What we're looking at is a simulation of dust moving through gas.
Astronomers actually spend a lot of time thinking about dust because dust is ubiquitous
in the gas and medium between stars. In fact, pretty much all the heavy elements
in the universe are actually concentrated in little dust grains.
So if you care about the things that made us, or the things that made planets,
or pretty much anything that's not boring hydrogen and helium, you have to care about dust.
We've discovered a new instability, or mechanism by which dust and gas
interact across a tremendous range of physical scales and different types of systems.
Everything from atmospheres with rain and clouds and dust and smog,
through seeds of planets, through gas and dust that fills the space
between the stars and galaxies. We've been trying to understand for
literally hundreds of years how planets form, and the biggest challenge has been
understanding how you build the seeds of the planets. And that's one area where
this could really dramatically change our picture of how that happens.
It's a lot easier if I can draw it on the board and show a sort of visual example of how this works.
You can start with tiny grains of dust, and these might wander
into each other and co llide and stick into a bigger dust grain, but then the
problem is this gets bigger and bigger until you get to maybe a centimeter-sized object,
and they don't stick together. Gravity doesn't pull two pebbles together
very strongly, so you need some huge object in order to pull
things onto it with its own gravity. And everything in between here is just a big mystery.
In the new picture, none of this in the middle actually happens
and the idea is that you just skip basically from these tiny dust grains
all the way to this giant object. So you get lots and lots and lots
of these little tiny dust grains, clump them all together until they become so many
in such a small space that gravity just pulls them all together into this giant object.
So how does this actually happen - how do the dust grains actually clump together?
To explain that, let's think about stuff moving through our atmosphere
where we have some intuition. Let's ask what happens
if we have a giant object like a bowling ball moving through the air.
You drop it out of a plane, so you push the atoms and molecules in the air out of the way as you fall down
towards the ground. And if there were just the one bowling ball,
that would pretty much be the end of the story. The difference is you've
now got a whole fleet of bowling balls that are all falling down at the same
time, so there's another one over here and another one over here. And this one's
falling down and pushing the air out of the way, but so is this one - it's also
pushing the air out of the way as it falls down. And so is this one pushing
the air out of the way as it falls down. And so this all streams sort of
funneled in this direction, between the two bowling balls. Now this one sees this
huge headwind coming up at it and gets kind of knocked out of the way by that
giant headwind. So maybe this one gets knocked over here but now that means
that I'm starting to line up my bowling balls all preferentially over here.
Which means they're generating an even stronger headwind, knocking the gas out of their path.
So this process runs away and then potentially, if this were
instead of our atmosphere with bowling balls, a swirling disk of dust around a new star
form a giant planet or asteroid seed by having all these bowling balls
collapse together under their own gravity. The next steps from here are
really working out the consequences of this for this broad range of systems, and
we're trying to convince a lot of other people in the community to start
exploring this because there's so many different aspects of these that we can't
hope to explore them all ourselves. We've started talking to people in volcanology labs,
looking at what they think is an example of this instability that
they'd seen as volcanic ash settles out of the atmosphere.
There's really an infinitely large family of different instabilities that can all be characterized
through a very general formalism, but are really ubiquitous everywhere.