An introduction to modern astronomy's most important questions. The four sections of the course are Planets and Life in The Universe; The Life of Stars; Galaxies and Their Environments; The History of The Universe.
Exoplanets
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Confronting The Big Questions: Highlights of Modern Astronomy
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Are we alone in the Universe?
Planets and Life in The Universe - Exoplanets searches, exoplanet census, astrobiology
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Adam FrankProfessor
Physics and Astronomy
Welcome back everyone.
Now that we've covered the solar system- our
solar system- it's time to look out to the stars and
begin thinking about planets around other stellar objects, other stars.
And so we want to talk about the idea of exoplanets,
and this is incredibly exciting field.
The question of whether or not planets
orbit other stars goes all the way back to the Greeks.
You can see people writing about it and thinking about it 2,000 years ago.
And remarkably we didn't know the answer to that question or the question of are
other planets orbiting other stars until around 1995 or 1996,
and I remember very well when this discovery was made and it was really
remarkable to have this millennia old question suddenly be resolved.
And in that time, in that brief time since 1996,
we've gone from not knowing whether there were any planets orbiting other stars,
to now knowing that- having you know direct detections of more than a thousand of them,
and being pretty certain that almost every star has planets orbiting around them,
then that's really a remarkable occurrence that we've gone from not knowing anything to
now being quite certain that the galaxy is teeming with planets.
Now, the question is whether or not those planets are teeming with life.
We're trying to get to the answer to that question, we still don't have it.
But it's important to understand how far we've come and how quickly.
All right so the questions we'd like to ask
about planets- obviously we want to know if there's life on planets,
but we have to work our way up to that.
And we first have to understand what these exoplanets are telling us in general
about how nature creates
the conditions under which life might possibly exist in solar system.
So we really want to do a census of solar systems.
So how common are planets,
and in particular how common are different kinds of plants.
I've already told you that pretty much it looks like
almost every star has a planet orbiting around it.
But how are those stars arranged?
Where are they?
How common are Earth like planets?
That's a very important question.
Planets that have about the right size as the Earth does and also are in
the right kind of orbits such that they could have
liquid water and all what you need to have life start.
How common is that?
And of course, which of these planets actually get life going and managed to maintain it.
So those are the questions we'd like to answer.
But there are problems here, you know there are difficulties,
real difficulties in answering this.
Individual planets may be difficult
to spot especially if we want to do something like direct imaging.
It's like you know trying to see a planet orbiting its star
is like trying to look at a fire- find
a firefly that's flying next to one of
those giant klieg lights you see outside of
a movie opening and doing this looking from New York.
Right. You know the planets shine by reflected star light
and they are very very dim compared to the star that they're orbiting.
So what we want to actually learn directly by studying planets,
some data that we need to gather,
is we want to know there period.
We want to know how long it takes for them to orbit their star.
We want to know- and that also tells us how far away they are from their star which means
how much star light they're receiving and that's going to
tell us what the temperature on the planet may be,
at least gives us a beginning estimate of the temperature on the planet.
We want to know about the planet's mass and we want to know about its radius.
We'd like to measure those directly because we can put those together,
we can get a density and density is all important because if we find the density of
a planet is much like the density of rock then we know it's a terrestrial planet.
If the density is much like the gas giants,
we know it's going to be a gas or an ice giant.
And it's, you know, as we've seen those planets don't have any surfaces.
So we don't expect life to form on a gas giant,
because there really is no on the gas giant,
may form on moons around a gas giant,
but the gas giant itself is unlikely to get life.
So we're interested- we want to know the planets period,
we want to know it's mass, its radius.
We'd like to be able to directly measure the presence and composition of an atmosphere,
and that may be remarkable to you that we can do that,
but it is remarkable.
It's remarkable to me that by just by attaching glancing
star light passing through these planets atmospheres,
we've actually been able to a),
see that there is an atmosphere and b),
get some sense of the composition and
conditions on these atmospheres- of planets that are light years away.
It's a remarkable thing.
And then you know the big picture- can we put this all together and try and get
understand the conditions on these individual planets for life, in particular,
might we be able to use information about the atmosphere to see whether
there are what are called biomarkers- special spectral lines,
special indications from the atmosphere that life must exist.
For example, we believe that if you could find oxygen in an atmosphere,
that would really tell you that life has to
exist because if it wasn't for life on the earth,
all the oxygen would very quickly combine and drop out of the atmosphere.
So seeing actually oxygen as a component of a planet's atmosphere should tell us very
much- we believe that life has formed there, or it's ongoing.
OK, so now I'll ask how do we actually find exoplanets?
There's a bunch of different ways to do it.
We're going to go through them one after the other.
Of course, what you really would like to do is do a direct imaging.
You'd actually like to take a picture of a planet.
But as we talked about that's very hard to do because planets are
so faint compared to their stars.
But one way to do this is to actually look at young planets,
because when a planet first forms,
it's still pretty hot, particularly gas giants.
We've seen that gas giants emit
radiation- more radiation than they absorbed from the sun,
so if you catch a solar system very young,
a young Jupiter will glow particularly in infrared and here's a picture of Fomalhaut-b,
it's that little dot there,
and Fomalhaut-b is where we now have good evidence that this is a planet,
a young Jupiter that is being imaged very early in its lifetime.
So we can do a bit of direct imaging,
usually the planet has to be, you know,
pretty far from its star and it also has to be fairly young.
Now, the other way we might want to go about looking for planets is to actually look for
the dance between the planet and the stars.
So when two things orbit each other,
it may seem crazy,
but even the star,
that the Earth orbiting around the sun actually makes the sun wobble a bit.
So when we think about two things orbiting each other,
if they had the same mass,
they actually orbit around a center their common center of mass,
which would be in the midpoint of their orbits.
As you increase the mass of one of the objects,
then what happens is that center of mass moves closer and closer to the heavier object.
So if this was the sun and this is
the earth what happens is it looks like the earth is going around the sun,
but in fact, actually the sun is sort of
wobbling in response to the earth, just slightly.
So if we had a large enough planet,
we might actually try and see this wobble of the star on the sky.
That's what's called- these are called Astrometric observations and those haven't been
particularly successful because you're actually trying to see
this tiny motion on the sky of the planet.
But in fact, actually you still can use the motion of
the orbit in what are called radial velocity methods which we'll turn to next.
So there we go, those are at least two methods of looking for planets.
These have not been the most successful and what we'll do next is look at
two very successful methods for finding planets around other stars.
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