PSR 1257+12 is a millisecond pulsar about a thousand light years from the sun in the direction of Virgo. It is one of nine isolated millisecond pulsars known in the disk of the galaxy.
In 1991, precise measurements of the timing of PSR 1257+12's pulses indicated that the pulsar's motion was being affected by the presence of at least two Earth mass planets. Announced four years before the discovery of a planet around 51 Pegasi, this was the first serious claim of an extrasolar planet. As the planets had been discovered orbiting a pulsar rather than a sun-like star, the discovery took astronomers by surprise.
In the months and years that followed, other pulsar planets were announced, but all were retracted as their signals were found to be noise or due to the orbit of the Earth. The planets of PSR 1257+12, however, could not be explained away so easily. It seemed that they did, in fact, exist.
In 1994, further observations indicated the existence of a third inner planet with a mass about that of the moon. Then, in 1996, a fourth, weaker, signal was found, indicating a Saturn mass planet further out.
In 1997, the inner planet claim was questioned. Its orbital period of about 25 days closely matched the rotational period of the sun. It was reasoned that the apparent motion of the pulsar was actually due to the modulation of the solar wind in that direction. However, if this was the case, similar signals would be seen in other millisecond pulsars, but such signals have not been found. Additionally, the oscillation of PSR 1257+12 does not change with the radio frequency it is observed in as would be expected if the oscillation was caused by solar plasma. So, it appears, the inner planet really exists as well.
Interestingly, the claim of a fourth planet was withdrawn in 2002. A strong signal does exist with a three year period, but only when observed at 430 MHz. A planet would cause the pulsar to oscillate at all frequencies. But, something must be orbiting the pulsar to cause the signal at 430 MHz. It was speculated that a planetoid or comet one fifth the mass of Pluto might account for the observations. Such an object would be ablated by radiation from the pulsar, forming a coma that would be detectable at 430 MHz, but would be too small for it's gravity to otherwise affect the pulsar.
In 2004 an alternative to the comet theory was proposed. Two clouds of ionized gas orbiting 4 AU from the pulsar might better fit the observations. Each cloud would have a seven year orbit, but with two clouds, the signal would appear to oscillate in half that time, three and a half years. The two clouds might be held in the L4 and L5 Lagrange points of a large asteroid and might contain smaller asteroids as well. Instead of a comet, PSR 1257+12 may host an asteroid belt.
As of this writing in early 2005, the asteroid belt hypothesis is still being examined and has not yet been officially published. The nature of the fourth signal is still up for debate.
The discovery of planets orbiting a pulsar begs the question as to how such planets could have formed in the first place. Were they present before their parent star became a pulsar? If so, how did they survive the supernova? Or did the planets somehow form after the pulsar was created? In the years since the planets of PSR 1257+12 were discovered, many theories have been put forward to explain them. But most had significant flaws.
One of the many ideas proposed was the supernova recoil theory. Before becoming a pulsar, PSR 1257+12 was a giant star orbited by, it is conjectured, a close stellar companion. When PSR 1257+12 went supernova, it received a recoil in the direction of this companion and captured a disk of material. The momentum of the recoil then carried the pulsar away from the companion. In the disk of captured material, shielded from the intense radiation of the pulsar, planets formed.
A recent study in 2000 seems to support this idea. The low probability of the pulsar being kicked into a companion star suggests that pulsar planets are rare. In 1996 another pulsar planet was found in the M4 globular cluster, but it seems likely that that planet was captured by the pulsar. Thus PSR 1257+12 is currently unique as a pulsar around which planets actually formed.
Furthermore, the likely amount of material captured from the companion to form a protoplanetary disk is consistent with the amount needed for that disk to survive ablation from the pulsar's radiation, protect the newly forming planets from ablation, and produce planets of the masses and distances found around PSR 1257+12.
It appears, therefore, that instead of being the exposed cores of giant planets that survived a supernova, the three terrestrial planets of PSR 1257+12 are just that, rocky terrestrial planets rather like Mercury or the Moon.
View a VRML model of the PSR 1257+12 system. Please be patient while the file downloads.
For a VRML tour of our galaxy's exoplanets, check out Extrasolar VR.
Skymap created with John Walker's online Your Sky Virtual Telescope