What does it show? The bright disk, as seen from a moderately oblique angle is dust and smaller particles, warm because of illumination from the host star. The dark lanes in the disk are the fascinating thing, though, because they indicate that larger bodies are using their gravitational influence to sweep up the dust at certain radii from the star. Those bodies are planets, and while they are too small to be seen in the image, their handiwork is evident.
What is wonderful about this image is how inclusive it is. Extrasolar planets have been discovered around many stars, but there is no particular indication, when one or more planets are discovered orbiting another star, which other planets might exist without having been observed. This image, however, shows the whole disk, and any large planet would have to clear a lane, so we are seeing the whole system to a pretty good resolution, and this includes at least eight clear lanes, plus a ninth if you count the outside of the last dusty region.
Does this mean there are eight or nine planets? Perhaps. There are at least two ways a planet could clear a lane. One, by being in the lane. In that case, the planet would sweep up dust that comes close to it, and scatter away some dust, to leave its immediate vicinity relative dust-free. A second way that a planet could clear a lane is to clear a lane that is some distance away from it, where the ratio of orbital periods between the planet and dust particles is the ratio of small integers (2:1, 3:2, etc.), and then the planet could clear a distant lane by tugging its dust particles away, a little at a time.
To investigate this possibility, I quickly checked the period ratios that are implied by the distances of the dark lanes from the star. Counting out from the center to the edge, with gap A closest to the star and I representing the outer boundary of the disk, these periods are (in arbitrary units):
A 0.12, B 0.44, C 0.67, D 0.91, E 1.29, F 1.67, G 2.27, H 2.73, I 3.34
By and large, the ratios of these periods are pretty sparing in compelling small-integer ratios. The ratio C:B is about 3:2, D:C is about 4:3, and I:F is about 2:1, but otherwise, the ratios seem pretty arbitrary and therefore seemingly unrelated to long-distance lane clearing. This could mean that all or almost all of the clear lanes represent single bodies, which is an exciting possibility, because it would mean a system of 8 (or almost 8) planets in orbits that are currently non-overlapping and therefore might continue to represent individual planets as the system evolves and all the dust is swept up by the planets.
If these are eight planets, they are distributed quite differently from our solar system, which has much larger distances between the outer planets than the inner ones. Perhaps what we see at HL Tau are the cores of what will eventually be larger planets, with the accretion of smaller, inner planets yet to become evident.
Whatever the details yet to be discerned, this remarkable observation indicates that we are gaining the ability to map other solar systems, at least those with easily-seen dust disks, in remarkable detail.
