The discovery of the first extra-Solar system planet was made by Wolszczan and Frail (1992). Precise measurements of the radio pulses from the 6.2ms pulsar PSR1257+12 showed systematic variations in the arrival times of the pulses. The regular and predictable variations in the pulse arrival times inferred to those authors that the neutron star is orbited by two planet-sized bodies of masses of at least 2.8 and 3.4 times the mass of the Earth. The initial data suggested also the presence of at least one additional planet whose existence has been confirmed by Wolszczan (1994).
Because neutron stars are formed through a supernova explosion, it is not obvious how these planets came to be in orbit around the pulsar. If they formed before the supernova event, how did they manage to survive the explosion? Could they have been formed out of debris from the shattered star? Could they have been captured from another star?
Check out Alex Wolszczan's page on planets around pulsars
Suggested readings:
"A Planet Orbiting the Neutron Star PSR1829-10", Bailes, M., Lyne, A.G. and Shemar, S.L. 1991, Nature 352, 311. Note: This paper has been retracted; the observations initally were not properly corrected for the motion of the earth around the sun.
"The Origin of the Planet Orbiting PSR1829-10", Podsiadlowski, Ph., Pringle, J.E. and Rees, M.J. 1991, Nature 352, 783.
"A Planetary System Around the Millisecond Pulsar PSR1257+12", Wolszczan, A. and Frail, D.A. 1992, Nature 355, 145.
"An Observational Test for the Existence of a Planetary System Orbiting PSR1257+12", Rasio, F.A., Nicholson, P.D., Shapiro, S.L. and Teukolsky, S.A. 1992, Nature 355, 325.
"Confirmation of Earth-Mass Planets Orbiting the Millisecond Pulsar PSR B1257+12" Wolszczan, A., 1994, Science, 264, 22 Apr 1994
"On the detection of mutual perturbations as proof of planets around PSR1257+12" Peale, S.J., 1994, Astrophys. and Space Sci., 212,77
"Lost and Found: Pulsar Planets", Bruning, D., 1992, Astronomy June, 36.
"Pulsar's Planets Confirmed", in News Notes, 1994, Sky and Telescope May, 10.
You might also wish to take a look at a reference on the timing of the binary pulsar PSR1913+16 such as: "The Binary Pulsar: Gravity Waves Exist", Will, C. 1987, Mercury Nov-Dec, 162.
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