So, let's start the story of how planets from others stars were discovered,
and it started in the most unlikely way imaginable. Let's take our minds back to 1990.
Back then, no planets had been found around other stars and, basically, no one thought it was possible.
For all the reasons we've just gone through, it seemed to be really, really hard.
People might have thought that maybe 50 years from now it would be something you'd solve,
but it wasn't on anyone's near-term horizons.
I remember, it was my first year of graduate school and yes, I said well, someday we'll be able to do it,
but certainly not during my lifetime.
But then everything changed, and it changed in a really unlikely place.
So, Brian, we've got all the different sorts of stars in our galaxy. We've got a dwarf like our sun, red giants, blue super giants.
Of all these stars, which one would you say is the least likely type to have planets?
The one place that you can guarantee would not be planets.
Well, I studied these massive stars that explode as supernovae, and so I would think that the remnant of a supernova, a neutron star,
this incredibly dense thing that the centre of a massive star collapses down to,
so that's a star about 10 kilometres a gross that weighs one and a half times the sun,
that would be a place I would struggle to imagine how you might possibly get a planet there.
So, these neutron stars are spinning balls of neutrons 10 kilometres across,
we can see them because, for some reason, it's not really well understood, often they omit beams of radio waves.
As the radio waves spin around like a lighthouse, every time they pass towards us we see a flash.
So, you see something go "blip blip blip", flashing at radio waves,
that's a pulsar, which is another name for a neutron star that's pulsing.
But why were you so sure that these things would not have planets?
Well, I think that there's a couple of things that, first of all, the stars that create these things only last for about 5-10 millions years,
not even clear if that's long enough to form planets in the first place around the first stars.
But when the stars die, they explode as a giant supernova,
so they have 10 solar masses of stuff that are shooting out at a high range,
and whatever planet is there is going to be obliterated
and if it's not obliterated it's probably going to no longer be bound to the star and go out into interstellar space.
So, there's so many reasons why they shouldn't be there, I can't really count them all.
Okay, so we all agree that the one place where you should absolutely not find planets,
one sort of star you should not see planets around, is pulsars or other neutron stars.
Of course, that is the very first place where planets were seen around other stars.
So, let's tell you this rather unlikely story. It all starts with this telescope, the Green Bank 300 foot telescope in the United States.
So, that's one of the largest radio telescopes in the world.
It was, but then one morning, the astronomers found that it had turned to this.
That would have been a bad day.
It was, indeed, a bad day, and you can imagine what effect this news had on the rest of the astronomers around the world.
Yes, so we always like to be safety conscious,
so when I would have seen that, if I was running and observatory, I would have said
"I think we'd better check the joints and everything in our radio telescope to make sure it doesn't end up in a heap of rubble."
This, indeed, is exactly what happened for this, the worlds biggest radio telescope, the one in Arecibo on Puerto Rico.
It's an amazing telescope, what they've done here is actually find a limestone valley and completely carpet it with a telescope.
It only looks straight up, but by moving this focus cabin around a little bit you can look a little bit left and right.
Right, and so instead of being steerable, like the Green Bank telescope is, this one is literally stuck to the ground,
but it allows it to be much, much bigger by putting this mesh of metal which goes through and,
essentially, fills this perfectly-looking spheric parabolic shape, and so that will then focus the radios' antennas,
depending on exactly where the radio waves are coming down,
the antenna focuses the radio waves on a part of the sky where you can move this giant collector of,
essentially, a little radio antenna itself that you can move around the top of the dish
and so you can point to parts of the sky, but this dish doesn't move by moving this around
because the radios will go into the different part of the sky depending on where the light waves are coming from.
So, normally, people competed fiercely to get time on this telescope, and they'd say "oh, look at this object"
and so this dish would be slid over to the relevant place so you can see a particular object.
But, the managers here were paranoid, could this collapse happen to us?
So, they decided to take the telescope offline for a few months
while they went over and double-checked every beam, girder, cable and make sure that nothing horrible was going to happen here.
But, this provided an opportunity.
In some sense, the telescope still works while they're checking all of the girders.
The trouble is it can only point straight up, and this guy, Aleksander Wolszczan, if that's how it's pronounced,
thought "hmm, well this can be kind of useful. So, if I don't care where I look,
I can get an awful lot of time on the worlds biggest radio telescope, I'll just look as the Earth rotates straight overhead,
so there'll be a ring around the sky, we can look over and over again at this, and we can use this to look for pulsars."
So, that's what he did.
He had a huge amount of time he would have never otherwise have got, had there not been this maintenance,
and found a fair number of pulsars, including a millisecond pulsar.
These millisecond pulsars are kind of interesting because they flash, instead of every tenth of a second or something,
they're flashing at literally hundreds of times per second, and that's a funny thing to figure out how you can get one in nature.
So, here's a simulation put together by Australia Telescope National Facility, that is you start with a binary star.
You have a supernova, which creates a pulsar.
The second star in the binary later turns into a red giant and it feeds gas into the fist one.
As the gas spirals in, it gives it angular momentum and makes it spin faster and faster, you see it going around faster.
That's the classic conservation of angular momentum.
Eventually, the second star will come to the end of its life, as in this simulation,
it might end up as a pulsar itself, but a slow-moving one, but it might also just end its life as a white dwarf.
So, this is the normal idea about how you form a second pulsar.
You have a normal pulsar in a binary system and it forms a normal, slow-moving pulsar after this explosion
and then the second star swells up at some later stage, it must be the less-massive star or otherwise it would have exploded first,
and dumps gas onto the surface and spins it all up.
So, what you're left with here really depends on how much stuff there's left in a star.
If it was a little star that spun this up, then obviously it's not going to supernova,
you have to be lighter than eight solar masses to explode into a supernova.
But depending on how much stuff gets dumped up, you can end up with anything here from a white dwarf to a pulsar.
So, a normal theory is that whenever you see a millisecond pulsar, it should be a binary, with something near it,
there's something that's spun it up to these enormous speeds.
You've got to imagine something spinning at a thousand times a second, it needs a lot of spinning to make it happen.
The trouble is that when people studied millisecond pulsars, it turns out that some of them do indeed,
like this one, have a second pulsar or white dwarf near it, but a lot of them don't.
They seem to be by themselves. This is a puzzle that was well known in the early 90's, it's a real puzzle.
How do you get these things spun up to the enormous speeds without something nearby to feed them?
Well, we're almost positive that there's no way in a supernova explosion to get them going that fast on their own.
Even if you did, they would slow down, it turns out, due to the magnetic field slowing things down.
So, Aleksander decided to try and see if there was another binary companion, a white dwarf or another neutron star near this one.
Now, how do you do that?
Well, here is a pulse from this pulsar. It was pulsing every 6.2 milliseconds,
and they can measure the time of a pulse to an accuracy of about 12 microseconds.
So, this pulse is how many radio waves, how much energy in radio energy you're getting arriving here on Earth is a function of time.
So not much, not much, WOAH RADIO, down a bit, up even more then drop down, nothing, nothing, nothing.
The thing to bear in mind is you can measure when this pulse is coming at an accuracy of microseconds, and this is just one pulse.
If you have a pulse every six milliseconds, you get thousands of millions of pulses.
What this means is that you can measure with exquisite precision the timing of these things and that gives you a clue as to what's going on.
Let's say it is in a binary system, so now you've got a pulsar with, let's say, a white dwarf around it,
because of the reflex motion we've already talked about, the neutron star, which may be more massive than the white dwarf,
is still going to be moving backwards and forwards.
When it's further from us, these pulses have a longer distance to reach us than when they're nearer, which means that they arrive a bit later.
So, instead of all arriving at regular, when it's further away they'll arrive late, when it's nearer they'll arrive early
and given when we can measure when these pulsars are arriving to a microsecond accuracy,
light only travels 300 metres in a microsecond,
so that means that we can tell the changes of distance to accuracies of hundreds of metres, or even less.
Because you can take so many of these measurements, you can actually average and so a single pulse might do 300 metres,
but I'm going to have a million pulses because this is doing a thousand seconds, you get a million pulses.
That means that I can average down and probably measure things to better than a metre.
Yes, in fact, some of these things they can measure relative changes to actually a centimetre accuracies,
better than you can do surveying on Earth and this is a neutron star they're at the other end of our galaxy.
So, here's how Aleksander thought "we're going to find out if this thing has, as predicted,
something like a white dwarf from another neutron star in it, we'll look for it to move towards us and away from us."
So, we expect a giant sine wave being many hundreds of metres per second, would be my sense of the scale.
Yes, because whatever it is that's near it, it's got to be very close because it's transferred matter over to it,
so we're looking for something very close to it, orbiting quite fast, maybe on timescales of days,
so you're expecting oscillating days and very large because it's the massive things that's going to cause it to move quite a lot.
That's what was predicted, here's what was actually seen.
Geez, Paul, that doesn't look like a sine wave, it looks like a bit of a dog's breakfast, as we'd say.
Yes, so the first problem is that it's not a clear sine wave, it's all over the place.
Second problem is, we're talking about your 2000 microseconds,
about two milliseconds, so we're talking again your kilometres.
So, it's a kilometres variation, that's not very much over a year because this is almost a couple of years worth of data.
That's too slow, so we're talking about timescales of months, not timescales of days,
and we're talking about movement of your kilometres, not kilometres per second.
So, it is moving, but it's not moving at a sine wave and it's moving too little and on too slow a time scale.
So, what's the way? What's the solution in here?
Well, if you look hard at that, maybe you can see a bit of a pattern. Well, there is a bit of order in the chaos there.
So, Aleksander was playing around with a model fitting software and playing with it,
and it clearly doesn't fit a single sine wave, but it turns out that if you added two sine waves together...
Oh, so they beat in and out.
Yes, sometimes they'll line up together and give you a big pulse, sometimes they'll go against and give a small one,
but it seems that this thing is doing two oscillations.
One slow oscillation back and forth and a faster oscillation back and forth superimposed.
I don't know if I can do that sort of dance, try and do two dances back and forth at the same time,
I thought it'll be too embarrassing to try, but that's what we're talking about here.
So, what's going on here?
Okay, so if you have a little planet and a big planet going around different orbits, it's sort of a spirograph that I used to play for,
you'll get interesting little patterns like that.
It needs to be a planet, not a binary star because if it was another star, the oscillations are going to be much bigger than this,
so you need something that's further out, which means that the oscillations happen on timescales of months rather than days,
and it needs to be much smaller than a star, like a planet or two planets.
So, not only do we have one planet where we shouldn't have one, we have two planets.
Okay, so let's go and se if we can actually work out what the properties of these planets are.