So comets seem to be out. How about if we just take two big objects
100 kilometres across, and smash them together. That's surely going to make a lot of dust.
Well, this is how dust comes about in our own solar system. You smash asteroids together typically
and that produces dust. So this seems like a good idea. We just take some big rocks
bang them together, and we keep supplying dust.
But we're going to have to keep doing this, because whenever we make dust, it doesn't stick around for very long.
So that means we're going to have to crash together a lot of rocks.
That's true.
But one rock can have a lot of mass, and it won't show up in the infrared because it's not blocking very
much of the light. So, in fact we could have a situation where most of the mass is
in a few big things. A few being like, a few billion.
And they're very rare collisions, but each collision of two things that are ten kilometres across will produce absolutely
enormous amounts of dust. See, you probably could get away with these things going on
for, hang on, hundreds of millions of years, slowly grinding down the big collection where
most of the mass is to produce a constant supply of small dust things that then get blown away.
But I thought this is how we ended up making planets in our own solar system
where they come together and form big things.
Yeah, that's the problem. I mean we just talked about how you had lots of rocks going around, but instead of smashing them to make
small things, we joined them to make big things. Why doesn't that happen?
Why don't the astroids fuse together to form planets? (Yes.)
That's what we believed to produce the Earth, after all.
Well if all the objects were moving around in nice circular orbits all the same
then their relative speeds would be very slow. So that means their gravity probably could push them
gently together, until they merge and form big things. As they did collide, it'd be a
very gentle collision, just like two pillows gently merging together and so it would make
something bigger. So obviously we can't have something like this. We need them to be having
more chaotic crazy motions so they bang into one another.
So we don't want them to get organised. We need it to be anarchy, so that when they come together they smaaash!
we get all the dust. And so we really need to keep anarchy going, and don't allow this
sort of organisation where we have none of those collisions. (Yep.)
We want just the right amount of anarchy. If we have too much anarchy, all the big things will smash together and produce
a huuuuge! amount of dust that gets blown away and that's the end of it. (Ok.)
So we want sort of mild anarchy.
Ok.
And how do we do that? Well we need to stir these up as you say
and the obvious way to do that is just what happens in our own asteroid belt
which is to have a planet nearby. Our own asteroid belt hasn't coalesced to form a planet because
there's Jupiter just outside it, and Jupiter keeps it stirred up enough that it can't all coalesce.
So maybe that's what we need. And there's a bit of evidence there are planets
near these things. For example, if you look at this image of Beta Pictoris and the disk
around it, if you look closely you see it looks like it's a sort of a bit of a slanty
bit coming out down there. (Oh, yeah, there's a little flare there.)
So that's saying maybe there's a little planet stirring things up.
Yep. So this is a warp. So maybe there's a planet on an inclined orbit
that's pulling it up on this side down other there, or something like that.
Ok. Do we see anything like this on other systems that we can see better?
Well here's Fomalhaut, and in this case you can see when you image it
once again there's a coronagraph blocking out the light from the middle.
This is again a Hubble space telescope image. You can see the dust is actually in the form of a ring.
(Ok.) Which is a bit wierd.
You'd imagine it to be all the way in and out.
So it could be that there's in fact a planet inside the ring which is clearing out the dust in
the inner bit, and keeping the ring stirred up.
Ok. So that is a potential way to solve our problem.
And there's a third, Epsilon Eridani, your favourite star. And in this case we actually now there's a planet there
because of the radial velocity approach.
It's a little bit controversial, but it seems from radial velocity that there is a planet
which has an orbit just inside this ring of dust that we can see.
Ok. So this is an image taken in essentially sub-millimetre in that, you know, some ways between radio and infrared
and there's a little hole in the centre where the planet seems to be clearing things out directly.
So, again it seems to be holding together that these debris disks are something
a little different than what we have in our own solar system, or maybe they're a much
bigger analogy to what we see, where things collide, come together, it's very violent, and we can
keep on making dust which reflects, and we can see here on Earth.