There was an interesting (non-climate) paper in Science, by Laura Perez and colleagues, about Spiral density waves in a young protoplanetary discs. Essentially, they used the Atacama Large Millimeter/sub-millimeter Array (ALMA) to observe a young stellar object (Elias 2-27) in the Ophiuchus star forming region and found – as shown in the image below – that this particular system had a circumstellar disc with quite prominent spiral density waves.
A great deal of my research over the last 10 years, or so, has involved studying spiral density waves in very young protostellar systems, and so I was invited to write a short Perspectives piece about this paper (I should be getting a link to a free copy shortly, so will provide it when I do). There are a number of reasons why this result is interesting. One is simply that observing structure in discs around very young stars is difficult; so it indicates that it is now becoming possible to do so. The system is about 139 parsecs (pc) away (1 pc is 206268 times the distance from the Sun to the Earth) and this image has a resolution similar to the orbit of Neptune around the Sun – this may not sound all that impressive, but the system is a long way away, and the observations are at quite long wavelengths, so it is really quite remarkable.
Another reason this is interesting, is that this disc is quite massive relative to the mass of the central protostar (the disc mass is about one-quarter that of the central protostar). This means that the spiral density waves could be due to a gravitational instability. If the disc mass is high enough, then its own gravity will try to cause the material in the disc to clump together. However, the velocity in the disc varies with distance from the central protostar, which then causes these density structures to shear out, producing spiral density waves, very like what is seen in a typical spiral galaxy (it also depends on the temperature on the disc, but this is the basic picture).
The movie on the right is from one of my simulations of a massive disc around a young protostar (the properties are not the same as those of Elias 2-27, so this is just illustrative) showing the development of these spiral density waves. There are two main reasons why this is interesting. One is that these spiral density waves can act to drive mass inwards onto the central protostar, and may indeed be the main driver of mass accretion during the earliest stages of star formation. The other is that it’s fairly clear that the growth of planet building material must start when the system is very young, otherwise there wouldn’t be enough time to build the cores of the gas giant planets (Jupiter and Saturn, for example) before the disc gas dissipates. Also, there is some meteoritic evidence that differentiated objects had started forming when the Solar System was less than 1 million years old, and this means that kilometre-sized objects formed quickly (differentiation is the sinking of dense material – iron/nickel – to the centre of an object).
One issue with the growth of solids in a disc is that – if the disc is smooth and laminar – the solids should spiral inwards due to aerodynamic drag, and may fall into the central star before growing large enough to decouple from the disc gas. However, if there are any structures in the disc, then the solids can collect in these structures, rather than spiraling in to the central protostar. Therefore, in addition to driving mass accretion, these spiral density waves may also provide sites where planet building material can collect and grow. So, they may be important for both star formation and planet formation.
However, there are a number of fairly large caveats. The disc around Elias 2-27 is quite extended (i.e., it has a large outer radius) which makes a gravitational instability less likely than if it were more compact. Also, some work that myself and collaborators did about 10 years ago suggests that two-armed spirals (like that seen in the disc around Elias 2-27) are only likely in discs that have masses at least half that of the central star. The disc around Elias 2-27 is massive, but not quite that massive. So, it is possible that these spirals have a different cause. It could be embedded planets that are driving spirals out into the disc, or maybe it’s a tidal perturbation from a passing star. Even mass infall onto the disc could drive spiral density waves.
However, whatever it is, it is still an interesting result. It shows that there can be structure in discs around very young protostars and that we are now in a position to probe these structures, so as to understand how they influence both star formation and planet formation.