So, assuming that the planets can actually survive the supernova explosion, we have yet another problem.
As it explodes, most of the mass is going to get blasted out into space.
How much mass is left from a typical supernova explosion?
Normally the neutron star weighs about one and a half solar masses after the explosion,
but the star that exploded was 10 solar masses before it exploded.
So, you've only got maybe 15 percent of the mass left, but bear in mind, the planets before the explosion were in a nice orbit.
You'll see a circular orbit like in our own solar system,
which meant that they were going just the right speed for centrifugal force to balance gravity.
The trouble is, the gravity now is 10 times smaller.
These things have now been travelling 10 times faster than they need to be. So what's going to happen to them?
Well, it means that they have higher than the escape velocity, of course, they're just going to go off forever,
and even if they don't, their orbits are going to be really perturbed and be made into big, highly eccentric, elliptical orbits.
Exactly what we don't see.
These things are forever going in beautifully circular orbits, and you might worry that this star also has gravity,
so that will counteract but we know that this star weighs less than this one, because the first one exploded first.
Yes, the more massive stars explode first.
But then the second star will swell up at some later stage, possibly engulfing the planets yet again,
and feeding matter onto the neutron star to make it into a millisecond pulsar.
That's another problem, where's the remains of this? There should be a white dwarf left over, where did that come from?
That's right, so we really have a problem of where that star went to.
So, the obvious thing to do is to come up with a different solution here, I think.
Yes, I mean, really the odds against the planets surviving have been stacked up here, there are just too many problems.
They'd have to be too far out, they get swallowed in the red giant phase,
they get flung into an elliptical orbits or out into deep space during the explosion,
they get disintegrated by the blast wave and where's the secondary star that should be there?
This is looking pretty dead to me.
So, if we form it afterwards, how are we going to do that?
One obvious possibility is that there were no planets before, they all got destroyed, but somehow we acquire planets after the explosion.
It could be that you get the thing exploding, leaving the neutron star in the middle, and some of the mass gets flung out into space.
Maybe some fraction, it doesn't need to be a very large fraction, goes out but then falls back in again and then forms a spinning disk,
a bit like a protoplanetary disk and then this protoplanetary disk turns into planets.
Okay, I can imagine if maybe the first object explodes and does this,
but the problem is that we have an angular momentum problem here because the supernova is a big, round ball,
things explode so I know that in this case I really don't have much angular momentum to get rid of because I didn't have any to start with.
So, remember, in solar system formation, the trouble was that you turned a big, blobby cloud of gas which had motion
as it got smaller and smaller, angular momentum was conserved
and therefore had to spin faster and faster until centrifugal force balanced gravity.
But, in this case, everything has come out from the middle. It was probably spinning rather slowly then,
but it certainly didn't have enough angular momentum to hold it out, otherwise it wouldn't have been a star in the first place.
So, the gas is meant to come out and fall straight back in again, so why does it end up in a spinning disk?
I can imagine maybe a little bit of turbulence or something, but there's another problem here as well. There's still another star.
Yes, where's the other star gone? There should be another star. So, maybe we can kill two birds with one stone.
We've got problem number one, "where are these planets coming from?"
and problem number two, "where has the second star gone?" So, possibly we can combine these in some way.
One possibility is that we need to get rid of the second star.
Maybe it feeds matter onto the neutron star and it somehow gets torn to pieces or disrupted
and so some of the mass end up in the neutron star but some ends up in a spinning disk and out of the spinning disk, planets form.
Well, that actually kind of makes sense because imagine that you're a star and you're next to this little neutron star,
and you start giving it more and more mass,
so it has a lot of gravity to begin with because it's small and compact and pretty heavy,
and you're making it heavier and you're becoming lighter.
I can imagine that, for example, since the size of a star is really a relationship between how much gravity can keep it compact
and how much pressure will push it out, that is, if I make it lighter and lighter, it'g going to have less and less gravity
and the thing could, presumably, potentially rearrange itself and become larger and larger,
making it more susceptible to the main star's gravity ripping it apart.
Yes, so normally you would think that if you have something, like a planet that took some of it away, it would be smaller.
But as Brian said, this doesn't necessarily work for some types of stars,
sometimes with stars it does, but just like I imagine a pile of cushions.
The cushions on the bottom are going to be compressed by all the weight of the cushions above,
so if you take cushions off the top, it can actually get bigger because the bottom ones will spring up.
Some sort of the stars can be like that. The stuff in the middle is compressed by the weight outside,
so if you strip off the stuff outside, the stuff in the middle can expand.
So, you can imagine a star that gets bigger, feeds gas, gets bigger still, feeds more gas,
until eventually there's nothing left but a little bit of gas which might form planets.
I like the sound of this!
But it only relies on certain sorts of stars, fully convective stars and certain sorts of white dwarf will do this,
so you'll need a very specific sort of star.
Another possible theory is that when you feed all of this mass onto the neutron star,
that mass is going to fall down the intense gravity and generate huge amounts of x-rays,
because x-rays will come out and zap the nearby star and that might cause it to swell up, and maybe even disintegrate it.
So, it donates material and sort of gets penalised, not dissimilar to a male Black Widow I guess,
where you try and help out and you get killed for your efforts.
Indeed, this is called the 'Black Widow Model',
the idea that this could be why so many of these millisecond pulsars don't seem to have companions
because the neutron star ate its neighbour and destroyed it for the benefit of feeding it. Pretty nasty.
That's just one possibility.
Now, how about another idea which is that we know we have two stars,
and you can imagine that one star is a neutron star and one star is a normal large star.
Yes, so let's assume that the two stars were very massive stars and the planets were a long way out.
The star will explode and form a neutron star, the bigger one, and let's assume that these things are so far out that they can, more or less,
survive that and the second star is so massive that it has enough gravity to keep these things bound.
They're not in a circular orbit anymore, but they're still bound there.
Alright, so now we have a big star orbiting a massive star orbiting a neutron star.
It turns out that in some situations as this starts feeding that, what will actually happen is that the two will merge.
The neutron star will fall into the middle of the massive secondary star producing what's called a Thorne-Żytkow object.
So, that would be a really interesting star that's not a normal star.
Instead of having this normal core that's undertaking nuclear reactions in its core, instead here we have a neutron star,
the core of a dead star, incredibly dense, and so you're going to get this kind of shell around it
where things are going to be really hot and all sorts of wacko nuclear reactions will be occurring.
It turns out that the prediction is that around this core, the rest of the star will swell out to absolutely gargantuan proportions,
so big that, even planets far out as they are, are going to be inside it.
Now that they're inside it they can continue orbiting, there's going to be a little bit of drag
but it turns out that this drag will move them in and turn the orbits into circles.
So, they'll go closer and closer in and they'll become more and more circular which sounds like just what we want.
It has to be pretty well-timed, doesn't it? Because they're going to keep going in and the closer and closer they get the denser it is,
the more they're going to be dragged, and the faster they're going to want to fall in.
You're going to have to turn off this process somehow.
The idea is to have these things spiral in, nice circular orbits and then ends its Thorne--Żytkow phase
and just ends up as a neutron star at just the right moment to freeze them, like a game of musical statues,
just frozen when the music goes off in just the right places.
Hmm, so it's a very tuned model, although it does sound like it could do what we need it to do.
Except that maybe the neutron star in there isn't going to be spinning fast enough.
Ah, that's probably a problem because it has to be spinning at almost 1000 times a second and this isn't going to be easy.
There's going to be a lot of drag and all sorts of problems here to keep that star spinning.
So, those are our three models, which of them is true?
Well, in time honoured fashion of astronomers, we clearly need more data.
What we'd really like is to see some more planets around pulsars.