AAS Day 2 : Dust, dust everywhere
Imagine being in our solar system, standing just where the Earth is now, roughly four and a half billion years ago. Around you would be the detritus of star formation, left over material forming a protoplanetary disk from which the planets are coalescing. Understanding just how this disk of dust and gas became the eight planets of the Solar System is one of the fundamental questions of astronomy, and in the penultimate plenary talk today David Wilner from the Harvard-Smithsonian Centre for Astronomy took us through the latest results from this fast moving field.
There are just two major problems with looking for these disks. The first is that most of the disk is cold, dark and not shining, and the second is that they are very small and thus difficult to image. For a nearby example, you’d be lucky if your disk appeared to have a size of a 1/4000th of a degree.
Battling manfully against these challenges, astronomers have had extraordinary success to understand these systems. To get around the first, we can look for comparatively rare chemicals such as hydrogen cyanide (HCN) or Carbon Monoxide (CO) and use these to trace the structure of the disk.
Through these and other methods, we can now measure the mass of a typical disk, and find that 1/100th of the mass of the Sun is typical; that’s a good number because you have enough stuff to form a Jupiter and a few other planets besides, but it does illustrate how small a proportion of the whole we’re talking about. We can also work out a lifetime for the disk. About half of the material disappears in the first 3 million years, and most of the rest in another 2 million; the blink of an eye in astronomical terms.
The next stage is to try and see these things; to do that, we string together light from many separate telescopes, producing images which are as good as telescopes many times larger than we could possibly produce. Unfortunately, perhaps, the disks tend to look just like blobs, but at least we are seeing the places where planets are forming for the first time.
So how do these disks form stars? Well, we have to first get dust grains to stick together. As the clumps get larger, then their gravity can attract further material to keep them growing all the time. If there’s enough material, then gas will be attracted too and you can form a nice new Jupiter. Remarkably, we can actually see evidence for each of these processes in the details of our observations of protoplanetary disks.
One nightmare researchers like the speaker have is their inability to prove that the disks they’re seeing are the same as those which go on to produce planets. The best evidence that they’re getting it right is the presence of large gaps in some of the studied disks; just as small moons of Saturn can create gaps in the rings, so small, newly formed planets still embedded in a protoplanetary disk might produce the gaps we see.
In some cases it’s even possible to make this connection directly. TW Hyd is a star which is known to have a disk, with an observed hole. Earlier this year, a large planet (about 10 times Jupiter’s mass) was detected orbiting the star, and while this planet isn’t the one creating the observed hole at least there’s a system with both a disrupted disk and a planet.
Our early Solar System, then, will have gaps in the disk where Earth and the rest are forming. For now, we can only see such things in detail in our mind’s eye, or in the distorted world of computer simulations. That might chance soon though, with the advent of the ALMA array high up in the Chilean Atacama desert. ALMA’s high resolution images might reveal not only the gaps in the disks, but also material streaming into those gaps to form smaller dust disks around the nascent planets. In viewing these secondary disks, we will be seeing not the birth of planets, but the birth of systems of moons like those around Jupiter. Now that’s a view worth waiting four and half billion years to get.
