There's been this huge waves of projects looking for these transits. It sounds pretty easy.
You just photograph stars repeatedly and look for very small dips in brightness every now and then.
You were involved with one with Dan, Brian, is it as easy as that?
Well, unfortunately, there's a lot of things that can make a star dip in the way that mimics a planet
and so we have to worry about getting rid of things, for example, of two stars, a bigger and a smaller one orbiting,
where you just sort of clip the edge of the one star with the other.
It turns out that puts a little dip, and the reason why the dip is so small is because you just clip a little bit of one star with the other.
So, we have to get rid of that effectively by looking at the velocity of this star,
because this system will have a much bigger radial velocity change,
so it's a lot easier than measuring with the way you find planets, so you can get rid of that type of thing. But there are other problems as well.
So, in this case, we have an eclipsing binary again where the star really gets eclipsed, but there's another star that is behind or in front
and, of course, when we look through the Earth's atmosphere, one pixel is much, much bigger than this screen,
so you're averaging or adding up everything that you see here.
So, these two, if just by themselves, are making quite a big dips because they're big things partly in front of each other,
but because their light is diluted by this star over here, it makes it look very small.
Right, and so that's a real problem and so there's a couple of ways we can get rid of this.
One way is just to make sure that, for example, the star that we're looking at doesn't look like two or three different stars,
the other one to make sure that this star isn't a giant star because it turns out that when you're looking for transits,
a giant star has a huge radius compared to the planet and so you just can't see transiting planets in front of giant stars.
So, how many candidate planet things do you see and what fraction of them turn out to be real planets?
The fraction that we're having to get rid of, it's almost a hundred to one in the end.
I mean, we're able to pretty quickly get it down to sort of ten to one,
but in the end we have to go through and do radial velocities of the transiting objects we see to be confident that they are planets.
So, this is a bit of a nasty surprise. It looked like a great method.
You could do it with small telescopes and the trouble is, because of this huge contamination of things like this,
you had to actually still go to the big telescope for the follow-up.
So, huge amounts of big telescope time, once again required to follow all of these things up.
But it's been done, and continuing to be done and here now is our discovery chart.
Once again showing the mass of the planets against the orbit,
only now we are going to start showing red dots for the ones discovered using the transit method.
There's the first one, and what you can see is that these things are primarily hot Jupiters.
Right, and you should realise that it's not that suddenly we've discovered a bunch of missing hot Jupiters,
it's just that this method is really good at finding objects here in the diagram, and it's really bad at finding objects down here.
The basic reason for that is, let's imagine that Brian's head is a star and we have got a planet it over here,
if it's close in, if it's edge-on it will cause a transit. It can actually be at quite an angle and still cause a transit.
But if it's way out over here, it doesn't have to be very far a line before it goes above or below Brian's head.
So, basically the probability of seeing a transit goes down quite deeply as you go further out,
simply because the odds of it being edge-on enough to go in front becomes very, very small.
So, Paul, we can find all of these kind of wacko hot Jupiters, we already know they existed,
it seems, well I won't say it's pointless, but it's not obvious why it's so interesting.
Well, why it's interesting, I mean yes, we knew hot Jupiters existed, but now we actually know the mass,
not just M Sini because it's got to be pretty much edge on, more importantly, we should know the radius of these things.
We're always assuming, when we call these things hot Jupiters,
that they were gas giants like Jupiter, with a density of about 1000 kilograms per cubic metre.
But maybe they were actually giant rocky planets.
In our own solar system, the things of mass of Jupiter are, there is only one of them and it's made of gas, primarily, no density,
but maybe there can be masses of things that are more like the Earth, the density of more like 5000 kilograms per cubic metre.
So, how can we tell?
Well, here's a plot of the transiting objects, what we're plotting here is their radius in units of Jupiter radius,
that's one Jupiter radius, and up here is the mass in units of Jupiter mass.
There's Jupiter, there's Saturn and Neptune and this line here is what you'd expect from something that's primarily made of gas,
so about Jupiter's composition, and these error bars up here in purple are the actual planets we're seeing.
So, you can see that there is an object that looks about like Jupiter, but most of the objects are a lot bigger in radius,
and remember, the volume, because it's the cube with the radius,
being a bit bigger means you're a lot less dense, you really are puffed up.
This is the first big surprise, it looks like these things are actually mostly,
not all, but mostly considerably bigger than you'd expect from their mass. These things are very low in density.
Actually incredibly low densities that some of these things up here,
I mean, Saturn is actually a low enough density that it would float if you had a really big bath tub,
these things are more like the polystyrene type densities, incredibly low.
So, we do know that they're very near to their stars, so maybe they're being puffed up by the fact that they're close to their stars.
Well, this is the first idea people had, you take gas and then you make it that it's heated up,
and then as it heats up it becomes less dense, so these things are very close in.
So, just how hot are these Jupiters anyway?
Now, if you remember a while back, we derived an equation for the temperature of a planet in equilibrium.
We balanced the radiation in from the sun with the radiation out, assuming it absorbed and emitted perfectly
and was all the same temperature, which is not very realistic,
and we found out that the temperature is the fourth root of the luminosity of the star
all over 16 pi Stefan Boltzmann constant sigma and D squared, where D is the distance between the star and the planet.
So, we can use this to work out the temperature of a hot Jupiter.
Let's do an example, so let's say we have a solar luminosity star, so in that case that's the luminosity of the sun,
let's assume that it's 0.05AU out, then if you plug numbers in, we end up with a temperature of around 1240 kelvin, so pretty hot.
If the star is less luminous and the planet is far away, the temperature will be less.
So, Paul, to explain why these objects are so much less dense than Jupiter,
and you kind of expect Jupiter to be how a planet would form, you would think.
Let's plot it as a function of temperature and have a look at what we see, because that's one possible idea to explain things.
Here we have the planet radius, versus the planet temperature and there does seem to be an effect.
You also see that the symbols here indicate, for example, the size or the abundance of elements in these planets
and it doesn't seem to correlate with any of those, so it really looks like temperature is driving it. So, problem solved, isn't it?
Well, yes, to me it seems like if you're going to shine heat onto the planet, it'll puff up because that's the way gas is.
You heat it up it puffs up, but there may be a problem.
Yes, I mean, the trouble is that we think that a planet like, for example, Saturn, a gas giant like this,
Here's an optical picture of it, but if you look at it in infrared, it looks much more evil.
When these things formed, you had a big cloud of gas that shrank and shrank and shrank
until it formed these things, much like when a star forms, and in the process a huge amount of heat was generated in the middle.
It turns out that, for gas giants, this heat is still mostly there.
The inside, as you can see from this infrared image, is glowing intensely.
The middle of the planet is much, much hotter than the outside.
So, when we talk about the heat coming in from the outside and warming it up,
it might be quite hot on the outside, a thousand degrees or so, but that's nothing to the heat in the middle already,
so it's very hard to see how anything you do to the outside of the planet, it might only affect the outer half a percent or so,
but the middle of the planet is so hot anyway, it's almost not going to care what's going on out there.
We sort of need to somehow get what's going on on the outside into the inside somehow.
This is currently very controversial. Here's three possible models for what's going on.
If you remember earlier on, we talked about when they might have started off in an elliptical orbit
and had huge tides raise when it came closer, that's a possibility.
Maybe when this thing came close, it was pulled out of shape by the intense gravity
and as it spun and was pulled all out of shape and that stirred it up and heated it up in the middle.
So, that's sort of like what makes Io's volcanoes happen, where Io is getting bent out of shape by Jupiter
so it gets heat in its centre, so that's one way to do it.
A second possibility is that there could be winds. Now, we generally think that, because of this tidal effect,
the planet's are always going to face one side towards the star, so just imagine that Brian is a star, I'm going around him,
I would always, as I moved around, rotate so I'm looking at him from the same direction.
That's sort of what the moon does when it goes around the Earth for the same reason.
Exactly the same reason, but this means that if I'm staring intensely at the sun,
this side of me is going to be very, very hot and that side is going to be pretty cold,
which could well drive very strong winds from one side to the other.
I mean, on Earth, the equator is hot and the poles and cold.
They're nothing like as much of a difference as we get on these planets, and that drives the winds on the Earth.
Maybe temperatures can drive winds here, or maybe they don't.
But, if you're talking about these very strong winds, once again they're at the surface level,
but maybe a very strong wind can dredge up and cause some turbulence and stir up the stuff underneath it
and that will start the stuff further and further down so you might have some cascade of turbulence further down to warm it all up.
Hmm, yeah I don't know, it sounds a little dubious to me.
What about magnetic heating? I mean, we have our friend, the magnetic field,
I bet you that we can do some magic with it here too.
So, of course, we know that planets in our own system have magnetic fields, gas giants have very strong magnetic fields.
Maybe if you have this gas on top that's ionised, that means it will pull the magnetic field lines along with itself
and they will then pull on the stuff inside, so maybe that's a way to communicate maybe the winds at the surface down into the middle.
Alright, so we have some models here, but maybe not a solution yet.