so it’s complicated. Simulations are
really going to be key to answering a lot of
galaxy physics questions, star formation just
being one of them. And I think really, in terms
of the star formation paradigm, we’re going to be able to move
towards a predictive theory of star formation,
because we understand the fluid dynamics better. Because we understand turbulence
and magnetic fields in a better way. So thank you very much. -So our next speaker
is Sarah Rugheimer. She is originally from
Montana, got her doctorate at Harvard 2014, and
is now a Simons Origin of Life Fellow at St. Andrews. I should also mention that
Blakesley and Sarah do podcasts, and do outreach work. And in fact I was listening
to one of Sarah’s podcasts– you also do this
with Sarah Ballard– so I was listening
to one of them, and she was saying for the
mental health of an academic, you should try to
keep your desk clean. And I looked at my desk
and I was thinking, OK, well, I have
some work to go. -I don’t think I
said that, though. -Oh? -I think it was the other Sarah. -Oh, the other Sarah, OK. OK. -My desk is a mess. -I feel better now. OK. OK, so Sarah’s going
to talk about how detect life on another planet. -Thank you. [APPLAUSE] -Well thank you for
inviting me here today. I’m really excited to be talking
to you about my research, which, I’m interested
in this question of how can we detect
life on another planet? So I just wanted to start first
with a question to you guys. How many of you
think that we will be able to detect life in the
next 10 years on an exoplanet? Raise your hand. 10 years. How many of you think it’s
going to be more like 20 years? Raise your hand. And how many of
you think 30 years? 30 years? And like, say, greater than 30,
50 years, something like that? Maybe never. All right. Well, we’re going
to talk about that. We’re going to talk about
what makes this difficult. So I’m interested in this
question of are we alone in the universe. And we’ve looked for
various signs of life before, with SETI, the
Search for Extraterrestrial Intelligence. And you know, so far, we
haven’t found anything. We’ve just really had silence. And so my question is still,
are there are aliens out there? I want to know this. And we haven’t yet had any sort
of greetings, earthlings sign, and so we’re still looking. And I want to be
really clear here, that when I talk about
are there are aliens, I’m not talking about aliens. I’m not going to be talking
about intelligent life today. I’m really only
talking about microbes, like these single
celled organisms that we might be able to
detect the signs of their life in the atmosphere
of another planet. And so how could we do this? Well, one of the thing to do is
by looking for biosignatures. And I would argue that
the strongest biosignature is something like an
oxidizing and reducing gas in combination. On Earth, that’s something
like oxygen and ozone, or ozone in combination
with methane. But individually, you can get
either of these abiotically. For example, UV light can
split water and carbon dioxide, and you can be left behind
with oxygen or ozone, and you get methane
from vulcanism or from hydrothermal vents. And so these by themselves
are not good biosignatures. On Earth, though,
you have biology giving large fluxes of both
gases, of oxygen the methane. But without biology,
these gases would destruct in the atmosphere,
and you wouldn’t necessarily see them together in the
atmosphere of an exoplanet. So this is why we go
back to that definition. Also, biosignatures
require context. Things like, can
we get constraints on the surface temperature? Is it a rocky planet? Does it have bio vital
indicators like CO2 and water? Now CO2 and water by themselves
are not biosignatures, because they can be around
in the whole universe just abiotically, but
they are useful for us to detect because
they indicate food and habitability and
greenhouse gas on the planet. Other biosignatures that you
might have heard talked about are things like N2O,
methylchloride, or dimethyl sulfide, and maybe ammonia in a
hydrogen dominated atmosphere. And these, though
they’re produced in much smaller abundances,
are useful to think about because they don’t have
any abiotic known source. So my research,
though, is how could we detect these biosignatures
around other stars? And I love this
pale blue dot image, which I’m sure many of you are
familiar with, when Voyager went out past the orbit of Pluto
and turned around, and took this snapshot of our planet. And here it is. And in that image, you can take
a spectra the light from that and see various signs
of life on our planet. In particular, you
would be seeing that combination of methane
and oxygen and ozone. And also we have surface
vegetation signal as well. Right now, we’re not there. I just want to be
clear we’re talking about future observations. So we’re in indirect
detection, not transmission, but in direct
detection we’re now detecting hot, giant planets. And for transmission we’re able
to do superearths, warm, really warm or hot superearths. So really, these sorts of
observations are future. And this just shows
you the spectra of three terrestial planets
that we all know and love, the Earth, Venus, and Mars. And you can see that those
spectra are very different. And we expect the
universe to provide us with an abundance of
different types of planets, and we are hoping
that we’re going to be able to distinguish the
different types of planets by looking at
something like this, and getting spectra
from the planets, and being able to tease out how
does one planet look different from another. Our first opportunity
to be able to do this is going to be with the
James Webb Space Telescope, launching in 2018, as well
as with some of these large, ground based observatories
like the GMT, the E-ELT, and the TMT that
are being built in the 2020s. And even more so that
the next generation of missions, something that
in the astronomy community we term LUVOIR or HDST is
this future mission. Something that’s more like a
12 meter space based telescope. This would be able to
get the direct light from these planets, of
habitable, Earthlike planets much more easily. And so that’s kind
of the trajectory of where our technology
is going and where we’re hoping to aim our
future research efforts. So my work, though, is
interested in how does the star impact the atmosphere,
the spectral features, and the biosignatures
around these planets. And so I’m going to talk a
lot about FGKM stars today, particularly the M stars. By and large, you’re going
from a high UV environment for F stars, down to low
UV, though then M stars also have a spike in UV environment. And these– FGKM is,
again, just bigger, hotter down to the cooler
ones for those of you who are not familiar
with that terminology. And UV destroys
some biosignatures. So it could destroy,
say, methane for example in the atmosphere,
and that might make it harder to detect for us. But on the other hand, UV
produces other biosignatures like ozone, making that
one then easier to detect. So UV is kind of a
mixed bag in our ability to detect the biosignatures
around these future planets that we’re going to observe. In addition to
that, it’s the ratio of the far UV, so the shorter
wavelength, higher energy UV, to be near UV, or
the lower energy UV. That matters. And you can see this
in the reaction rates for the production and
destruction of ozone. So here, you can see that
the production of ozone is dependent on this
far UV energy photons. But the destruction depends
more on the near UV light, and can be produced that way. So it’s not just how much
UV you have in general, it’s where in the UV
spectrum you have. We want to have accurate
observations of the star UV in order to understand
our future observations. So I do modeling. This is me, before
I dyed my hair red. And as Blakesley said,
we can learn a lot from computer modeling. This is the only time
I’ll be like a movie star. And so we do this to model
these atmospheres of exoplanets, and were interested in
particular questions like what wavelengths
should we be looking at? What resolution
should we be looking to build our spectrometers
for to detect these features? And how big of a
telescope do we need? Do we need that LUVOIR
sized telescope? Can we go, you know,
instead of 12 meters could we do 10 meters? You know, and these
sorts of questions are the type of
questions that I hope that my work will help answer. So I mentioned the FGKM stars,
and today just due to interest of time, I’m going to really
focus on these M stars, because these are very
interesting stars. So the M stars are the
coolest stellar type, and they’re much
smaller than our sun. But they make up
75% of the stars in our universe and
solar neighborhood. So in the about the 300
or so closest stars to us, 246 of them are M stars. And so these are going
to be our nearest targets and are just very common places
that we would expect to look. I love animation
by Elizabeth here, where this is the night
sky as you normally see it. And then now, if you
turn on what it would look like with the M stars. I don’t know if you
can see the contrast. Now they’re starting to
see more red dots there. You get to see how
many more stars we would see if we could actually
see these M stars in the night sky. M stars have a couple
of key advantages. One is because they’re smaller,
a similarly sized planet crossing in front is going to
make a bigger transit depth. That’s very useful for us as
astronomers to detect planets. So you can see here is say an
Earth sized planet crossing in front of an an
M star, it’s going to create much bigger depth than
an Earth like planet crossing it in front of a star that
something similar to the sun. And so, this is
one reason why we want to also look at M stars. In addition these
planets around M stars, because they’re
cooler, are going to be orbiting at
much closer distances. So that means their habitable
zones are closer, in right. And so this is a
diagram of stellar type here, and the habitable zone. So here’s say, our sun,
and here’s the Earth, that’s in the habitable zone. But for an M star,
the habitable zone is much closer to that star. So it’s orbiting maybe every
20 days instead of once a year. And if we’re trying to add
up especially transits, that’s much more
convenient for us, because we don’t have to wait
for observations once a year. We can get them every
10, 20, 30 days, depending on the star type. And the most exciting
thing, I think, in one of the
discoveries recently is that one in four M dwarfs
have a habitable planet. That’s amazing,
because M stars are the most abundant
stars in our universe and in our solar neighborhood. And it appears that terrestrial,
earth like planets form quite frequently around them. So they’re good places
for us to follow up. They have some problems. One is they have
a lot of flares, and they might be
pretty difficult places to live around. In particular, early
M stars remain active, even early M stars, which
are the hotter M stars here, remain active for one
to two billion years. For comparison, our
sun remained active for, say, half a billion years. And later M stars,
past M4 or so in type, those later M stars
remain active for six to eight billion years. So this is the amount of
time that they remain active with spectral type. And so that makes our follow
up and understanding what’s going on is going to
be more difficult. Also, M stars are just very
hard to characterize in general, and there’s a lot of
work being done on that. So I’m going to go through
kind of a little cartoon drawing of the
differences between what I’m going to call an active M
star and an inactive M star, and then some of the
observations that we have, and talk about how
that influences our modeling of these planets. So this would be something like
an active M star spectrrum. And this is the theoretical
minimum limit of activity. So that’s just assuming–
going back to Blakesley’s point that magnetic fields are really
hard– the UV in a lot of stars is driven by magnetic field
activity in the chromosphere. And say you didn’t have
that around an M star, you only had just
the temperature in the photosphere
radiating, then this would be the minimum
amount of UV that you would get. But in general, when we
measure these older M stars, we get something more like this. Something in the middle. And we actually don’t yet know
what the floor of the UV is. How low can we go. Because for some of
these stars, we only have the emission
peaks observed. We actually don’t know where
the floor of the continuum is. So for example, could it
be down here, or maybe it’s all the way down
to the photosphere. We don’t yet have
that, answer and we’re trying to– we need more
observations of these stars to probe the range
of expected UVs, and particularly the
lower range of UVs that we might expect to get. Also, I’m going to
talk a little bit about the known false
positives for oxygen and ozone. So oxygen on Earth, all
21%, is produced by life, but we have thought
about some ways that you could create
it without life. And so these are just a handful
of the six or so commonly talked about false
positive mechanisms, and what’s really
interesting about this is about a majority of them
really are most common, or we expect to be most
common, are only around planets orbiting M dwarfs. So again, understanding
M dwarfs and their UV is really important. And all of these mechanisms
revolve around the UV environment of these stars. So I’m going to go
through one example of a false positive
generations of an M star. So M stars can be
really– before they join the main sequence,
they’re really luminous. They have this
super luminous phase that lasts quite a long time. They’re very active. And during that phase, if
the planet that’s orbiting it has water, that water is going
to be photolyzed from UV light reaching it. And it’s going to
break apart the water, and the hydrogen is
going to escape to space, and it’s going to be done. And you’re going to permanently
maybe desiccate that planet. It’s going to turn
into like a Dune world. And unless you have later
water delivery, of course, but this is a concern. And if you were to
observe such a planet, you might see a buildup of
oxygen in that atmosphere, depending on how
much of that oxygen is going to then later
react with the surface. Which is something
that a lot of research has been going on into
that problem as well. Another example
of that, was this was the first
example, I think, that proposed of abiotic
oxygen, was for planets that are on just
on the inner edge– before the inner edge
of the habitable zone, so they’re all a little
too hot, shall we say. And they go through a
runaway greenhouse effect. So you can imagine
this scenario, it’s basically the same thing. You have the UV light from the
star breaking apart the water, and you get the hydrogen
escaping to space, and your oxygen’s left behind. So again, this goes
back to the point that I made at the beginning,
is that to detect life on another planet,
is we’re going to need a combination of gases. One gas is probably
not going to be enough. We’re going to need context
and multiple gases in order to understand what we’re seeing. And so this goes back to
the idea that was first proposed a long time ago
with Lovelock and Carl Sagan, and a lot of people
have talked about this with the combination
of oxidizing and a reducing gas, and
context to eliminate these false positive mechanisms. And in addition to this,
because most of these mechanisms rely on the UV of the
host star, we really need to use real UV
data in our modeling. In particular, because of
the complex magnetic fields, we can’t yet predict what
that would be around a star, so we rely on
these observations. So work is being done
on that as we speak. So here is the real data
instead of the cartoon, where you have this is
a bright young flaring star, AB Leo in the black line. And this green line is
typically quiet M star, and then the red line is, again,
that lower theoretical limit. And we yet don’t know how
low can this go, so to speak. And this difference is
10 orders of magnitude. So understanding what the
lower floor of the UV is is going to be really important
for us, especially since we only have one UV telescope that
can make those measurement, which is Hubble right now. And Hubble’s not going
to last forever . So when we look at
these M star models, these are for all the
different types of M stars, going from the hotter M
stars to the cooler M stars. The only thing I want you
to take away from this here is this is
on a linear scale, and this shaded
region is the UV part. So it doesn’t seem like it’s
a large part of the flux, but it’s absolutely
dominating what’s happening in the atmosphere. Because all of
your photochemistry only cares about
the UV light that has enough energy to
break apart these bonds. So if we compare two extremes,
like most maximum amount of UV that we would expect and the
most, absolute minimum, maybe even less than we would expect
for an Earth like planet orbiting an M5 star, you
can see that the spectrum might look very different. So this is the same planet just
put around two different stars. So with different UV, it’s
the exact same stellar type. It’s the same size
and otherwise star, except for just different UV. And so on the black line,
you have flaring, young star, and and in the red line you
have this theoretical minimum. And different sorts of gases
build up in the atmosphere and create different
features, and you could have a very different spectra. Which is why characterizing
the UV of these stars is going to be vital
for us to interpret the signs of biosignatures,
and understanding the context of what we’re
observing in the future. So I hope I’ve convinced
you that stellar matters, and that UV matters, and for
the second part of my talk, I want to talk about
how planets change. So we know that planets evolve
through geology, through plate tectonics, through
life, and we’ve on Earth had many different
phases going from the Hadean– Hell, as I promised I would talk
to you about– to the Archean, to Snowball Earths, and
even Jurassic period. Our planet has gone
through many changes. And I like thinking
about the history of life on our planet to
understand the scale. It’s really hard for us to
wrap our head around the scale of these timelines. So I’m going to present it as if
the whole history of our planet was in one hour. So we started at zero minutes
with the formation of Earth, so that was 4.6 billion
years ago or so. Then you have the
origin of life. We don’t exactly know when,
there’s a big question mark there. But somewhere around 3.9
to 4.4 billion years ago. That’s roughly nine
minutes into the hour. Then you have the
oldest signs of life, that’s around 3.5 to 3.8,
depending on which papers you believe, billion
years ago, around 14 minutes into the hour. You have oxygenic
photosynthesis, definitely has been around since
around 2.7 billion years ago. That’s 25 minutes into the hour. But this always blows my mind
every time I think about this. Multicellular life, so anything
more than just one cell, wasn’t around until
one billion years ago. Very recent. That’s 47 minutes into the hour. And land plants weren’t around
until 0.5 billion years ago, or 53 minutes into the hour. Humans, anatomically modern
humans, not hominids, but humans, in our
anatomical form have been around
250,000 years ago. So that’s literally in the
last second of this timeline. And the atmosphere,
of course, has changed through all of this. So here we have
no units, notice, because we have very
poor constraints on the early atmosphere. But the time, this is Earth’s
formation all the way up to present, and just kind of a
cartoon of what we might think has happened. So we expect there was
more carbon dioxide right after the planet
formed, and before oxygen took hold there was methane as
methanogens kind of dominated the planet. But then once the
oxygen rose, the methane started declining, because
again, those react together and the lifetime of
methane goes down from being 1,000
years to 10 years. Once you have oxygen around,
and then carbon dioxide is decreased, and
now of course it’s increasing again due
to human activity. So that’s sort of what
we have going on here. And oxygen has rose kind of in
two distinct little steps here, and there’s been some other
complexities along the way. But that’s the broad picture. And so I’m going to take a
look at sort of four time points in this Earth history. From something that’s a
pre-life sort of planet, something that has no
necess– just geological gases in the atmosphere, to
something with 1% PAL. So, this is Present
Atmosphere Level. So that means 1% is 1%
of our 21% of oxygen, not 1% of oxygen. And then likewise,
10% PAL of oxygen would be 2.1% percent
oxygen in the atmosphere at 0.8 billion years ago,
and then the modern Earth. So those are four time points. And this is where we get a
little bit into some graphs. We have four slides, of a lot
of lines and a lot of graphs, and then we’ll get back to
some of the big picture stuff. So here is the pre-life
spectra of a planet orbiting different types of stars,
going from the purple colors to the red colors and
different stellar types. And I’m going to go through the
different geologically epochs. This is the first rise of
oxygen, second rise of oxygen, to the modern atmosphere. Now there’s a lot
of lines there and I don’t expect you to take
it all in right now, but I want to
highlight a few things. One is that the ozone level
pops out very early, actually, for the planets orbiting
the hottest stars. For the planets
orbiting the F stars. Because F stars have so
much more UV radiation, that they make ozone
more efficiently around those planets for
the same levels of oxygen. And then you see it starting
to form for all stars here, and methane is of
course changing as well in play
with how much oxygen there is and, how much
flux we’re assuming is coming from biology. And so it might be easier to
detect, for example, oxygen or ozone in combination
with methane actually earlier in history than
in our modern atmosphere. And then you have the
modern atmosphere. So that’s again where
we want a combination ideally of something
like methane and oxygen. And this is easiest
to do in the IR, though there’s
also reasons why we want to look in the visible as
well, which we’ll talk about. Also the CO2 feature
has changed as well. So the amount of CO2, of course,
depends on the future strength. But interestingly enough, you
look at the bottom panel here, you have this peak. And that’s due to our
temperature inversion. So ozone heats the
stratosphere, and then that causes an emission peak in
the center of the CO2 feature. This is something that we
might be able to observe, and then it can be like
a secondary indicator of an inversion later, or
ozone in the atmosphere of a terrestrial planet. So I wanted to highlight
the oxygen feature though, and this is in the visible,
because this is the one that a lot of people talk
about and a lot of missions are focused on. Can we detect oxygen
around the planet? And whether we detect
oxygen, just as going back to your question
about the media, whether detecting oxygen as
a technological goal I think is very valuable. But that’s not necessarily
detecting life. Because we need to keep
in mind that there’s a lot of false positives. So this is something that I
hope the astronomy community is going to do a very good job
as we start coming and getting these first detections
of potentially biosignature molecules in
the atmosphere of a planet. So here we have oxygen
going from pre-biotic to the 1% again, PAL,
10% PAL to modern earth. This is just kind of the
absolute future strength. But when we look
at what we would see with our telescope, when
we add in all the fancy stuff, it gets a little harder to see. But in a clear sky model
you can still see those dips pretty well, right. Now the interesting
thing is with clouds, it really disappears. So there’s a lot of lines here,
but I just want you to compare. Look at, there’s no real– you
can’t really tell the dip here compared to here, and it’s much
more minor there than there. Whereas for the modern
Earth atmosphere, you can tell no matter
what the cloud coverage is. And so this is something
that my thesis work showed. Is that clouds are really going
to be a confounding factor, and yet we’re
already seeing this in observations of exoplanets. That clouds can really block
parts of the atmosphere, and this is going
to be something that we’re going to have
to try to figure out how to get around. But it is a potential problem. And if we look in the
IR, though, clouds are different, because
it’s more based on the temperature difference
between the absorbing and emitting layers. So if we look at ozone the IR,
you have the feature strength. Again, this is just the
absolute absorption. Then you have the clear sky
model, versus the 60% cloud model. They don’t look, you know,
qualitatively all that different. And so in some ways,
ozone would be much easier to detect through
geological time than oxygen. And so if you compare just
the oxygen versus the ozone, again here, you can see that it
pops out much faster in ozone than it does for
oxygen. Though of course once you get to modern
levels of oxygen, it’s just it’s just there. And interestingly enough,
the feature for ozone is actually deeper in the past
for some of the hotter stars, because the hot stratosphere
actually washes out the feature. So these are all sorts
of things that we’re going to need to keep
in mind when we’re looking for these features. I just kind of want to end with
like this global sort of idea of we’re now at the stage, the
first stage in human history, that we’re able
to start thinking about answering this question
of are we alone in the universe. And if we just think about the
history of humans in one hour, we had the agricultural
revolution– I’m starting there, I could’ve
started 250,000 years ago, but then the clock
would be really skewed. So we have the agricultural
revolution roughly 10,000 BC. Then you have the first evidence
of metallurgy around 5000 to 6000 BC, or 20
minutes into the hour. Written language around 3200
BC, 34 minutes into the hour. And then the Industrial
Revolution 1760, 58 minutes and 43 seconds. The internet in 1969, 59
minutes and 46 seconds. And the first exoplanets
discovered in 1992 and 1995, 59 minutes and 54 seconds. So we’re really
approaching this time where I think
we’re going to find a lot of interesting
things about our universe, as you’ve heard from
the previous talks, and hopefully about
our place in it. We have TESS launching and
CHEOPS launching in 2017. These are going to be great. TESS is going to find
thousands of close by planets, dozens of these are
going to be rocky and in the habitable
zones of these cool stars. CHEOPS is going to get
that accurate radii of known planets. And this is important
because then we can get the bulk
density, and start to figure out what are these
planets actually made of. We’ve had some amazing
discoveries this summer. The first one was this temperate
planet, the TRAPPIST planets you might have heard about
orbiting a very late M star, or brown dwarf. And these planets are
something that maybe we could follow up with
JWST, and one of them is potentially habitable. And that was very exciting. And then I’m sure
you heard last month, there was even the
more amazing discovery of the terrestrial habitable
planet around Proxima Centauri, the closest star to Earth,
which which is just amazing. And I’m so excited to see
what the future’s going to hold for that. Ultimately we want to be finding
these molecular fingerprints of life orbiting other planets. And it’s a
technological challenge. It’s like looking for a firefly
in front of a spotlight. Imagine that spotlight’s
in California and we’re in Massachusetts. It’s really hard. But we’re going to try to do
it, and at the first opportunity we’re going to have as with
James Webb Space Telescope and with these large
ground based observatories. And we live in a big galaxy,
and so I’m pretty hopeful. We’re one star
amongst billions, we have a lot of
places we can look, and we have billions
of galaxies. And hopefully we’re going to
find some planets like Earth, and be able to characterize
those planets in the upcoming decades, and finally answer
this question of are we alone in the universe. Thank you. -Hello. This might actually be a
little bit outside your topics, but from what I read, the
planet in the solar system other than Earth is
most likely to have life is supposed to
be Europa, because of its water and
its internal heat. But obviously that’s not
in the habitable zone. It’s not Earth like,
obviously, so how would that change–
like, you’re not looking for the same
kind of– you’re not looking for a model
of Earth to determine whether it has life. So how does that
change the process of looking for signs of life
on a planet like Europa, if we were going
to look for that? I think we’re launching
a craft over there at some point in the future? -Yeah, I think that’s
a great question. It’s actually a question
that’s very insightful. Because I would
say that where life can exist in our own solar
system and in the universe is much in much more
wide areas than where we’re planning to first target. If you take the
example of Europa, yes, I think it is a very
habitable environment because of the things you mentioned. There’s water, you have
geothermal activity, you have rocks and minerals. You have a very
stable environment for billions of years. I’m very hopeful there. But it doesn’t
have an atmosphere. It’s not interacting
with– like, we couldn’t observe that on
around another star system. That’s way beyond our horizon. And so when we say
habitable zone, sometimes it’s a
shorthand, I feel, for the remotely
detectable habitable zone. So if we can’t remotely detect
life on Europa from Earth, how could we ever
detect a Europa analog around another star system? It’s just way too
difficult. So when we’re talking
about biosignatures around other planets,
around other stars, we’re really talking
about planets that have an active
biosphere that is like Earth, where there’s
just a lot of life on it. That’s what we’re hoping
to be able to detect. Whereas like Mars,
for example, you know, also could be habitable,
but we can’t even tell if there’s life on it yet,
and we’ve sent a lot of rovers and we have observations of the
orbiters and stuff like that, and we’re still like [GROAN],
trying to figure all that out. So I feel like for
these types of planets that are outside the
traditional habitable zone, they’re certainly interesting
for our solar system search to probe the where
life lives and how does it arise in
different environments, and can we tell if it’s
a second origin of life or if it was transferred
from meteorites within our solar system. All of these things
are fascinating for our solar system, as
well as like with Titan, if we find life in the
liquid methane lakes. It’s not based on water. So interesting, but
again, how would we tell if that life– what
are those biosignatures? So that’s why
we’re first looking for life that looks like ours. -So, I guess, does
that mean like– so, let’s say we are looking
on Europa just– oh, I’m sorry– we are looking on
Europa or Titan, or Mars. Does that mean’ we’re
kind of reduced to just, kind of base soil samples,
like other than conjecture, like, we we don’t have
much else to work on, since we’re not working with
the Earth model as much? [INAUDIBLE] an Earth model? -I mean, we can go to
those planets and moons, which is amazing. And we can get samples there
and do measurements in situ, and so that’s what makes those
like much better targets. And so it’s kind of–
there’s this kind of exoplanet science for planets
outside of our solar system, and that’s for a
solar system planets. I think they’re both two prongs
of very interesting questions, and I’m excited about all of it. -Are there other questions? Dimitar. You’ll have to get
the mic, though. -This is a question
for Blakesley. Blakesley, I wanted to
ask you a question which combines a little bit of
your work with that of Sarah, in the sense that
now that you’ve shown that the magnetic field
difference really matters, do you expect that there
will be serious differences in the amount of mass in
those broad [INAUDIBLE] discs around the young
stars that form when the magnetic field is stronger? And do you expect this
will change the planets architectures and what planets
form around said stars? -So the answer is yes. Yeah, I mean, this is
still ongoing work, and the simulations I showed,
sort of one of the first times that we’ve been able to do
multiple different magnetic field strengths. Usually those
numerical simulations are so expensive in terms
of the computing time you need to run them,
that it’s very difficult to do a parameter study. So we’re just now moving into
this area where we can actually change all the
relevant parameters and see how that affects things. Like what kind of stars
will ultimately form out of those different simulations. And we suspect that in
the high magnetic field strength, if you remember
you could see that filament, that large filament,
so more mass is concentrated
into that filament. That’ll change how
the gas ultimately fragments into stars. And so you might get higher
mass stars in that case than you would in the cases
where the field was weaker, and you have more filamentary
structures, like smaller filaments, more
fragmentation, you might have smaller
stars forming. That will ultimately change
what kind of star you form, which then leads
into what Sarah was talking about with these
different M dwarfs or G dwarfs or K stars. That would then change to
have habitability as well. So the magnetic field matters. -It always does. -So one question for Sarah
and the other for Blakesley. For Sarah, I guess the question
of extraterrestrial life, it has multiple layers. Your signature of gas is
based on the assumption that extraterrestrial
life has a similar biology to that on the Earth. That’s a big assumption. Unsubstantiated. Secondly, due to inflation in
the beginning of the universe, there may be parts of the
universe in which there’s intelligent life,
but we will never be able to communicate
with them, because of the vast distance
separating them. Under current physical law,
there’s no way to communicate. -Yes. -And for Blakesley, because you
didn’t specify the time points over the evolutionary
course of the universe. If you included the [INAUDIBLE]
and the [INAUDIBLE], I wonder if it would
make a difference in the simulation of star
formation on a large scale. -You mean from, like,
cosmological scales, are? -Yeah. Yeah. -Well. So I was talking
very specifically about star formation
in the galaxy. But you can also
think about how stars form within galaxies as a
function of like longer time scales. We’re talking about like
many, many gigayears. And actually the
universe, we know, from measurements and
also from simulations, the star formation rates were
much higher 5 billion years ago than they are in the
present, you know, modern Milky Way type galaxies. So the universe back then
was actually producing stars at different rates, and we
really don’t understand why. So there’s a lot
of evidence that it has something to do with
the accretion on to galaxies was higher in the
early universe, but it’s still an open question
about what kind of galaxy physics we need to
really consider in order to see the increase in
the star formation rate at earlier cosmic times. -Right, and only face of
the universe [INAUDIBLE] -I’m going to answer
the questions as well, that you asked first. So, yeah. I think they’re excellent
questions actually. Because why would we expect
to find similar biosignatures as what we find on Earth. And a large part of this,
we don’t know, first off, but a large part of this relates
to chemical energy gradients and things that we know work on
Earth that we might expect to, through Darwinian
evolution, arise. So if we look at the path
of life on Earth and just in the universe, I would quickly
mention that CO2 is abundant. Water is abundant. So these sorts of, you
know, raw materials for life are really common. Silicon is less common. So these are sort of things
why we might expect life to use them, as well
as water has some very interesting properties. And we have literally a
lot of places to look. So we’re going to look for
places that we can recognize, because I just
don’t think we would be able to tell
whether it’s life or not if it’s a totally
different biochemistry. But I want to go a
little bit further into that is if you look
at oxygenic photosynthesis. It relies on CO2 and
water and starlight. Those things are going to be
on other planets, you know, because those are both common. And so it seems to
me likely that if you have some sort of life that’s
able to take advantage of it, it would. Because you have a lot of
energy source from the star, and then you’re using
things that we know are common on other planets. So that might lead
rise to oxygen. And then on the flip side of for
why oxygen is important for us, you get twice the
amount of redox energy from oxygen, about, as from
the next chemical gradient. So this is what led to the
rise of complex life on Earth. As for the second question,
the vast distances, of course that’s going to be a problem. Not just with other galaxies,
even within our own galaxy. I mean, even talking to
Alpha Centauri is going to be a long conversation. -Up here. We have two questions
up here, we’ll start with one, yeah,
and then the other. -First of all, thank
you both for the talks. Very enjoyable. Question for Sarah, and
I’ll start from very far. As an Italian, we sent
Galileo to the trial. We have our own
history of believing we are the center
of the universe, and so I always kind of
rejected anthropocentrism. So for me the question which
was the last slide of your talk, it’s kind of boring. Like, just
probabilistically I believe there is life out there. So I guess my question
is, what happens next? So if you think of your
field in 10, 20 years, I guess that you don’t
want to just put a check. OK, there is life, I’m
done, retire early. So what can you do after that? What can we learn besides
a binary yes or no answer? -OK, well first off, getting
the binary yes or no answer is going to be difficult, right? Because with these
microbial signs of life, are we going to know 100% sure? We’re not sending a probe there
with a little microscope seeing the things, you know,
swimming around in the ocean. So that’s actually going to,
in and of itself, I think, is going to be really difficult. Because we can only
maybe say, this planet seems like it has life. We can’t think of
any other reason through geology, chemistry,
and physics alone that would make these sorts
of gases in the atmosphere. So I think getting the binary
answer is already actually pretty difficult. And we’re
only going to be able to say, oh, maybe, we think this
is our best candidate. And then I think getting
that answer is exciting, because we don’t know, right. I mean, we think life is
common in the universe, and I would say I base
my personal opinion on that based on how
quickly life arose on Earth. We have evidence of life
going back very far. So it seems that microbial
life started very quickly, and so this is why
I’m optimistic. But I’m very excited for
us to map out not just is there life out there,
but also getting back to frequency, how common is it? How common are planets
that have these sorts of atmospheres or active
biosignatures or biospheres? All sorts of things that
we can do with that. And then on the
future, future horizon, I don’t know, when we have
like Starship Enterprise, looking at maybe different
possibilities of biosignatures. And we’re starting
that work on Earth right now, with looking at
alternative biochemistry. Stuff like what Steve
Benner is working on, to see what other types of
biochemistries could life use, and what kind of biosignatures
would those produce, and are we going to all distinguish that. That’s much more
difficult, but certainly I think that’s a very
interesting area of research. -As a total outsider
to the field, though, I can easily imagine
if we one day get to the point where we can travel to
one of these planets. And actually it’s
going to happen, I think, within our lifetimes
even, with Alpha Centauri, if Breakthrough Starshot works. So if that’s the case, we need
to be able to somehow know something about where we’re
going before we go there, because it still will take 20
years for Breakthrough Starshot just to get to the
nearest star system. So you don’t want to
somehow stop and turn around, because you realize
that the planet is not any good. We need to be able
to characterize these planets from a distance. -Yes. Yes to all of that. -I have a question for Sarah. So I was wondering,
obviously, how do you take the spectrum
of the planet’s atmosphere? Because from
earth’s perspective, it’s going to be right
in front of the star, and obviously the star is
billions of times brighter. So how can you tell
like that you’re getting a spectrum
of the planet, and not the start behind it? -It’s a very good
question, because it’s a bit like you’re getting
10 billion photons, and then you get one
photon from the planet. That’s a very difficult problem. And when we’re talking about
gravitational waves and all these other very
difficult measurements, this is why it’s
difficult. This is why it’s like looking for a
firefly in front of spotlight across the continent. So there’s a couple different
ways, really shortly, two ways. One is you take a picture
when the planet’s in front, you take it when it’s
behind, you subtract the two, and you’re left behind
with the starlight. You look at the light going
through the upper layers of the atmosphere as
it’s in transmission. And you can measure
how it appears to change at different
wavelengths due to that. And we’re already doing
that, actually, we’re already doing that for planets
that are superearth sized, and that are a bit
warmer than Earth. So we’re already
getting the measurements of atmospheres for
planets that are very close to what
we eventually want to get for Earth like planets. So we’re nearing that
technological capability. And then the next
way to do it, which will allow us to search for
even greater number of planets, is to just black
out the starlight completely with like a
starshade, or something on the telescope to block
out the light, a chronograph. And then you just
directly get the light from the planet that’s
being reflected off. -Awesome. Thank you. -You have a question over here? Two questions over here. -Can you give us
a little bit more of an idea of the time scale to
make the kinds of measurements that you could even
be able to see, let’s say an ozone line
in an Earth like planet at a distance of 10
light years or something. I mean, we’re talking about
500 orbits all stacked on top of each other, and so forth. And then I have a follow
up, if the answer’s yes. -Yes. So there’s a couple of
papers, Deming, 2009, and Kaltenegger, 2009, look
at calculating those features for wiht JWST. And so we’re looking at
hundreds of hours for say, 10 parsecs of telescope time. So adding up multiple
transits, maybe even looking at a planet for the entire
lifetime of JWST every time it transits to get a
full characterization of its atmosphere. So for JWST, we’re
only going to be able to do a handful of
these terrestrial planets. We’re going to be able to do a
lot of other planets with JWST, and it’s going to be very
exciting for planetarian science in general. But for habitable, Earth
like planets, you know, I think we’re going to get
the first glimpse there. We’re going to get another
glimpse with the ground based telescopes, and then really
the next generation with LUVOIR is going to be where we’re
going to hopefully get more statistical sample
of Earth like planets. -OK, thanks. -I would like to ask– maybe
I didn’t quite understand– but you mentioned that for
the first 2 billion years we didn’t have any oxygenic
photosynthesis on Earth. Would you be able to detect
life on a planet like that, where there’s no oxygen
in the atmosphere? -Yeah, so, I mean. We might have had small amounts
of oxygenic photosynthesis, it just wasn’t
widespread until, you know, especially the
first rise of oxygen 2.4 billion years ago. People argue about when oxygenic
photosynthesis first started. So it would be hard. I don’t think we
would be able to tell, because if it was just
like methane and CO2 and how much methane would
you expect from geology on a different planet, that’s
going to be very difficult. So, you know, that’s why it’s
a combination of biosignatures. The other gases, like
N2O, and methylchloride, and dimethyl sulfide
and these things, they also might be around. But they’re produced in
such smaller quantities, that they might need a much
more massive telescope to see, and they’re very usually very
individual to certain organisms rather than kind of
more general to life. So yeah, I think it’s
going to be difficult. Certainly the easiest time
to detect life on earth was after the rise of
oxygen when you also had methane in the atmosphere. So, the last 2 billion years. -Some other questions? I guess that gives me the chance
to ask the last two questions, if that’s OK. So one for Blakesley,
and one for Sarah. So I’ll just ask them both,
and you can take them. So for Blakesley, I
guess the question about going back in time
to the earlier universe. I was thinking this the star
stuff, Carl Sagan quote, kind of pinged this in my mind. And I was wondering,
did the composition of the protostar clouds, if you
change the composition in terms of the metallicity,
have you have you examined that dependency
and if so, what’s the result? And then for Sarah,
you concentrate a lot on CO2– I mean, on ultraviolet
as being a main bond breaking driver, and I don’t honestly
know about ionizing radiation, but I was just curious
if ionizing radiation is a factor of the magnetic
fields of the planets that you’re looking at? Or weak, or strong,
or what we know about possibly the magnetic
fields of exoplanets? So I don’t know, so maybe
Blakesley, you can go first. -Yeah. So you raise a very good point. So if you think back to
this gas cycle, where you’ve got warm gas
transitioning to cold gas, transitioning to denser,
colder, self gravitating, and then at some point,
poof, a star is formed. So the issue you raise
about the metallicity affects that gas
cycle very strongly, because in order to go
from the warm, ionized gas to the colder, denser,
self gravitating gas, you need some sort
of cooling channels in order to let
the gas cool down. And that is a strong
function of the metallicity. So metallicity being how
many heavier elements like iron or carbon or
oxygen that you have. And so that, the
metallicity is a function of how many generations
of stars you’ve had. And we know that in
the early universe, we don’t have a lot
of heavy elements. We have mostly hydrogen
and a little bit of helium and lithium. So the very first stars, the
very first generation of stars were probably extremely
different from the stars in the present universe,
like or in the Milky Way. The first stars had to have
different types of cooling channels, and most
likely they were much more massive than
the types of stars that we can easily form today. And so that would greatly
affect the types of simulations that you would do. So the simulations that I showed
today, the sort of parameters are very geared towards
local Milky Way type star forming regions. Sort of similar to the
Hubble Space Telescope images that I showed. There are other types of
simulations, other groups that are performing simulations at
very, very low metallicities, so more towards the regime
of the early universe star formation. Also a very active area
of research, and you get very, very different
types of stars. -Interesting. OK. -So detecting magnetic
fields is going to be very hard for
terrestrial– especially terrestrial earth like planets. One mechanism for maybe
bigger planets and [INAUDIBLE] would be like bow shock
for the stellar wind as it’s coming
around in transit. But I think that’s going to
be very hard to constrain, actually, and that would
be so interesting to know. Because like you
said, also, the kind of protection from higher
energy particles and whatnot. As well as the
ionizing radiation that reaches the surface, if
you look at earth’s history, most– the damaging radiation
would reach the surface until you’ve built
up an ozone layer. And we know life still
flourished and arose, and it did fine. Whether that was under a
layer of water or soil, or we know also that UV drives some
important pre-biotic reactions, as well as damaging others. So I think UV is really complex. And ultimately though, at
detecting the magnetic fields on planets is going
to be quite difficult. -OK, well, thank you. So we’re done with the session,
but we have a reception outside with refreshments and some
food, so please avail yourself, and the speakers will hang out
and answer residual questions. So thanks for coming. [APPLAUSE]

Joseph Wolf

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