A field with a future

Over the last 10 years there has been a steady increase in the number of planetary systems that we know about. Indeed, as we have heard, there are already over 100 planetary systems known. One thing is clear from what we have found so far, and this is that none of the systems found so far is similar to the Solar System. In Figure 1, I show an updated version of the plot given in Beer et al. (2004). This shows eccentricity plotted

Semi-major axis (AU)

Figure 1. A plot of eccentricity versus semi-major axis of the planet that induces the largest velocity semi-amplitude in each of the observed systems. The diamond at the bottom right represents the Solar System in the form of Jupiter.

against periastron for the largest contributor to the radial velocity detection for each of the known planetary systems. Also shown is the point for Jupiter's orbit to represent the Solar System. It is quite evident that in this plot the Solar System is an outlier. None of the planetary systems discovered so far have their major planet in the right place or in the right orbit. Compared to the Solar System, the planetary systems found so far have their planets significantly closer to the central star, and with significantly higher orbital eccentricity (except for those so close in that tidal circularization has taken place). In terms of some form of the "Anthropic Principle" (summed up by the words of the First World War song "We're here because we're here because we're here because we're here"), one could make a prima facie case that the Solar System is special in some way. But I think it is fair to say that at the moment there is no one who really believes that, when the Domesday Book of planetary systems has been completed, the Solar System will look out of place. However, in science, belief is not enough, and what this implies, therefore, is a general belief that our roll call of planetary systems is seriously incomplete.

We have heard a number of talks at this meeting about the continued search for new planetary systems. It is clear that there are strong observational biases which explain why the planetary systems discovered so far have been found at small radius, because for those systems the radial velocities are higher and the orbits shorter. Waiting for longer orbital periods to be found may just be a matter of time, but it is clear from the talks by both Marcy and Mayor that we are approaching the limitations of the radial velocity technique, not least because of intrinsic stellar properties such as pulsations. In this context, Doug Lin put in a plea that some effort be made to see what constraints can be set by the large number of null, or near null, results already obtained, and the talk by Bob Brown gave some suggestions as to how progress might be made in this area. However, in order to extend the parameter space of planetary systems, it is evident that other techniques will be required. Of these, the direct techniques involving transits will again find mainly very short-period planets, whereas the micro-lensing searches will tend

Semi-major axis (AU)

Figure 1. A plot of eccentricity versus semi-major axis of the planet that induces the largest velocity semi-amplitude in each of the observed systems. The diamond at the bottom right represents the Solar System in the form of Jupiter.

to find planets at intermediate separations of around a few AU. So it may be some time before the picture can be much extended, let alone completed.

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