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.
What Exoplanets Have Taught Us
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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.
So now we've reviewed quite a bit about exoplanets and th
question we really want to ask now is what have they taught us?
And taken as a whole, what have we
learned from actually going out and seeing exoplanets.
This is a really interesting point about science in general.
You know, you can conjecture all of, all you want right,
you can play arm chair scientist as much as you want.
And think about well where life forms and how it forms and what kind
of different civilizations are they and you know do they all have.
Prosthetic foreheads and you know but until
you actually go out and measure things
you really you know you, just don't know and then when you do measure things
and we do open up a new window on the universe usually what we
find is that we're completely wrong you
know that in general nature is more clever.
Nature is more fecund nature is more creative then we could possible imagine.
As someone once said anything
that the laws of physics and chemistry allow
is going to happen so let's see what sort
of surprises were in store for us a
when we actually went out and measure exo planets.
A so okay so we're just going to get some statistics right now but
its important to understand that a
these statistics are changing there's new results
coming a coming out all the time the statistics maybe not hopefully you
know a won't change that much we'll a but the actually numbers would
have changed.
So as of January 7th, 2014 there have been 1,015 confirmed planets.
Found around 758 stars okay.
A there's been 174 systems with multiple
planets which is great so we're actually seeing
solar systems and there have been 3,603 planetary
candidates And confirmed planets altogether so that's this
larger number is or perhaps particularly from
the Keppler a satellite mission where we
have lots of candidates and we've gotta go back and and a look at
them again to confirm that they are in fact candidates and more being found
everyday now you may wonder wait a minute I just said at most I have.
3,000 candidates you know add candidates plus
confirmed how beforehand was I talking about
billions well its what the great thing about the a the science that is involved
in the exoplanet studies is by going out and making these
measurements we can infer from them what we think is the.
The general rule.
So by looking at 3,000, we now have 3,000 planets.
And 758 or at least yeah, 758
stars with planet, confirmed stars with confirmed planets.
We can generalize from that.
We can actually use that statistics to argue up to what is the conditions
over the entire sol Solars or the entire galaxy so that's how we end up
with those numbers we talked about in the last last lecture of billions of stars.
Okay so that's the just general numbers what about the sizes so what
we found is that planets so far come in a range of sizes
from very big Jupiter its from several times the mass of Jupiter all
the way down to half the size of the Earth that's what we've.
Found so far.
There are lots of super Earths.
And by super Earths we mean an Earth a planet that has
you know maybe five, six, seven times the mass of the Earth.
And it's not really clear yet what a planet like that would look like.
Would it be a rocky planet?
Or would the pressure on the body from all from all of that mass actually
force it you know force a different kinds
of internal structure some people have talked about
some of these as being sort of like water worlds even you
know mostly liquid a but its not clear its really unclear right now.
So a most of the planets that have been found are
sort of in the range of masses of Neptune but that's also
partly because their easy to find so there's going to be lots more
surprises as we get better and better looks a at these systems.
Now let's talk about the surprises let's talk about the
things that we didn't understand if you grew up say in
the 1970's like I did.
What you were taught was there was a solar system, which was ours, and it
had the terrestrial planets on the inside, and the gas giant on, on the outside.
It was all very tidy and orderly.
Is that what other planetary systems look like?
The answer is no.
In fact, what we have found in an
abundance of what are called hot planets, and what
that means is that their orbital radius is
less than, a tenth of a, an astrological unit.
Or about a third
of Mercury's orbit. We found lots of planets [UNKNOWN].
A lot of different sizes that are orbiting, you know,
almost like the outside parts of the atmosphere of their star.
They're very, very, going to have to be very, very hot.
For example, 51 peg, a sunlight star with a planet
that has a period is a of just four days.
It takes just four days to go.
That's its year, is it takes only about four point two three days to complete.
So, [UNKNOWN] And the reason why we found so
many of these, is they were the easiest to find.
With the radial velocity method, the closer you are, the closer the planet is
to the star, the more it's going to make the star wobble, in its orbit.
In their mutual, dance together.
So, the first ones we actually discovered were Jupiter's.
We, we'd, the, the, the first [UNKNOWN] we discovered were
Jupiter-like planets in very close orbits, or so called hot Jupiter's.
Now, the crazy thing about this is there's no way
that a Jupiter could form, you know, where Mercury is.
Is just from our understanding of the discs out of which planets form and
we talked about this the Jupiter's have to form past the snow line so
the Jupiter's must form fairly far out in the a a disc that forms
planets around the star and then migrate inward something has to happen to allow.
The a Jupiter to spiral inward so that it ends up parked on this very hot orbit and
we think we understand some of the processes that go into the migration its
probably the interaction between the planet and
the disc the gaseous disc could also
be interactions with other planets gravitational scattering
as we call it a planets that
almost crash into each other but one way or the other Jupiter's migrate inward.
The other surprise that we've seen is that if you look at the
solar system, almost all the planets are going around in fairly circular orbits.
Most of them have orbits that don't, the inner part, the closest
approach and the farest apporach from the sun are not that different.
We know that for example, comets have very elliptical
orbits, where they'll come in in very cigar shaped
orbits, they'll come very close to the star, and
then zoom back out and spend a long time
out in the depths To space so a we thought we sort of thought that oh all planets
will have these very circular orbits but what we
found is that there are many planets on very
a very eccentric orbits very cigar shaped
orbits and this must happen because of interactions
between planets when when a solar system forms
and there is lots of planets in there.
It may be that there are periods where the planets almost collide.
And every time a planet, two planets come
close to each other, they scatter each other.
And if they were on circular orbits they
now are, are left on very highly elliptical orbits.
So that tells us that solar systems especially
early on must be pretty violent places. There's a lot of interactions.
So we have for example Mars has an eccentricity
of 0.06 which is very close to a circle.
And that's one of it's a, for the
solar system is some, somewhat of a high eccentricity.
But what we've seen in the exosolar planets is that there are 80
cents, 80% of planets that have eccentricities
Greater than .1 greater than .1 remember
Mars is .06 so its clear that there is a lot of
activity going on out in the solar in these exo solar planets so
the main result from this is that we are the weirdos right
the solar system is not like all these other systems that we're finding.
The other systems have very much different a have a very.
Different architecture then what we see in our
own solar system so we're we're weird we're
strange and what that means for life and for
civilizations we'll have to see as we go on
and as we learn more so a we could
expect however that there's going to be a lot more.
Variety as we get better and better views of
these solar systems we're still at very much at the
beginning but but there's been some real real surprises
from what we might have though 30, 40 years ago.
Okay.
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