Learn about the science behind the current exploration of the solar system in this free class. Use principles from physics, chemistry, biology, and geology to understand the latest from Mars, comprehend the outer solar system, ponder planets outside our solar system, and search for habitability in our neighborhood and beyond. This course is generally taught at an advanced level assuming a prior knowledge of undergraduate math and physics, but the majority of the concepts and lectures can be understood without these prerequisites. The quizzes and final exam are designed to make you think critically about the material you have learned rather than to simply make you memorize facts. The class is expected to be challenging but rewarding.
Lecture 2.18: Inflating hot Jupiters
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From the course by Caltech
The Science of the Solar System
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From the lesson
Unit 2: The insides of giant planets (week 2)
Meet the Instructors
Mike BrownProfessor
Planetary Astronomy
We now know that some of these hot
Jupiters around stars that have been discovered are
under dense, they're bloated, they're bigger than they
should be if they were just a hydrogen-helium atmosphere.
The question is, how did they get that way?
There's an obvious answer that's so obvious that the first time
I ever heard of it I thought well that's the obvious answer.
And the obvious answer is you move them in close
to the star, and they heat up, and they get bigger.
And, that answer's so obvious that it's also obviously wrong, because if you just
heat them up, if you put them close to the star, you heat up just the outer layers.
Those outer layers get hot enough that they radiate more heat back, and
unless you can somehow get that heat into the interior, and heat the entire
planet, and puff up the entire planet, you've just heated the miniscules amount
of the atmosphere on the outside and have done nothing really very useful at all.
In fact you can make very detailed models of how
that would work, and they would look something like this.
All of this again from this very nice paper
that we've been talking about results from awhile now.
And these are the same plots we saw a minute ago.
We have the mass down here, the radius up here.
And here the curves that we already knew from before.
These are the curves of pure hydrogen helium.
These are the curves of adding some am, amount of heavier materials in there.
And you can see Jupiter and Saturn sitting there.
Uranus and Neptune down here.
And, here are some of the planets that we
were looking at, a minute ago, including this one here.
One thing they've done is added age and temperature.
Temperature of the star, age of the star which we could determine.
Younger stars, presumably younger planets can be bigger.
They take a while to contract.
Something like HD29458B though is five billion years old similar to the
solar system, so the fact that it is big, still not understandable.
Also, that hot star does indeed make a difference, in fact if you have a
2000 degree surface temperature on your planet versus
say a 1500 degree surface temperature on your
planet, you can make yourself thicker and, and even having one of those two surface
temperatures is enough to change you from these
lines down here, up to these lines here.
So, it's not entirely true what I
said, that the heating doesn't make any difference.
It just doesn't make enough of a difference.
It does make a small difference, heating up like
that, but it can't make a difference like this.
And, HD29458B is the worst case.
It's not as hot as the other ones, and it's older than some of
the other ones, and yet it's still bigger, than some of the other ones.
All of the objects that we're just a little bit above the Jupiter Saturn
line though fit very nicely into these new models of an irradiated hot Jupiter.
Still, 29458B and again many, many more discoveries
since then are these inflated, these bloated hot Jupiters.
They needed some sort of explanation.
You can't just dump the energy into the
upper layers and expect the thing to get bigger.
You could, as I said, somehow take energy from the star.
You have an abundant source of energy in the star there
and you need to get it into the interior in some way.
If you could even get something like 1% of the energy that's hitting the surface
of the planet into the interior instead and heat up that interior by some amount.
You would end up with something that could like HD29458B.
How could you it?
Well the first proposal, in fact for
HD29458B itself was that, perhaps, HD29458B is not
in a circular orbit around it's star,
but and ever-so-slightly eccentric orbit around a star.
I'm drawing an exaggerated version.
Normally, these things are so close to their stars
that over time, their orbits have become perfectly circularized.
Having an eccentric orbit causes you to dissipate energy as
you get closer to the star you are more in
the star's magnetic field you get squeezed as you get
further away you expand, squeezing,
expanding, squeezing, expanding, expends energy.
Expending energy like that eventually circularizes your orbit.
So of a very short amount of time, this hot Jupiter should be in circular orbits.
And indeed when we look at them they're almost all in completely circular orbits.
Now, it could've been that it used to be at a much more eccentric orbit.
In fact, we see things at much more eccentric orbit.
And it has been circularizing and it just now arrived
at circular orbit and it got heated up from that.
It's possible, but if the heat is no longer being dumped
into the interior then the planet would get smaller very quickly.
So if this is the case that means that HD209458B had to
arrive at its new circular orbit yesterday on a cosmic time scale.
It's possible, but when we started finding more and more and more
of these the probability that all of these just arrived seem pretty small.
What if it's on an eccentric orbit right now?
I said that the orbit should quickly turn circular unless one thing happens.
Unless there's a second planet out here.
We're just perturbing it the entire time.
This process, by the way, is exactly what happens on Jupiter's moons.
Jupiter is orbited by Io, Europa, Ganymede, and Callisto.
And Io is on an eccentric orbit, even though
it's very close to Jupiter and should be circular.
Why is it on eccentric orbits?
Because it's being forced by Europa, which is being forced by Ganymede, which
is being forced by Callisto and it's keeping Io on this eccen, eccentric orbit.
That squeezing and releasing, and squeezing and releasing that
I described up here for the stars is happening to
Io right now, and Io because of it is
the most volcanically active body known in the solar system.
All of that energy that's being dissipated from Io
is being spued out in the form of volcanoes.
It wouldn't take much of an extrinsicity to cause heating.
There would be enough to make HD29458b inflate and
yet, with very, very, very careful measurement, no eccentric
ever been measured and, more importantly, no hint of
a second planet in that system has ever been measured.
And third importantly, more and more of these things up here have been found,
suggesting perhaps a more common process than these sort of one off ideas.
Okay.
Here's my ugly, crude drawing of the things
that I think are important going on here.
[SOUND] In this access we have things that have not very much stellar fluxating it.
They're not very hot compared to things that are very hot.
[SOUND] On this access we have, well, we
have grouped it, grouped into three different mass ranges.
Things that are between zero point three and one Jupiter mass.
Between one and one point five, between one point five and four different masses.
So it show, it shows those three separate things.
On this axis, we have the radius between Jupiter radius to up to
one point five, up to two, and in each of these three categories.
[SOUND] Now and finally, I have in this green line what you
would have, you just clearly a radiated model that I showed you before.
And you can see it clearly [SOUND] that all
of these things are well above this green line.
But a couple of interesting things are going on that
I think you can see even from my crummy drawing here.
One is that in all three cases, [SOUND] the more flex
you get, the hotter you are, [SOUND] the more inflated you are.
More inflation, more inflation, more inflation.
That is a big hint that inflation is caused by stellar heat.
[SOUND]
There's a second really good hint which is that if you
notice these lowest massed things are much more inflated up to
one point five two [SOUND] than these which barely make it
up to one point five, and these are in the middle.
The smaller you are, the more inflated you are.
This again makes sense, if it's the inflation is caused by
stellar heating, that means a certain amount of stellar heating will get
into the interior but if the object is small, that amount
of heating will call to, cause it to puff up much bigger.
If the object is, is massive already, it won't puff up very much at all.
[SOUND] Again, very good hints on what's going on.
Neither of these things would happen if,
for example, it was caused by eccentricity.
In the case of eccentricity causing puffing up it
would simply be a function of the other planet.
What the other planet was doing.
[SOUND] You'd never get these nice correlations like this.
And these correlations look really good except maybe when I draw them.
How then do we get that stellar heat to get into the interior of a planet?
You can't just cook it from the outside,
we have to somehow heat that interior part.
There are a couple of different ideas there
have been many ideas proposed over the years.
The one that's popular these days, I, I
kind of like it, it goes something like this.
Imagine that there's a planet, very close to a star which is over here.
We know that these planets, when they are so close to stars that these, the stars,
the heating from the star drives these massive
winds on the planet that circulate around the planet.
These winds presumably penetrate down quite deep on the planet.
The other thing that we think we know is that
in addition to neutral materials there should be ions in here.
There's an ionosphere just like the Earth has an ionosphere.
molecules, atoms that were originally neutral.
Remove an electron suddenly their charged and that this ionosphere the ions
in the and that unlike the earth's atmosphere where the ions are mostly
just in the very top, there are enough ions down in the lower
parts of this giant planet atmosphere that they can actually have and effect.
Here is the effect that they would actually have,
these winds would like to drag these ions along.
But, there is also a magnetic field, and magnetic fields, ions like to
become attached to magnetic field lines as the magnetic field rotates the ions want
to be attached to the rotation and go with it, or if the
magnetic field is not rotating very fast, if this planet doesn't rotate very fast.
Yet the winds rotate very fast.
The ions are being held by the magnetic field as the winds are blowing past them.
This is friction.
Friction can generate a lot of heat, heat can make a big puffy planet.
There are a lot of details to work out on the
physics, things like, do these things really have strong magnetic fields?
Do they really have ions?
Are the ions deep down?
Do they really have these strong winds that we say?
A lot of details to work out.
And yet, this seems like a pretty good explanation for why
the more flux you have, the stronger your winds are going to
be, the more inf, inflation you're going to have, the smaller of a
planet you are, the more that this friction can heat you up.
It's a nice tiny explanation.
Is it true?
I don't know.
This is the answer we often get when we get near the end of these units.
There are many questions that still remain to be figured out.
I think it's amazing thing that these days we know enough about
these planets around other stars, that we can start to see these trends.
We can start to see that increasing the flux makes them bigger.
We can start to see that increasing the
mass, decreases the amount that they get bigger.
We are learning tremendous amounts of things about
the way these hot Jupiters work, and now
at this point, there are some more detailed
physics to work out to really understand it.
But I think that we're getting there, and I think that we are getting to the stage
where we really are learning about what's inside of Jupiter-like planets.
But now Jupiter like planets that are around other stars.
That, is a pretty amazing thing.
[BLANK_AUDIO]
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