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Diversity of Planetary Systems
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Do curso por Princeton University
Imagining Other Earths
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Formation of the Earth and the Origin of Water
This lecture discusses the physics of star and planet formation. Because of the conservation of angular momentum, collapsing gas clouds form disks that then fragment to eventually form planets. We observe a tremendous diversity of planetary systems which implies that planets, unlike Stars, have a much wider range of properties.
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David SpergelCharles Young Professor of Astronomy on the Class of 1897 Foundation and Chair
Department of Astrophysics
Great, so now we're going to talk about this remarkable diversity of
planetary systems that we are discovering.
We're finding lots of things, that we didn't see in our own solar system.
There's a class of planets that we've gotten very excited about
called Super-Earths.
These are rocky planets with masses who's masses are greater than the Earth,
and less than Jupiter, or Saturn, or Uranus, or Neptune.
Uranus is about ten times the mass of the Earth, and
in our own solar system, we don't find any two or three Earth mass planets.
Yet when we look at what Kepler has found,
these are the number of Earth size planets and
these are the number of super Earths and Neptune size planets, we can see a lot
of what Kepler had found is planets, whose radius ranges from about one and
a half times to six times the Earth's radius.
This class of objects, super Earths and
Neptune size planets at the orbit of Earth or Venus.
Is a class of objects that we've not seen before.
And one of the things we're trying to understand,
what would these planets be like?
Trying to imagine these objects larger than earth, do they contain lots of water?
Do they have plate tectonics?
Do they have hydrogen rich atmospheres?
What are their properties going to be like?
And this is an area of very active study.
We've not yet been able to characterize many of these, super-earth systems.
So what we've done so far is build theoretical ideas,
about what their properties might be like.
So I want to talk about some of these possibilities.
And to do that, what we do is we construct planetary models.
We make different assumptions, about the composition of the planet, and
then construct models, where we ask what if we make a planet entirely out of iron?
Our planet mostly out of water.
Here's the track of planets that would be 75% water, and 25% of magnesium silicate.
And here's some intermediate cases, where we makes planets.
I have some mixture of iron and intermediate materials.
And what's shown here on this plot are where Earth and
Venus are, and some of the systems discovered by Kepler.
And you can see that the Kepler planets are occupying a very large region,
in this relationship between mass and radius.
There are Kepler planets with a wide range of densities based on that we
suspect they have a wide range of properties.
So, we make assumptions about the planetary's interior composition.
We make assumptions about the composition of the atmosphere.
Whether it's an atmosphere rich in water, and oxygen, or
ozone, are an atmosphere rich in carbon dioxide, and
based on these models, we try to construct possible planets.
We think of the Earth as a watery planet but in fact, it's remarkably dry.
The Earth lost most of its water early in its formation history.
The other hand, there's a possibility of true water-worlds.
Planets like Uranus and Neptune are very much icy giant planets.
They are made up of a lot of water.
And if a planet like that migrated, from an orbit outside the snow line,
a place where a lot of the water is preserved in comets and
then accumulates into the planet.
So these icy planets migrate in during plant formation.
The ice would melt and these would become enormous water worlds.
Planets that would be radically different from the Earth,
perhaps have oceans hundreds of miles deep.
We can think of a planet the size of Uranus doing this, or
even one of the giant moons of Jupiter and moons like Europa.
We could also think about plants like Titan, with their ammonia atmosphere and
ask what they would be like, if they migrated to earth's orbit, and
had temperatures comparable to room temperature.
Our solar system.
Our solar system planets represent only a small fraction
of what is possibly out there, and just by thinking about taking
components of our own solar system, Titan, Europa, Neptune, and moving them
to different orbits, we will have planets whose properties are very different.
And one of the things that we very much have discovered in our
exploration of extra solar planets, is that the possibilities out
there are far more diverse than what we've seen in our solar system, and
in many ways far more diverse than we had imagined up till now.
Other possibilities are carbon rich planets.
People sometimes talking to them as diming planets.
The two very important elements that make up after hydrogen helium.
The most common things in our universe.
Are carbon and oxygen.
Both carbon and oxygen are produced in stars,
massive stars, during the later stages of stellar evolution.
If you recall our lecture on stellar evolution, we talked about how stars burn
hydrogen to helium, and then helium to carbon, and then carbon to oxygen.
Depending on the star, some massive stars end up
producing more oxygen, and some produce more carbon.
The earth formed out of a nebula that was enriched,
a young protoplanetary nebula, that was enriched by a lot
of oxygen from a young star.
Most in the case of the earth,
most of the carbon that was around was bound up in carbon dioxide, and
carbonates, because there was more oxygen than carbon, so it grabbed all the carbon.
Once all the carbon was grabbed.
You can think of the oxygen binding with other things,
forming silicates, where you have silicon combining with oxygen.
Magnesium oxides, iron oxides, all these things form because
we've got in the case of the earth more oxygen than carbon and that determines.
A lot of the earth's composition and and lot of its geology.
But what if you have a star, whose proto-planetary nebula
was more enriched by a carbon star than an oxygen star?
That, if formed in a region in our galaxy.
Whose massive stars produce more carbon than oxygen.
We sees regions like this, when we observe our own galaxy.
And in systems like this, the carbon and
dioxide binds up.
But the carbon, now binds up all the oxygen.
And when other things form, they form without any oxygen around.
So the silicon, binds instead of to oxygen, to carbon.
To make carborundum, silicon carbine.
And things like that.
With all that extra carbon, you'll form graphites and lot's of tars.
We think this planet would have perhaps a wax surface.
Covered with all that carbon.
Intriguingly underneath that surface, you take all that carbon and
you start to compress it, the interior of the planet might be made of diamond.
So we think there are perhaps giant diamond planets,
encrusted with tar and carborundum.
These planets would have atmospheres that are dominated by gases like methane
or you know, cH4, because we've got carbon binding with a hydrogen,
rather than having the hydrogen bond with the oxygen to form water.
So.
There might be starts whose compositions are quite different.
Some planets whose compositions are quite different from our own.
Another intriguing possibility,
when we think about Super-Earths are habitable hydrogen planets.
If we had a planet.
Who's mass was say, three times the mass of the Earth.
It would be able to retain a lot of molecular hydrogen in it's atmosphere.
In the case of the Earth, molecular hydrogen escapes.
The hydrogen molecule is so light, that it doesn't stay very tightly bound to the Earth.
And it is evaporated by the sun's ultraviolet radiation.
And this is from a model by Pierrehumbert & Gaidos.
Where they constructed a model of a planet that was a 3 Earth-mass planet.
And they put on top of the 3 Earth-mass planet.
40 bars of pure hydrogen.
That means that the pressure at the surface of the planet,
was 40 times the pressure in this room.
So roughly 40 times as much atmosphere on that planet as on ours.
And that atmosphere was made of pure hydrogen.
What they found is that molecular hydrogen was such an effective greenhouse gas.
That you can have quite a comfortable planet
at 10 times the distance 10 of the earth.
At 10 AU from a star like the sun, they were able to construct and
model a planet that could be habitable.
Well you have this thick hydrogen atmosphere on a super Earth planet,
and beneath this thick hydrogen atmosphere, there could be liquid water.
So, the range of possibilities of what could be
a habitable planet is probably much larger.
Then we've imagined up to now.
What are the lessons we've learned so far from our study of extra solar planets
is that what's out there is much richer than we've imagined.
We thought all solar systems would be like our own.
And what we're discovering is that there are a range of possible planets
that are a much larger range than we see in our own solar system.
When we think about planetary properties, we can think about
a planet's distance from the Sun and a planet's mass as the two minimal things.
That determine its properties, but even given its distance from the sun and
its mass, different planets will have different compositions.
Some planets might be dense like Mercury, and from
the heavy materials that can survive in the inner portions of the solar system.
While other planets might have formed.
In the outer regions of the solar system, so it might be ice-rich or
even hydrogen-rich and then migrate in.
And when they migrate in you could find a planet, at one radius that has a given,
from the star, that has a given mass.
One might be made primarily of water, another primarily of.
Carbon like materials, carbonates another like the earth filled with oxides.
And, the properties of these plants might be quite different.
And I think we've only begun to explore the properties of planet,
external planetary systems.
So far with telescopes like Kepler.
We've been able to infer things like mass and radius, and
I think we're going to have to wait, until we have the next generation of telescopes.
Telescopes that are able to start to characterize the atmosphere as a planet.
Start to characterize the properties of the surfaces,
before we can begin to answer this question.
Of, what are we missing?
What other possibilities has nature realized that's gone
beyond our imagination?
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