Rocks around the Clock: Limits on Pulsar Planetary Systems

The apparent discovery of a planet around the neutron star PSR 1829-10 raises the question: are planets around pulsars rare or merely difficult to detect? To address that issue, a survey for low-mass companions in archival timing data of about 185 pulsars was conducted. The sensitivity of an algorithm for two timing-quiet pulsars, PSR 1919 + 21 and PSR 1937 + 21 is demonstrated. PSR 1919 + 21 is a typical long-period pulsar (P = 1.337 s) with an extensive history of timing observations. It has no earth-mass planets with orbital periods between tens and thousands of days. More stringent limits are derived for PSR 1937 + 21, which is a strong, millisecond pulsar. Planetary companions as small as 0.001-0.01 earth masses are excluded for orbital periods between 10 and 100 days. If the solar system were found around PSR 1937 + 21, it would be possible to detect all the planets except Pluto and Neptune.