The recent detections of extrasolar planets have stimulated the imagination of many astronomers and members of the public as well. The existence of places possibly similar to the earth brings up the question of our uniqueness and all the implications thereof. The results to date, however, are surprising in that all the detected planets appear to be many Jovian masses and to lie well within 1 a.u. of their parent stars. It is difficult to understand how they could have formed in such tight orbits without tidal disruptions or have been perturbed into stable orbits that close to a more massive object. It may be that such planets are rare and have been detected by observational selection but maybe the solar system is the unusual case. The method of detecting cyclic variations in radial velocity of a star to indicate orbital motion, requires a large mass for the planet and a small orbital radius in order to produce a detectable shift in the star's motion. Detection of decameter bursts similar to those from Jupiter might offer a way of detecting lower mass planets at any distance from their stars.
Five solar planets have magnetospheres although only Jupiter produces strong decameter bursts. The intensity of these bursts should depend on the magnetic moment of the planet which is likely proportional to the mass and also the rotation rate. Injection from the stellar wind is at least partly responsible for the magnetospheric particle population; this, in turn, is proportional to the inverse square of the distance from the parent star and the stellar activity. Finally, the passage of satellites through the magnetosphere affects the dumping of the particles into the ionosphere. It is clear that we need to study Jupiter thoroughly with LOFAR to find the location of its burst radiation which will help us to more thoroughly assess the emission processes and better choose other candidates for detection of similar planetary systems.
As a quick assessment of the relative possibilities for detection of bursts from extrasolar planets, we note that Jupiter has the strongest magnetic field but the other planets also have different geometrical effect: the earth is closer to the sun but does not have any satellites within its magnetosphere; Saturn does have weak hectometric bursts detected by Voyager but its magnetic axis is aligned with its rotational axis rather than tilted and it is further from the sun; Uranus and Neptune have highly tilted magnetic as well as rotational axes so both their stimulating and viewing geometries may be unfavorable.
One good planet in the solar system with decameter bursts readily detectable by LOFAR is very promising, however, particularly if we wish to begin the search for extrasolar planets like those detected to date by the radial velocity techniques. They have larger masses than Jupiter (and thus likely larger magnetic fields) and are closer to their parent stars (with thus a greater cross section for the stellar wind). A reasonable detection rate is certainly possible.
It should be noted that the sun produces noise storms of comparable brightness in the same frequency range but they are much more sporadic and do not have the precise repetition with rotation rate. Thus, it will be necessary to monitor each candidate for a few months to search for periodicities appropriate for a planetary rotation, say a few hours to several days.
With typical bursts of about 106 Jy at 20 MHz for a planet at Jupiter's distance, (they can be a factor of 10 higher) we can expect 0.4 mJy at 1 pc or 4 microJy at 10 pc. This may make detections marginal as we have only about 5 MHz bandwidth and a few minutes per burst.