So it seems there really are small things, asteroids and the like in these extra solar systems.
We don't see them directly, but they smash into one another and generate all this dust.
So, given that our solar system formed probably in a way not dissimilar from those
we must expect to find the remnant in our own solar system of something like these debris disks.
Yes, I guess we've got the asteroid belt in our own solar system which certainly
generates dust, and the zodiacal light that we can see at night.
However the amount of dust generated by the asteroid belt is vastly smaller than the amount of dust generated
by these extra solar debris disks.
And also, the ones we're seeing around other solar systems are much further out.
Typically seeing things maybe 40, 50, maybe 100 astronomical units out
which is beyond the orbit of Neptune in our own solar system.
So they're really not asteroid belts so much as ice-dust belts.
Ok. But there is a bunch of stuff out there beyond Pluto, which we call the Kuiper belt.
So the Kuiper belt is this impressive collection
of rocks and debris that's out there and we sort of inferred the fact that Pluto was there
and we think that Pluto was created from, you know, the conglomeration of all these
rocks and pieces, and for a lot of time we figured there wasn't much stuff out there
because Pluto was thought to be much larger than it really is. Now we realise
it's actually quite a puny little dwarf planet instead of a full-sized real planet like the
Earth is, and indeed it's actually even smaller than Eris, a new object that we've seen.
So the Kuiper belt is full of this stuff we need of these icy bits of junk occasionally conglomerating
into planets, and it strikes me that that may well be what you're talking about. A debris disk.
Yes. It turns out we've got two asteroid belts in our solar system. We've got the rocky
one between Mars and Jupiter, and we've got the icy one beyond Neptune of which Pluto and
Eris and Sedna and all these other things are the biggest items. And in fact, presumably
they collide together occasionally and produce dust. We can't see it here, but that is actually
an analogue of the outer icy disk. The Kuiper belt is what we're actually seeing in other solar systems.
So how how big is our Kuiper belt? I mean, I know about the Kuiper belt
but how big is it relative to the ones that we're seeing around these other systems?
Well the amount of dust that we predict that should be generated in the Kuiper belt is maybe
ten times smaller than anything we're seeing elsewhere.
Alright. But would we be able to see our Kuiper belt elsewhere?
No, we wouldn't.
Ah. So maybe it is the remnant. That sounds like a good match.
So Paul, how common are these debris disks like our own Kuiper belt, around other stars?
Well the latest data from the Herschel space telescope indicates that if you look
at the spectrum of a star... so it goes up and comes down, and what they do is they look
for whether the light at the infrared is higher than what you'd predict from the spectrum.
And in about 20 percent of stars it is significantly higher. So that they can say with confidence
that there's some sort of debris disk out there.
Ok, so what you're saying is that 20 percent of stars have debris disks, and 80 percent don't?
Well, what we can say is that we can detect debris disks in about 20 percent.
However given the sensitivity of current space telescopes
the debris disk has to be about 10 times more massive, 10 times more dust
than on our own Kuiper belt. So we're only picking up mega Kuiper belts if you like, and we couldn't
see our own Kuiper belt. So it could be that 20 percent have these mega debris disks out
40 astronomical units or beyond and the other 80 percent have nothing, but more likely perhaps
20 percent of these mega ones and the other, some fraction of the rest, maybe half maybe
all of them, have smaller debris disks more like our own solar system might have.
So we're really going to need... I mean, Herschel is a brand new really powerful telescope and
it's sadly run out of coolant, so it's sort of ending its lifetime, and we don't really
have anything until the next generation of huge ground-based telescopes are going to
be able to do anything like this and they're not even that sensitive, where Herschel was.
Yeah so it might be that's going to be the story for a long time to come.
Alright. So these debris disks, they're out at large distances. But we know that there's going to be dust
there, and that that dust will disappear over time, so we need to keep on replenishing it
and we need to have some way to stir things up or else we just end up with the dust going away.
So in our own solar system that's Neptune. What's going on in these other ones?
Yes, so if the... as we said earlier, if these objects are just moving in nice circular orbits they
won't smash into one another, and if they do they'll merge rather than blowing themselves to pieces.
So we need something to put them into eccentric orbits. As you said Neptune does this in our
own solar system, it stirs up the Kuiper belt. So what this is telling us is not only do
we need large mega belts of these things out there, but we need probably some planets out
there to stir it up. So we're looking for big planets a long way out.
So every time, and those 20 percent of the objects that have these mega Kuiper belt objects, we probably
have something like Neptune or even bigger out there stirring things up.
So, indirect evidence for planets.
Yeah, but it'd be nice to actually see them directly to make sure our
theories are correct here. The trouble is both the radial velocity approach and the
transit approaches are only spotting things further in. They're not going to see anything
out at Neptune-type distances. Microlensing something that far out will show up as a free-floating
planet, and we know they saw some free-floating planets, but we don't know if they really
were miles out or whether they're actually out at Neptune-like orbits.
Right. So we need a new technique, a technique that allows us to get a long way out from the planets, because
all our other ones really are biased towards things close in.
That's what we're going to talk about next time.
Oh, very good.