So, Paul, at last we have a method for reliably finding planets.
All we have to do is monitor a star and its radio velocity to a few metres per second for a few weeks, months or years
and we can pick out its reflex motion caused by its planets.
It's not the way anyone thought this would happen, but it turned out to be quite effective and suddenly it became very trendy.
These people who hitherto were very obscure were suddenly showered with grant money and jobs and telescope time.
They got to built their own spectrographs and huge amounts of time on the world biggest telescopes, and so the data rapidly got better.
Yes, it really became a huge industry where there were multiple groups around the world
and they, of course, got a lot of time so they made a lot of discoveries.
So, here we've got a plot, which we'll loop over, showing the discoveries against time.
So, what we're plotting here is the mass and, remember, they don't actually measure the mass, they measure the mass Sin inclination.
On average, for things randomly distributed, it's real mass is going to be 14 percent above that.
To begin with, there were only a few things which were a pretty high mass close in.
But as time goes on, first of all they start finding things down here with lower and lower masses
and that became because they got better and better at their techniques.
They got better software, better at correcting all of these different problems we've talked about
and also they got to build their own spectrographs; which were purpose built for this purpose,
rather than just borrowing someone else's spectrograph, which gave them more precision.
The second thing that's improving is they're starting to see things out here, further from the sun.
When you get further from the sun, you're talking about longer periods.
These things might have periods of five, 10, 15 years.
So, you're not going to see them in a one week observing one, you have to actually observe over many, many months, years.
As time goes on, this is where we're at at the moment, at the time of writing this talk.
These are the hot Jupiters, these are the things very close in,
and you see that they range from maybe less than a hundredth the mass of Jupiter up to 20 times the mass of Jupiter,
much higher than that and they'd be a star, and you also begin to see more and more objects further and further out.
It turns out that these are relatively rare.
The bulk of the things, now that we got 10 to 15 years worth of data, are out here,
maybe about an Earth orbit out, so it would be a period of a year or thereabouts.
We're even beginning to see things with periods and masses close to that of Jupiter.
Well, so if we look at this diagram, we see that there are a lot of Jupiters where the Earth is,
so our solar system isn't obviously the norm. I guess though we can't yet see our solar system because we simply haven't looked.
We're getting close to the be able to see something like Jupiter, something with a sort of Jupiter mass or a Jupiter orbit.
We're not yet in a position to be able to see things like the Earth or Venus, let alone Mercury or Mars which are way down here.
So, these are still not like our own solar system.
We're still talking about the hot Jupiters down here and these things, we'll see in a moment, tend to be in the highly eccentric orbits.
You've got the hot Jupiters and we've got the eccentric giants over there.
So, Paul, we have all of these big, Jupiter-like planets in quite close to the sun, so not at all like our own solar system.
If you remember, that's a problem for our normal theories.
Our theories say that you should only be able to form these big things out beyond the snow line, beyond the asteroid belt where Jupiter is.
It's a real problem that we're talking massive things that should form a long way out, very close in.
So, let's not give up yet, maybe we can form them out where they're supposed to be formed,
and move them in rather than trying to form these big planets close in in the first place.
This leaves, for a long time, the standard theory for how these hot Jupiters got there,
and it had a benefit that it was actually predicted before the observations.
It's always, you know, have more faith in the theory if it comes on before the observations.
Theorists are very good at explaining things after the event. There's actually a theory of migration.
This came as a solution for some problems in our own solar system. The idea is that when the planets are first forming,
they're still embedded in this protoplanetary disk, which is a very massive disk primarily of hydrogen and helium,
and some smart theorists had calculated that there would be a reaction between these things.
There's not going to be much gas drag, but the planet will generate ripples in the gas
and the ripples would have a gravitational pull back on the planet and this then causes planets to move.
This was come about as maybe an explanation for where Uranus and Neptune come from in our own solar system.
Basically, the time to take them to form out where they are is so long they should never have formed.
So, the standard theory was that maybe they formed in close to where Saturn is now, and then moved out.
But the theories could get them to move either way. Here's some simulations.
This is a simulation of a fly-by of a protoplanetary disk, which is in red, with a planet forming.
What you can see, this is a fairly massive planet, and it's gravity has actually carved out a gap in the disk,
and you can see where it catches a ripple pattern coming in through there.
Gas is falling in here and spiralling around the planet to actually form the planet,
this is how you might actually form something like Jupiter.
We'll zoom in on this in a second, and you see the whole complicated ripple pattern out here,
and these ripples can either move it in or outwards.
In this case it will probably move it inwards, but depending on the exact density of the disk and its temperature and how many planets there are,
you can get this thing to move in or out, or even turn around.
So, since this is a very convenient model, you can expend anything you like from it.
It sounds very complicated.
It is a very complicated model, and because we know so little about what these disks are like in the first place,
it's very hard to say what it would do, but it's certainly possible for these things to move around in huge factors.
Now we're going to zoom in on the centre here, and we'll see a disk around the planet, and this might be the disk of gas,
like the disk of the whole solar system except smaller one around a planet which might have formed,
for example, the Galilean satellites of Jupiter, or the big satellites.
We know these ripples happen, because you can see them in our own solar system in the rings of Saturn.
You've got tiny, little moons forming gaps in the rings, just like we see in there,
and they form this rather complicated ripple pattern on the side, and so we know that when we do have a disk and you have a mass in it,
it can indeed open up gaps and produce these ripples, so it's confirmed in that sense.
Here's a simulation showing how one of the planets had actually been moved in.
So, here you've got the planet, it had actually been spinning around in an enormous time,
but they've cancelled that out and left it in the same position, and what you can see is,
as time goes on in this particular simulation, the planet's moving itself in.
The ripples over here and there produce a net backward torque, slowing down it's orbit and moving it in.
These other ripples were also caused by the planet as well, so, presumably,
a big planet like Jupiter would effect everything else in this system as well.
Of course, in reality, we might have several planets, each of which direct their own ripples,
the ripples start interfering with other ripples and it becomes incredibly complicated.
How can we form hot Jupiters?
Well, one idea is would be that we have the protoplanetary disc, and the planets start migrating inwards.
The gas giants form out where they're supposed to form and then fall steadily inwards,
and you've got an endless stream of planets all falling inwards until, at some point, the protoplanetary disk goes away.
This is thought to be about 10 million years after the solar system starts forming, and at that point it's like a game of musical chairs.
Everyone suddenly freezes where they are at that moment.
Because all the gas is gone and this rippling effect that's sapping energy from the system disappears
and so you literally get these objects coming in and whatever's left is left, as you say.
So, here's another simulation of that, so here you've got a planet and a star embedded in a disk and a planet orbiting around,
so as time goes on it's setting up ripples which, of course, gets to spiral in closer and closer, faster and faster,
and then at some point, there we go, it just goes away, leaving it where it is.
If you get it just right, you might be able to leave it like this is a hot Jupiter.
But the trouble of this is it needs a lot of fine-tuning.
These disks are supposed to hang around for about 10 to the seven years, so about 10 million years,
whereas the time to migrate something, that's only about 10 to the five years.
It sounds to me like you would probably lose almost all of your planets you formed into this scenario.
That is indeed the case. You'd expect that maybe only one percent of planets survive,
the one that happened to be just at the right place at the exact moment.
But that's kind of okay, because we actually only know that about one percent of planets have hot Jupiters.
The other 99 percent they didn't see anything. If you remember the survey, we can only see things that had massive and closer in,
and it turns out that if you did the sums, we're seeing that about maybe half a percent to one percent have hot Jupiters,
so it could be the case that maybe this happens everywhere.
All of the solar systems form planets and the vast majority is just destroyed or still a long way out
and then a half a percent or one percent is just caught at the right position.
So yes, it's very destructive, that's one possibility.
Another possibility is that, for some reason, there's a hole on the inside of the disk, so the things migrate in,
but then they stop migrating when they come to the inside of the disk, as there's not more ripples around them,
and so you just build up something really, really rather big here.
Okay, so in this case, you get the first planet that comes in, it quits migrating,
and then it sort of bumper-cars with everything else just piling in on top of it as they come in and so you get to build a great big object up.
But you have to make a big hole, so you say magnetic field, that's convenient.
Yes, whenever astronomers don't know what's going on,
they say "magnetic fields", "turbulence", we have our hands and look goofy, so, yes.
Well, let's magically assume that the magnetic field from the sun goes out and magically clears a hole in the disk
and magnetic fields can do whatever you like so maybe it'll work.
We know that stars have magnetic fields, we know it'll interact with the disks, so it's not maybe totally crazy.
(It's not completely crazy.) But it's fairly crazy.
This will have the benefit that these things coming in could be quite small because they pile up on the inside,
whereas in the previous model, you'd have to have a bunch of big things coming in.
But, you know, there's lot's of mass in the solar system.
Maybe there are, maybe our solar system did form a hundred Jupiters and 99 of them fell in and one got left behind.
The problem with this is that it's actually a bit too good a solution, this whole migration stuff.
I mean, bear in mind that the migration timescale is much too short, you can explain these hot Jupiters,
but what about our own solar system? We've got Jupiter out where Jupiter's supposed to be.
I mean, why didn't it migrate in or out or somewhere like crazy? It could do so very quickly.
In fact, it probably predicts the that most solar systems don't have any jump planets at all,
because they all might have migrated in long before they had a chance to stabilise anywhere.
Right, we've seen that, as we've found more and more objects, there are a lot of Jupiters and not really, really close in,
a lot of them are out by the Earth, and it looks like there may even be even more out,
sort of where Jupiter is supposed to be, so maybe our solar system isn't completely weird.
Yes, so this may be too good an answer.
You get things closer in but maybe it gets everything too close in.
So, we still have a mystery on our hands.