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PSR 1257+12 (also catalogued as PSR B1257+12,PSR 1300+1240 and PSR J1300+1240) is a pulsar located 2630 light years from Earth. It is believed to be orbited by at least four planets. These were the first extrasolar planets ever discovered.

The pulsar
As seen from Earth, the pulsar is located in the sky at RA 13 00 01.00, DE +12 40.00 declination, as seen from Earth in the constellation of Virgo, using J2000 coordinates, hence the alternate names PSR 1300+1240 or PSR J1300+1240 (using B1950 coordinates, it would be located at RA 12 57 00, DE +12 00 declination, hence the alternate name PSR B1257+12)

PSR 1257+12 was discovered by the Polish astronomer Aleksander Wolszczan in 1990 using the Arecibo radio telescope. It is a millisecond pulsar, a kind of neutron star, and was found to have anomalies in the pulsation period, which led to investigations as to the cause of the less than regular pulses. It has a mass of 0.29874213836478 M⨀ and a rotational period of 0.00621853193177 seconds, and is thought to be 800 million years old (as a neutron star). It is more characteristic of regular pulsars than other millisecond pulsars, and its state is thought to be caused by core coalescence rather than mass accretion from a companion star (as there is no companion star).*

The planets
In 1992, Wolszczan and Frail discovered that the pulsar has three planets and a possible comet. These were the first extrasolar planets ever discovered; as pulsar planets, they surprised many astronomers who expected to find planets only around main sequence stars. Additional uncertainty surrounded the system, because a claim of an earlier pulsar planet around PSR 1829-10 that had to be retracted due to errors in calculations.

The planets are believed to either be the rocky cores of former gas giants, or the result of a second round of planetary system formation resulting from unusual supernova remnants. If they are the remains of planets orbiting the star before supernova, they were theoretically gas giants with large rocky cores, whose atmospheres were stripped away by the supernova, and which spiralled inward to their current orbits. Any planets in the orbits they currently occupy that were there before the supernova would have been destroyed.

PSR 1257+12 A
Draugr, the innermost planet, has a mass of 0.020 ± 0.002 ME, with an average orbital distance of 0.19 AU, and an orbital period of 25.262 ± 0.003 days, on an orbital eccentricity of 0.0.

PSR 1257+12 b
Poltergeist, the second planet, has a mass of 4.3 ± 0.2 ME, with an average orbital distance of 0.36 AU, and an orbital period of 66.5419 ± 0.0001 days, on an orbital eccentricity of 0.0186 ± 0.0002.

PSR 1257+12 c
Phobetor, the third planet, has a mass of 3.9 ± 0.2 ME, with an average orbital distance of 0.46 AU, and an orbital period of 98.2114 ± 0.0002 days, on an orbital eccentricity of 0.0252 ± 0.0002.

PSR 1257+12 D
PSR 1257+12 D, the non-existent comet would have an upper mass limit of 0.2 MPluto and an upper size of R < 1000km.