So, since 1992 when this first planet was discovered, we've, of course, been out looking around all the pulsars,
and we're very good with these pulsar measurements of making very precise measurements.
We're not going to miss many planets. But, funnily enough, we haven't really been able to find too many.
But we have found a couple. (Here's one that was discovered.)
It's around the pulsar B1620-26, and this pulsar is a bit different than the one we've been talking about so far
because it's not in our own galaxy but in the globular cluster, M4, Messier 4, that orbits around.
This is an incredibly dense environment of close packed, very old stars.
There's about a million stars packed into this little fuzzy ball that we see up in the sky with your eyes.
It turns out that this is, rather, a different system from the first one we talked about, and what we have is a neutron star,
and this one does have a white dwarf orbiting it, just like you predict, and another planet is further out.
So, this planet is actually orbiting around this binary neutron star and white dwarf.
How do you form something like this? Well, the current theory about how you form this one
is that it relies on the very dense environment of the globular cluster.
Where we are in the galaxy, stars almost never come near other stars. It's just too much space between them.
The middle of a globular clusters stars do have close encounters with other stars quite a lot.
So, here's out current bet about what's happening. We started off with a planet orbiting a main sequence star,
much like the Earth around the sun, and we had a white dwarf and a neutron star.
They came close to each other and got a bit muddled up,
and the net result was, after they jiggled around and did their dance is that the white dwarf went out by itself
leaving the neutron star orbiting around the normal main-sequence sun with a planet left in the outskirts.
Then at some later stage the main-sequence star swelled up,
spun up the neutron stars into a millisecond pulsar and then turned into a white dwarf itself.
It's almost like the human relationship here, I guess, where we end up with a sort of new family after the fact.
This seems to work, we can do the numbers so it looks like it's a reasonably plausible thing to happen.
The main interest in this is that there are, in globular clusters, there are very few heavy elements.
So, people thought that the planets were made of heavy elements and that there shouldn't be very many planets here,
so the fact that we've seen a planet in this environment with very low heavy elements is quite interesting.
The second thing is that this globular cluster is extremely old, over 12 billion years old,
so that's telling us that at least some planets were capable of forming around normal stars
when the universe was very, very young, which you might not have expected.
So, this is an unusual system, maybe we should look at one which is similar to the first one. Have we found anything like that?
Well, this is the closest one, it was only discovered quite recently by Australian astronomer Matthew Bailes and his team.
J1719-1438, these are the coordinates on the sky. (They could come up with better names.)
They could indeed. Well, this one, in fact, is generally known as the 'Diamond Planet.'
Ah, that is a good name. (Yes.)
What we've got here is here's the motion and it's oscillating velocity by about the same amount as the first one we saw, so about a millisecond.
You can see that we have exquisite precision. (It's nice, it's like a single sine wave this time.)
Yes, this is what you'd expect from a binary, you have a second star, except the amplitude is much too small.
So, this thing that's orbiting around it doesn't have the mass of a neutron star or a white dwarf, it has the mass about that of Jupiter.
But this is going around every 2.2 hours. The Earth goes around the sun once a year.
This goes around in two hours, so its year is 10 times smaller than our day. It's incredibly close in.
In a sense it would actually be inside the sun to go that close in,
but luckily a neutron star is much smaller so it can actually be outside and going around that fast.
So, this would be quite an interesting place to be because you have the whole mass of one and a half times the mass of the sun,
crammed down into this tiny thing of 10 kilometres across. (This is much too big to scale.)
Then you have a planet orbiting it right next to it.
So, that is going to give you some interesting forces compared to what we're used to here on Earth.
You can do the calculations, what you'd find, of course, if it was something of about the mass of Jupiter,
we don't actually know if it's the shape, colour or composition of Jupiter,
but if it was, and every planet we know of that mass so far is a gas giant,
the nearest bit is going to be much closer to the neutron star than the further bit;
which means that gravity here will be much stronger than gravity over there.
Now, remember that to stay in orbit, the centrifugal force must balance gravity,
because gravity is much stronger here, this bit must be orbiting much faster than that bit.
That's not good if you have a planet and the near side of the planet orbiting you 10 times faster than the outer side of the planet,
that's not a planet anymore, it's been ripped to pieces.
Yes, so it's going to be ripped to pieces by these, effectively, tidal forces.
So, how can it be there? Well, the only possibility is if it's not the speed.
If we can make it much smaller, that means that the distances are more similar so the gravity is not going to be so different;
also it's going to have much stronger gravity so maybe it's got enough gravity to hold itself together.
So, you can calculate the absolute minimum density and maximum size for this thing, it turns out to be absolutely tiny.
What you need is something like this.
Ah, so imagine that you go through and the heaviest thing we could make out of it was an osmium planet,
that has a density 22 times of water, that's the heaviest element there is, but this thing is even denser than that.
Well, 22 times I'm prepared to do, so it could be an osmium planet.
Osmium, well okay, it's pretty crazy to do an osmium planet. (That's hard to imagine.)
But it may be more likely for it to be formed from the star that spun the planet up.
Remember, this is a millisecond pulsar so once again we've got the problem, where did the star come from?
So, maybe this very dense, Jupiter mass thing is actually the remains of that star.
So, possibly, it was a white dwarf and it fed the matter, spun this up and then we had the Black Widow phenomenon.
The x-rays, as the matter fell out, the x-rays came out and zapped the star and blew most of it away and all we're left with is the core.
Well, if you think about it, the core of a dead star, a white dwarf is, well the sun, for example, when it is becoming a white dwarf,
it's going to be making helium into carbon, and a little bit of oxygen happens on the outside, but inside it's effectively forming carbon.
So, the core of a white dwarf, you'd expect it to be carbon and this carbon, initially it's very hot when the sun becomes a white dwarf,
but over billions of years it'll cool off and it becomes this giant carbon crystal.
We know what carbon crystals are, those are diamonds. This really could be a diamond planet.
It wouldn't actually be a shape like this, it would be spherical,
but it would be made of diamonds, or maybe even some other forms of carbon even more compressed in the middle.
This is the idea that we might be talking about a diamond planet here.
That's what we've learnt so far, we've now found three planets around pulsars and they're all really wonderful in their own way.
Yes, but it's not exactly where I was expecting to look for life.