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Aleksander Wolszczan and Dale Frail discovered a planetary system around the millisecond pulsar PSR1257+12 through precise radio timing. The regular pulses supplied a clock whose observed arrivals varied periodically.

Those variations trace the changing light-travel time as the pulsar moves in response to its companions. Fitting the timing signal constrains the companions' orbital periods and minimum masses. The discovery established that the search for exoplanets could extend beyond ordinary main-sequence host stars.^[ref]

Dale Frail and Aleksander Wolszczan reported planetary companions to a millisecond pulsar in 1992. Their evidence came from small periodic variations in the arrival times of its radio pulses.

An orbiting pulsar changes its Euclidean distance from the observer, changing the travel time of its radio waves. Timing residuals can therefore reveal a reflex orbit and constrain unseen companion masses. This method found a planetary system around a neutron star, a very different environment from the Solar system.^[ref]