Alright, so we can see that there's this change of arrival times which we can then convert to, effectively, an orbit.
When you do this for all of the data as best as you can, you realise that there's not just two planets, there's actually three,
because these pulsars are amazing with how accurately you can measure things.
You can measure, for example, the masses of these things.
The innermost one is actually smaller than the moon, tiny.
So, two hundredths of the Earth in it's mass, that is tiny.
There other two are a bit further out, they are both about four times the mass of the Earth, so there's nothing like that in our own solar system,
these are big but not as big as the gas giant planets, but definitely bigger than any of the rocky planets.
But, when you say "far out", you don't really mean far out,
I mean this one is two tenths, much closer than Mercury is to the sun, and even these two are about, as I said,
they're sort of in where Mercury would be so they're all very close to the star itself.
We can also measure the eccentricity, this is the measure of how off-circular the orbit is.
Eccentricity of zero means the orbit is a perfect circle,
eccentricity of one would mean that it's a parabola, and these things are actually pretty close to zero.
We can't really measure it for the innermost one so we're assuming it's zero, but for these two we can measure it.
The eccentricities are very, very small. They're close in and almost perfect circular orbits.
You don't expect planets to be born with nice circular orbits, especially after the've been slammed with a supernova or something.
They could have been born, some of the planets in our own solar system have circular orbits,
but near after a supernova has gone off there certainly won't be any more.
This is an artist's impression of what these things might look like, not particularly accurate,
in fact, from one of the planets, the other ones look just like dots, they'd be too far away to actually see the disks.
This is an artist's impression of what a pulsar might look like if you look at the very strong magnetic fields,
but no one really knows if they're like that close up.
They've even put an Aurora, meaning that there must be some sort of atmosphere there,
but I don't think I'd be going for a vacation at this planet.
These are going to be cold, irradiated planets.
They're going to be zapped by huge amounts of x-rays from the pulsar and not much heat so these are going to be frigid,
radioactive, probably very unpleasant places to live, I don't really want to vacation there, it wouldn't be much fun.
Yes, not nearly as nice as this picture says.
But that leads us to our puzzle. I mean, we've just agreed that this, around a neutron star, around a pulsar,
is the one place where you cannot get planets because we know that planets form when a solar system forms.
But, after that, to get to a pulsar, you've got to get through at least one supernova explosion.
So, that's puzzle number one, how did the planets survive this immense explosion?
Yes, and the other question is why are their orbits so circular?
Almost certainly, they survive that supernova explosion, that's going to knock them into some eccentric orbit,
and so something would have had to make them go into a nice circular orbit again but the pulsar isn't going to do it. It's a long way away.
Then you've got a third puzzle, remember, to form one of these millisecond pulsars,
you need a companion star which will swell up and dump gas into it,
and will then leave behind either a neutron star or a white dwarf, but we're not seeing that.
These planets are nothing like the mass enough to do this,
so where is the companion star that got the millisecond pulsar sped up to these things?
So, let's look at these possible explanations for how you actually form these things.
Okay, Paul, so we have a bunch of problems here of why I didn't think there should be planets here in the first place.
So, how are we going to actually go through and come up with a scenario to explain why these things are here?
Well, let's talk through three of the theories that people have come up with that might possibly explain where these things come from.
The first theory is that they really are survivors. They were there before the star went supernova and somehow they survived.
So, let's see if this is, at all, feasible.
The reason we like this idea is because we know that planets form at the same time as stars.
Remember, when a giant molecular cloud collapses, it forms a spinning disk of gas and planets form then.
So, that's when the planet should form. Can they possibly survive?
We have a couple of problems here. First of all, we know we need to have two stars in the centre,
and if you're going to have planets going around two stars, they've got to be out a ways, or they're going to be ejected by these two stars.
I guess they've got to be much further out than the planets to actually see around the pulsar.
That's right, that's the first problem.
The second problem is that these are big stars, and so as they run out of their hydrogen in their core
and start burning helium and heavier elements and infusing it, they become huge.
They become red giants or even red super-giants, and they're going to expand out to huge distances
and, quite frankly, potentially engulf these stars or these planets, and when those planets get engulfed in the red giant,
they will get drag and they will sort of get consumed by this star.
The second star will also be in the middle but that has got enough mass to keep in orbit,
even inside the other star for a short period, but the planets probably don't.
That's right, so that's a pretty serious problem, and then that star explodes.
Okay, so let's calculate what happens, whether there is a planet that actually survives the explosion of a star.