Sculpting star systems
New simulations could explain how planets form from discs of dust swirling around young stars
This recently released image from ALMA of rings in the protoplanetary disc around HL Tauri is an instant classic, a snapshot of planet formation in action. And now a paper from a Zurich-led European team may be able to explain exactly how structures like these come about.
A lot of the excitement around the image is because the presence of rings and gaps seems to indicate that there are planets forming in the disc. Structure in the rings of Saturn is often due to the gravitational influence of its moons, and newly formed planets could play the same role here. The problem is that simulations which shed light on the behaviour of gas in a planet-forming disc had suggested that although rings such as these could form, they would be very short lived.
To get them to last longer than a hundred or so planet orbits – more than a few hundred years – has been tricky, but it’s this problem that the authors have addressed. It turns out that the presence of dust in the disc, not just gas, is critical. Dust – the word astronomers use for the tiny particles of silicon and graphite, each no bigger than a sand grain – has been the solution to many an astronomical mystery over the years and, given that it is the material from which planets begin forming, it is no surprise that it is important here.
The result depends on the performance of simulations of a disc similar to one our own Solar System may have formed from. One or two planets are introduced into the disc, and gaps quickly open up as a result of their interaction with the dust. It’s then that the magic happens – the change in the distribution of gas allows particles of dust to start collecting. The really exciting thing about the physics of this is that no matter what the conditions – whatever the gas is doing or whether there is one planet stirring things up or two – there is always some size of dust grain that gets preferentially trapped.
For the kind of disc we think may have formed our Solar System it’s larger particles, with diameters of millimetres or centimetres, that begin to clump together. Then, as the gas begins to dissipate, this collection of particles stays together as a long-lived dust ring from which planets can then, presumably, form. If this happens often, we should expect systems where a smaller planet, formed from the dust ring, should sit between two larger worlds. Such arrangements have indeed been found by Kepler, much to my surprise and confusion, but these simulations offer a natural explanation.
The authors resist the temptation to declare the case closed, however. Lots more work will be necessary to identify the circumstances in which this promising mechanism can operate. The main loophole to be closed is, I think, that the planets on either side of the ring were held in the same orbits throughout the simulation, whereas we know migration is common. Yet it seems to me an important part of a complex picture, and another stepping stone to understanding why the Galaxy contains such a remarkable diversity of worlds.
“It turns out that the presence of dust in the disc, not just gas, is critical”