The stable rotation of pulsars makes them the most accurate clocks in the Universe. The delay of the radio pulses as the pulsar moves in its orbit about its common centre of mass with its planet can be used to estimate the mass and orbital period of the planet. Two such planets with masses 2-3 times that of the Earth have been found in orbit around PSR 1257+12. Periodic time delays have been detected in at least two other pulsars, but as yet they have not been unambiguously shown to be planets. Some 700 pulsars are available for study.
Very long baseline interferometry (VLBI) at radio wavelengths provides the most accurate reference positions in all astronomy. Such astrometric measurements of radio sources on the longest Earth baselines can give precisions approaching 10 microseconds (corresponding to the diameter of a one pount coin on the Moon as seen from the Earth). One star, and possibly two others, have been shown to have a planet orbiting them. The geometry of the measured elliptical orbit of this star indicates a planetary mass equal to 50 times the mass of Jupiter.
SETI, the search for extra-terrestrial intelligence, is now a serious scientific discipline. A significant number of planets have now been detected around stars in our Galaxy. This underscores the likelihood that intelligent life may occur elsewhere in the Galaxy. Project Phoenix is using a multichannel received system with a resolution bandwidth of about 1 Hertz which is being placed on the world's largest radio telescopes. Sophisticated techniques have been developed to remove the confusing effects of terrestrial radio frequency interference in the 1-3 GHz band used for these studies. Over the next few years the 250ft Lovell Telescope will be used in Project Phoenix.