Mass distribution for extrasolar planets. [Geoffrey Marcy, University of California at Berkeley]
This small sample has already raised a number of questions. For example, 5Mjup planets are found closer to their stars than Mercury is to the Sun. There are also very few very massive planets, even though these would have been very easy to detect. Also compared to our Solar System, a number of very eccentric orbits have been found.
There is also a way to look for systems that might be forming planets. We saw earlier in this chapter that we think that planetary systems form from the disks that are a by-product of the star formation process. In Chapter 15, we saw how the presence of these disks could be inferred from the existence of bipolar outflows. That is, the disks collimate the outflow. Remember, for the bipolar flows, the presence of the disks has only been inferred in most cases, though there are a few examples where we see objects that could be the collimating disks. Even those disks are hard to see because they are small and subtend small angles. The disks that will form planets around a solar mass star are even smaller. For example, a 1000 AU disk at the 500 pc distance of the Orion Nebula, the nearest extensive star forming region, would subtend an angle of 2 arc sec. Of course, this is large compared to the size of Jupiter, so it would be much easier to see than even a giant planet.
These disks are best seen in the infrared for a number of reasons. They are cooler than the protostars they surround, so they give off relatively more radiation in the infrared than in the visible. Also, since they are often deep inside molecular clouds, with tens of magnitudes of visual extinction, they are hard to see in the visual. Of course, since they are small, we must use infrared observations with very good angular resolution. HST has provided the opportunity to carry out these observations, and a few samples are shown in Fig. 15.26.
These disks are important because they allow us to study the stage between the collapse of a molecular cloud to form a star and the formation of a planetary system. Of course, in order to study these disks in detail, we would like to be able to do high resolution spectral line observations, so we can trace the velocity structure of the disks. This is something that will be most easily done on the Atacama Large Millimeter Array (ALMA, Fig. 4.32) when it is finished.
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