So Paul, it strikes me that if we keep on getting rid of all the dust one way or another
the only way I could think to keep that dust to be in these stars we see, is to keep creating new dust.
Yep. So where does dust normally come from?
Well, obviously the star was, you know, had dust when it was formed out of the cloud it was formed out of.
And then red giant stars make a lot of dust and sopernovae make dust.
But this isn't a red giant or a supernova.
Alright, so that's not going to work. Now, I know you like comets.
They have some dust in them, don't they?
They sure do. Here's a picture of a comet I took in the Gemini telescope.
This is what most comets look like. They're little tiny fuzzy things that move, and you can see just a little bit of a tail coming out
which is actually dust-grains.
I think we'd probably better explain what's going on. That's actually a star, right? Where we've tracked on the comet
and so the star is being dragged along in the image.
Yes, the telescope is following the comet as it moves. This is over about 20 minutes or so
and the stars are moving across the background.
Alright. So that's one little comet. What about the big comets?
It's actually a big comet, but it's a long way out. This is out at about Saturn's orbit
However, most comets never get any closer than that, but occasionally comets like this one, Comet Hale-Bopp, does come close.
That's a picture I took about a day after it had been discovered, and it came closer and brighter still. And all these times what you're
seeing here is dust being blown away. What's actually happening is in the middle of the comet, these are
some space cropped images of the nuclei of two comets, there's this rather small thing maybe about in this case 16 kilometres long which
is probably a dirty snowball.
And that's Halley's Comet, right? (Yes)
So one of the grandaddy of comets.
And the radiation from the sun is coming in and melting it, and it's a mixture of gas and dust being blown out
and the big tale that we see is in fact primarily a tale of dust. Here's a little simulation showing this.
So the red dot is the nucleus and the blue dot is dust-grains, and you can see as it comes near the sun...
Wow. So, a comet when it comes by, really spews out dust in all directions. So this is what we need.
Yes.
Now unfortunately in our own solar system this is not going to work too well.
I mean, there aren't very many.
That's true. I waited for 15 years of my childhood to get to see
a decent comet. I was expecting Comet Kohoutek to be everything, and it was terrible
and I had to wait all the way to Comet Halley, and even it wasn't very interesting.
Yes, so comets are far too infrequent in our own solar system to produce anything like the
dust even in our own solar system, let alone these debris disks which have millions of times more.
However, it turns out that at least one of these other solar systems has
a lot more comets, or at least comet-like things. This is a spectrum of Beta Pictoris
the star in the middle of Beta Pictoris. And you can see absorption lines due to gas
and what you can see this is the main absorption line due to the gas, which is at rest relative to the star.
But you can see this little funny shape here. What's happened is this is the
spectrum taken earlier and that's the spectrum taken later, and the spectrum has dropped.
and likewise here, and likewise over there.
Ok, so it's like there was something made out of a substance that absorbed light at these particular wavelengths
and it was there, and then it wasn't.
Yes. So it turns out that for most of the lines you see in the star
you have another line redshifted by between 10 and 100 kilometres a second
so that means the gas is moving away from us more than Beta Pictoris is.
I see.
And it comes and goes on timescales of days. You see this absorption appear
a couple of days, then gone again.
So you could imagine if it was a comet coming in it will of course have a high velocity
because it's coming in on a highly eccentric or even a parabolic orbit. It's moving fast
it's made out of stuff, and so we know if it were directly at the same speed as the
star it would be here. It's coming in, it has a velocity relative to the star
and we can pick it up that way.
That's right. So it looks like what's happening is the star's being hit by probably about a thousand comets a year, raining down.
Which is much more our own sun which is... a comet this size might hit the sun maybe once every
a few hundred years.
So that would be a fun place to be an observer. You get you get to see all sorts of comets there.
Yes. But it turns out even here where you've got hundreds of comets a thousand maybe a year coming in, dumping their dust
it's still nothing like enough to produce the amount of dust you require to give us the infrared excess.
Ah, so were still looking for another explanation.