A good place to start when learning about circumstellar disks is understanding why we study them. The most important and exciting reason we study circumstellar disks is because they are believed to be the sight of planetary formation. Furthermore, by studying these disks, we are able to constrain theories of planetary formation and can develop timescales for the evolution of planetary development. Finally, addressing these issues allows us to compare our own solar system to others, placing our own solar system within a context of other systems.
It is now believed that our own sun, like all other stars, formed from a cloud of dust and gas in space. For some reason or another a local density increase occurs within these clouds. This causes that portion of the cloud to contract in on itself under the influence of its own gravitational pull. In the end, the cloud collapses in on itself to create what is known as a protostar. A protostar is a star that does not derive its energy from fusion; there is not enough gravitational force yet to cause fusion within the star's core. It still glows, however, because the gravitational contraction causes the gas to become so hot that it glows at optical and infrared wavelengths.
Though the star has collapsed in on itself, dust still remains. What is left of the cloud rotates with the protostar and begins to flatten into a disk. A significant portion of the left over dust and gas spirals into the protostar adding to its mass. This is known as accretion. There is still remaining material, however, that has enough angular momentum to stay in orbit around the protostar. The disk continues to flatten and accretion stops. The disk's thickness becomes very small compared to its radius, astronomers call this type of disk geometrically thin.
The disk is very dense. The grains are subject to many forces and collide with each other often. Some grains begin to stick together. Soon large asteroids form and start to attract other pebbles gravitationally. These bodies quickly grow to the size of small planets. The terrestrial planets in our own solar system are large accumulations of these bodies. Other planets also form further from the central star. It is cooler in this region of the disk which allows some of the larger rocky cores to accrete gas. These cores will become huge gaseous planets like the gas giants found in our solar system.
The picture of the newly forming planetary system heavily resembles that of our own. Unfortunately, it is the only one we can observe in detail, so our view of planetary formation is heavily biased. There is still a lot we don't know. For instance, are planetary systems like ours common? Do they have to have structures similar to our own, or is our solar system unusual? Might there be life on some of these planets?
So the question now is how do we study circumstellar disks and the planets that might be forming within them. In an ideal world, we could simply point our optical telescopes at a star with a circumstellar disk or planetary system and study its geometry. All we would have to do is take a picture and look for the planets. Unfortunately it is not that simple. The light of the star overpowers any energy emission a planet could produce. Furthermore, determining physical characteristics from an optical image of a circumstellar disk has many limitations.
Just take a look at the image of IRAS 04302+2247 at the top of this page. What you are looking at is a nebula that is cut in half by the dust of a circumstellar disk. The nebula itself is illuminated by the star but the dust of the disk, which is almost edge on to our field of view, absorbs all of the optical light from the star hiding it from our view. It is for this reason that infrared radiation is so useful for studying these objects. Although, the disk is completely dark in the optical range, it glows brightly in the infrared.
So, how might one extract more information from this disk? As was stated before, the disk itself emits heavily in the infrared. We can therefore use this to our advantage and develop ways of looking at the data which might allow us to determine some of its physical characteristics. We might also be able to determine where planets are forming, not by detecting the planets themselves but by using "tracers". By "tracer" we mean finding more easily detectable characteristics that planets produce, and using them to diagnose possible planetary structures.
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