Chris Stark, Carnigie Institute
Ben - We’ve reported on the discoveries of exoplanets before on the Naked Scientists, but what would our solar system look like to an extra solar observer? Dr. Chris Stark from the Carnegie Institute in Washington has been trying to find out.
Chris - Well, if we go back to sort of the beginning of the story, you know, many people think of space as a vacuum. In fact, in our solar system, it’s not quite a vacuum. There happens to be a lot of dust in our solar system and especially near the Earth, we’ve detected this dust. We can actually image this dust through its light that it reflects from the Sun or if we look in the infrared, it’s actually very bright. And the dust near Earth, we call the Zodiacal cloud and in fact, you can see it with your naked eye at night. And we think that this dust that occurs near Earth is actually created in the asteroid belts and due to comets that are whizzing past the Sun. But we know that there's another reservoir of bodies in our solar system out near the outer regions near the orbit of Pluto, and that reservoir, we call the kuiper belt disc. The logical conclusion is essentially that we would think that the kuiper belt objects would also be colliding and producing lots of small dust grains. And although we haven’t directly imaged the dust from the kuiper belt, there have been a couple of missions that have flown into the outer regions of the solar system with a dust counter on board. And so, we have a rough idea that dust exists out there and how much dust there is. And in fact, we believe that there's so much dust that if you were to image our solar system from afar, that dust would be one of the most easily imaged features of our solar system especially if you're to look at the infrared.
So the problem is, we know there's a lot of dust produced in the kuiper belt, but we really have no idea what it looks like. And so, if we want to of explore what our solar system might look like to an extra solar observer, we have to rely on theory then to infer what the dust from the kuiper belt would look like. That's where I come in. Recently, Mark Kuchner and I have produced some new models that predict what this dust distribution might look like. These theoretical models that are produced have to include certain dynamical effects. The dust grains are perturbed. Their orbits are changed by the gravitational influence of planets and if there's enough dust, actually, two dust grains can collide with one another. And so, if you want to produce a model of the kuiper belt dust disc, that includes both gravitational interactions with planets and collisions between dust grains, then you have to do what we call an n-body integration.
The problem that people thought was very daunting was that there are so many dust particles in our kuiper belts, on the order of 10 to the 20th or up to 10 to the 30th dust grains that people thought there was just simply no way to do this with current computational capabilities. And so, to produce a model that includes both collisions between dust grains and the gravitational dynamics that's necessary, it’s very, very challenging. Fortunately, Mark Kuchner and I came up with a tool that essentially allows us to accomplish this task and we call that tool essentially a collisional grooming algorithm. Basically, what we do is we produce a model of a dust disc that ignores collisions and we sort of include all of the gravitational dynamics of the problem. And then what we do is we go through and we “groom” that dust model that did not include collisions until it looks like what it should have looked like, had collisions been included in the first place.
Ben - So, what's the distribution of dust in our solar system actually like? Is it fairly homogenous or do we get bands? Do we get clumps? What would we expect to see if we were actually an extra solar species looking down onto our solar system?
Chris - Right. So probably the most notable feature that you would see if you were looking at our solar system from afar is what we call a resonant ring structure that Neptune ends up creating within the kuiper belt dust cloud. Basically, Neptune selectively places dust grains into specific orbits which we call mean motion resonant orbits and what this does is it creates this large circumsolar ring structure that goes all the way around the Sun, and it has a radius approximately equal to the distance from the Sun to Neptune. And it also creates a gap within that ring. And so, if you were an extra solar observer looking at our solar system, one of the first things you would notice is that resonant ring, and the gap in the resonant ring would tell you where Neptune was and possibly even the mass of Neptune, even if Neptune was too dim to directly detect itself.
Ben - Now this is just a modelling stage but you can use your results to inform what observations people should be making. What do you need to see out there in reality now in order to make your model a bit better?
Chris - It would be nice to confirm the existence of specifically Neptune’s ring structure that we predict. To see the dust out near the orbit of Neptune, you have to look through all of the dust in the inner solar system before you can see that. And so, something that we’ve suggested that may work is, instead of just trying to detect an image of the dust, look for essentially an alternating or a baring image. In other words, look for the asymmetries that we predict and looking for those clumps as they move.
Ben - One of the advantages with modelling is that you can roll it forwards in time and you can roll it back in time and make predictions based on your model. Do we think the solar system has essentially always looked like this?
Chris - In fact, no. We think it has changed quite a bit. In the past, the kuiper belt was much more massive and essentially, we think that it has been collisionally eroded over time. So in the past, this means that since it’s more massive, there was more dust, and this means that collisions between dust grains happened more frequently. And so, what we’re able to do is essentially tune our model and look at what the dust distribution may have looked like billions of years ago. And what we found was actually very surprising. It turns out that one of the first thing that happens when you go back in time to a more massive disc is that Neptune’s resonant ring structure disappears. If you're an extra solar observer looking at our solar system when it was very young, you would not see this circumsolar ring with a gap near the location of Neptune. Instead what you would see is you'd see a very circularly symmetric ring that is basically just tracing the location and the distribution of the kuiper belt objects that are colliding to produce the dust. Basically, Neptune’s gravitational imprint disappears.
Ben - And so, what's the next stage for you? What's the next step in your research?
Chris - Astronomers have already detected ring structures around other stars, but all of these ring structures that have been detected so far are around very young stars. This means that we’re not necessarily seeing the gravitational imprint or the resonant ring structure that I described earlier. Instead, what you're seeing is essentially the dust is tracing the distribution of parent bodies. So one of the things we’re interested in is trying to model observed ring structures around other stars and seeing what we can learn about the distribution of parent bodies in those systems.
Ben - Chris Stark, explaining how Neptune casts a distinctive ring out of the cloud of dust that pervades our solar system.