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Imaging Planets
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来自 Princeton University 的课程
Imagining Other Earths
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从本节课中
Optics and Imaging Planets
This lecture introduces the basic of optics, explains how lenses and telescopes work, and then discusses the challenges of imaging planets around bright stars.
与讲师见面
David SpergelCharles Young Professor of Astronomy on the Class of 1897 Foundation and Chair
Department of Astrophysics
Welcome back, and now I want to talk about the
future, and talk about some of the things that I think
we will be able to do in terms of characterizing
and detecting extra-solar planets in the next ten to 20 years.
And I think, as you'll see, we're on
the verge of a very exciting period of discovery.
Where we will not just detect the planets and
learn about their densities and masses through transits.
But start to learn in ever increasing amount about
the atmospheres and composition of planets around nearby stars.
So, how are we going to do that?
Well I think the next step forward in this field will be to go to space.
And start to image and characterize planets from space.
I now want to talk about in the next couple of
minutes in this lecture, our plans looking to the future.
And looking at some of the telescopes under construction, and under discussion.
That might be able to image.
Earth like planets around other stars.
The first telescope I want to talk about is the James Webb Space Telescope.
This is a model of the James Webb Space Telescope.
A telescope with six times the collecting area of the Hubble Space Telescope.
Here you can see the telescope it's very large sun shield that protects
it and keeps it cool so that it can observe stars and planets in the infrared.
And here's a crowd of people below.
These are standard size people so you can get
a sense of how big the telescope really is.
This new telescope, this Gaines web space telescope is scheduled to launch in 2018.
And when we launch it will be able, not
so much to image planets around nearby stars or Earth-like planets.
Through imaging, but the tech planets, like earth through the fake transit,
through transit spectroscopy, something that we
talked about earlier in the course.
JWST as we like to call it, James Webb Space Telescope will
also be able to detect planets through their effects on the surrounding dust.
And we're going to talk about that in the next lecture.
So here is this extrasolar planet transit again.
And the idea with James Webb is we'll wait for a planet
to pass in front of the star, take the spectrum of the star.
Before the transient, as the transient began and during the transient, and
by watching that variation that will give us a way of measuring the spectrum
of the planet and see what's absorbed in the atmosphere.
And here is an upcoming new NASA mission called TESS and what TESS will do
is find transits around nearby stars, and provide targets
for the James Webb Space Telescope, as it looks for these transiting events.
And here's a nice graphic showing transit spectroscopy working.
As a planet passes in front of a star.
Now let me come to a telescope that we're currently studying.
And this is something I'm very much involved with.
I'm actually the Co-Chair of the science
team for this telescope that we're exploring.
This telescope.
Has been called, or at least for
now, AFTA, for Astrophysics Focus Telescope Assets.
This is a telescope that has the same collecting
area as the Hubble Space telescope, it's a 2.4 meter telescope.
It's a telescope that was originally built
for other agencies in the federal government.
It was actually designed originally to look down.
It's a spy satellite.
But the program to use this telescope was cancelled.
And this telescope sat basically in storage for about ten years.
It's recently been handed over to NASA.
And we're all very excited about this because this telescope.
It's really very well designed for doing lots of exciting astronomy.
We're going to use it for several different purposes.
We're actually planning to use this
telescope for four different major applications.
It's a telescope that is the size of Hubble, but able to
take images that are 200 times bigger,
so imagine every Hubble telescope picture you've seen.
But now make that picture 200 times bigger so
you can gather a tremendous amount of information, and just
like we've been able to use the Hubble telescope to
learn a lot about Our own galaxy and extragalactic systems.
We hope to use this telescope to make exciting discoveries.
Like we do with the Hubble.
We also plan to use this telescope to survey large areas of the sky.
To study the nature of dark matter and dark energy.
Astronomers only know about four percent of the stuff that makes up
the universe around us, we only know the stuff made of atoms.
We've been able to detect gravitationally the presence of
this dark matter, dark energy, but we don't know
what it is, and one of the things we're hoping
to do with this telescope is use this to study.
Dark matter and dark energy, but then the other two applications
of this telescope are using it, to learn more about extrasolar planets.
We plan to point this to telescope towards the
center of our galaxy, toward the regions called botaswindow.
A region that doesn't have that much dust or
along the line of the center of our galaxy.
And we will use this telescope to look for gravitational micro lensing.
Recall a number of lectures ago we
were talking about Einstein and general relativity.
And how we could use generativity and gravitational microlensing
to find planets in the outer-regions of solar systems.
And find planets like earth out of the orbit of Jupiter.
We don't have other, we really can't detect planets
that are earth sized that far out using other techniques.
So we're going to use this for micro-lensing, and
we're going to put a Corona Graph on it.
And this will represent, we believe, the first telescope that will be
able to find planets like those we see in our solar system.
Planets like Jupiter and Saturn and Uranus, and if we're very lucky.
Urse.
But probably things what we call Super Urse.
Around nearby stars.
This shows, what we hope to see.
So this plot shows the kind of things we think about.
We think about imaging extra solar planets.
This shows the contrast ratio.
Relative to the star.
So up here were looking at planets that
are one millionths the brightness of their host star.
Here we are looking at planets that the
one billionths the brightness of the host star.
And here we are looking at planets that
are one trillionths the brightness of the host star.
Plotted on this axis is the separation from the star measured in arc seconds.
This is the separation of one arc second.
A tenth of an arc second.
A hundredth of an arc second.
The resolving power of the Hubble Telescope's around here.
What we're showing here is the range that we're
hoping to get at with this telescope and we're
pretty, hope sure we can get to around here
and we'd like to push further in this way.
And what's shown in these different colors are a
simulation of what we would be like if there were.
Earth and Jupiter and super Earth-like planets around the nearest stars.
So if they were out there, they would fill up this space here.
What you should see from this plot is if we put a coronagraph
on this telescope, What we can do is start to get into this interesting range.
And we should be able to image many planets like Jupiter and
Saturn around nearby stars, probably image a class of planets that we'll
talk about soon called Super Earth, and if we're really lucky Perhaps
we'll be able to get an earth like planet around the nearest stars.
And not only will we be able to image these, we will
fly a spectrograph on this that lets us tape the planetâs spectrum.
And we will be able to see if a star, a nearby star, the kind that we see.
Typically in the night sky.
When we look out at night.
Has a planet like Uranus, or Neptune
or Jupiter, and start to characterize its atmospheres.
But, you should also take from this plot, that this is
just the beginning of our exploration of planets around nearby stars.
In terms of characterizing properties that are atmospheres.
We really want to get into this region here, and
start to detect planets and characterize planets like earth.
We want to see if we're looking at an earth sized planet around a nearby star.
We want to determine whether it has an atmosphere that
has oxygen, that has water, that has signs of life.
And this is the direction of where we want to go, as we look towards the future.
In order to image planets around nearby stars, and block the
sunlight from the star, we've been
exploring another technology besides the coronagraphs.
A technology called an occulter.
The idea with an occulter is that you want to block the light from a star.
Well, what would you do if want to see something dim next to the sun?
You'd hold up your hand and block the sun.
That's the idea with an occulter.
We have our telescope here.
Our target star here and our planet here.
What we want to do is fly a big screen.
We call it a starshade.
And that screen blocks the light from the star.
And if we build that screen with the right shape it can create A dark region behind
the screen where none of the sunlight starlight from this star gets to us.
So then we have to fly two telescopes in space.
One telescope at a starshade.
Line them up just right, we could block the star.
And have all the light from the planet get through to us.
And what we've been doing over the last decade or so is developing some of
the technologies we need for the star shape
and thinking about how to make this happen.
Now this is not a new idea.
When we started looking into this, I was intrigued to
go back and learn of an old articles by Lyman Spitzer.
Lyman would've been 100 a few weeks ago and what
Lyman, is one of the great figures of late 20th century astronomy.
Often called the father of the Hubble Space Telescope.
Back in the late 1940s, it was Lyman Spitzer
who envisioned the idea of flying a telescope into space
and talked about all of the things we could
learn if we could build something like the Hubble Telescope.
And he devoted a part of his later career to making the Hubble Telescope happen.
But back in 1962, back long before we detected any extra
solar planets, in a conference on the beginnings of space astronomy.
Lyman Spitzer wrote that one of the problems
of particular interests that we emphasize here is
an important part of the long range program
is the detection of planets around other stars.
This is a matter of great philosophical,
and cultural, as well as scientific interest.
The detection of extrasolar planets is an extraordinarily difficult problem.
Fortunately there's a better way of detecting extra solar planets around other
starts, a method pointed out by Ron Danielson, who was here at Princeton.
Danielson, sadly, passed away at a very young age.
This method involves the use of an occulting disk far in
front of the telescope to reduce the light from the star.
And then goes on to describe that disk.
So, this is not a new idea but one that we're making real and happening now, and
one of the things we'll see later is an
actual occulter being deployed at the Jet Propulsion Lab.
Now one of the things we did to give a sense of
where what I think things are going in this field is for NASA
we developed a program of the kind of thing we could do if
we could fly an even bigger telescope, bigger optical telescope with occulter.
And we laid out what we could do with a telescope that we named THEIA.
It's a hypothetical telescope.
And the kind of things we could do with a telescope in our occulter.
First you'd like to detect it, search for it by measuring the color, seeing if
you could find the blue part of
the atmosphere, detect the signature of life potentially.
Look for things like oxygen and methane, and water, and ozone.
As we'll talk about soon, even detect, potentially, the
presence of life, and search for continents and oceans.
Well, how would we do that?
Well, let's go back to the Earth's spectrum.
I just want to give you a sense of how much information there is in the spectrum.
So, here is a Simulation of what we might hope to see, in the Earth's spectrum.
By measuring its color, by measuring how much
light comes in the blue, versus the green.
This depends upon the amount of pressure in the Earth's atmosphere.
The more pressure we have, the more air we have above us.
The more rally scattering the bluer the sky is.
So what, by seeing what the spectrum there rises sharply or less sharply, how blue it
is, that gives us a measure of how much air there is on the planet.
We can look for features like oxygen.
Here is oxygen lines.
In the planets atmosphere that tells us there's the planet is something that makes
oxygen we can look for thing like water and things like ozone, and
intriguingly we might even hope to find the signs of plants.
What's called a red edge.
And let me turn here to talk a little bit about the potential signature
of plant life that you, we might, if we're very lucky, be able to observe.
And what's plotted here is how much light is
reflected off of different planets as a function of wavelength.
Here we're looking at visible wavelengths
and here we're looking at infrared wavelengths.
What do you see?
Plants absorb most of the visible light that hits them.
Right, that's what plants do, they take sunlight they
absorb the optical light and they use it for photosynthesis.
Plants reflect a little bit more in the green, typically then
they do in the blue or red because of this little bump.
Plants are green, there are slightly more reflective
in the green there at the other wavelength.
But as you look at this plot, you'll see that plant reflectivity,
jumps up enormously and these are whole bunch of different types of plants.
And for all these different types, once you get beyond 700 nanometers
Beyond where our eye cuts off, plants are incredibly reflective.
If our eyes extended just a little bit further into the infrared,
when you looked out on a lawn of grass, or a forest,
that grass or forest, if our eyes could observe in
the infrared would actually look brighter than freshly fallen snow.
So plants absorb almost all the optical light.
That's what they use to photosynthesis.
But they reflect almost all the infrared light.
The reason they do this is to stay cool.
If you're absorbing all this optical light, it heats you up.
And you don't want to absorb the sun's infrared
radiation, you want to be as reflective as possible.
So evolution has favored plants that have
a very distinctive feature in their spectrum.
And this feature.
Is called the red edge.
And we don't know if there are plants
on planets around other stars.
But if there were plants and if the basic physics that governs how
they operate is similar to plants on earth We might see something similar.
We might see a plant that absorbs in the optical and reflects in the infrared.
It has this very sharp edge.
We don't really know of anything that's not alive that has such a sharp feature.
Evolution has favored this.
So one of the things we hope to see when
we image planets around nearby stars, and this will require a
bigger telescope then we have right now or even bigger
then this 2.4 meter after telescope is this hectic red edge.
So looking to the future what we hope to be able
to do is Identify planets that are earth like around nearby stars.
And then to characterize them.
When we characterize them, what we want to be able to do.
Is look at the properties their surface.
Determine if the surface is mostly say, ocean, or ice, or land.
Look at the properties of the atmosphere.
And determine whether we can find at, measure the atmospheric pressure.
Find things like oxygen and water in the atmosphere,
perhaps methane and characterize what the planet's atmosphere is like.
And not only measure the properties of the planet's atmosphere and its surface.
But even if we're lucky, look for signs of life.
And detect this red edge.
And be able to say that we think we're
seeing the signature of plant life around a planet.
Around.
On a planet around a nearby star.
Now to do that will require advances in our technology.
the telescopes we have today are not capable of seeing a signature like that.
But today we're also developing those technologies.
We're building bigger telescopes and we hope to fly bigger telescopes in space.
We're developing occulters and coronagraphs that will
let us image Planets around nearby stars.
And, one of the things I hope you do what, after this lecture's
done is click on the link that will take you to a YouTube video
that will show you the occulter unfurling at jet propulsion lab, and
this occulterâs just the kind of technology that we hope to fly in space.
So that someday, and perhaps someday soon, we can image a
planet around the nearby star, and find the signs of life.
