There’s not much point in running a science blog if you don’t discuss your own research now and again. The reason I had thought of writing about it is that, last week, I was at a meeting I had helped organise. The meeting was about discs around young stars; what we call protoplanetary, or protostellar, discs. The basic picture is that stars form from clouds of gas and dust that collapse under the influence of their own gravity. However, conservation of angular momentum means that if this cloud has any initial rotation, it will spin faster and faster as it collapses. Therefore material cannot simply fall onto the protostar in the centre, as it would end up spinning so fast that it would break apart. Instead, most of the material falls roughly perpendicular to the rotation axis to form a thin, disc-like structure – the protostellar/protoplanetary disc. Various processes in this disc then act to transport angular momentum outwards, allowing mass to flow onto the central protostar.
These disc are, however, also the sites of planet formation, and that’s what I was going to mainly focus on here. One thing we know is that big, gas giant planets – like Jupiter and Saturn – must form before the disc has dissipated, otherwise there would be no gas available to form their relatively massive gaseous atmospheres/envelopes. If a gas giant planets does form, then we would expect it to start opening a gap in the surrounding disc, as illustrated in the figure on the right.
In the early 2000s there was some evidence for systems with cavities/holes in the inner regions of their discs, which was interpreted as being evidence for a gap-opening planet. However, in some cases these systems appeared to still have mass accreting onto the central star, which seemed a bit odd if there was a cavity in the inner parts of the disc. However, what was really being detected in these observations was emission from dust grains, not from the gas itself, and so the cavity might have simply been a cavity in the dust disc, rather than in the gas and dust disc.
There had already been a couple of papers pointing out that, in the presence of a planet, the dust could respond quite differently to the gas, and that the response of the dust would depend on the size of the dust grains. Myself and some colleagues then published a paper suggesting that if a planet started opening a cavity in the disc, the dust could undergo a form of filtration. The really small dust grains (micron-sized) would be tightly coupled to the gas and, hence, if any gas was flowing through the cavity into the inner regions of the disc, it would also drag these small grains into the inner disc. Larger grains (mm-sized) would, on the other hand, be prevented from flowing through the gap into the inner disc. This process could therefore reduce the dust-to-gas ratio in the inner regions of the disc, which could explain why these young stars appeared to still be accreting despite there appearing to be an inner cavity/hole.After publishing the paper above, I started working on somewhat different projects and didn’t really think about it too much. A couple of years ago, however, I noticed that the paper was starting to collect many more citations than it had been in previous years. Then, at last week’s meeting there were a number of talks discussing observations of discs with possible cavities using the ALMA observatory. The ALMA observatory uses multiple anntennae to make high-resolution observations. It can also observe at different wavelengths, so can probe – in the case of discs around young stars – the structure in dust grains of different sizes.
What the ALMA observations appear to be showing is that in systems with inner cavities, there is indeed often a difference between the small and large grains, with (as shown in the figure below) the smaller grains extending closer to the star than the larger grains. This is largely what we had suggested in our 2006 paper, and so it was fascinating to see something that you had been involved in predicting 10 years ago, possibly being confirmed by observations that couldn’t have been done at the time.
As I said above, we weren’t the only group to highlight that gas and dust may behave differently in the presence of a planet. I think, however, that we were the first to propose the basic filtration process. It’s recently been pointed out, though, that dust filtration alone is unlikely to be sufficient, and that other processes may also be operating. This is no great surprise as what we did was pretty simple, and there are almost certainly other processes operating (grain growth, for example).
However, I think this is an still interesting illustration of how science often works. Early observations lead to models/theories that may then require a new generation of observations to actually confirm. By this time, however, the models/theories may have become even more complex and would require an even newer generation of observations to confirm. However, at each stage our understanding improves and, ideally, we converge towards some kind of consistent picture in which the models/theories and observations largely agree. It can, however, take time and it is great to be part of the process, even if your contribution is reasonably modest.