PSR B1257+12 A, also known as "PSR B1257+12 b" and Draugr, is an extrasolar planet, approximately 2,300 light-years away from the Sun in the constellation of Virgo. It was one of the first planets outside of the Solar System to be discovered. It is one of the three planets orbiting PSR B1257+12.
The planet is in the innermost object orbiting the pulsar PSR B1257+12, i.e. a pulsar planet in the dead stellar system. It is around twice the size of the Moon and as of January 2020, is the least massive planet known (with the mass accurately determined), including the planets in the Solar System.
When PSR B1257+12 A and its neighbors were discovered, scientists were puzzled on how the planets were formed. Normally, planets orbiting around a massive star would evaporate when its host star exploded in a supernova, due to the intense heat (up to 1,000,000 K) and radiation. The discovery of planets around a pulsar were unexpected, considering that a pulsar could be a host to planetary companions.
Several theories have been proposed for how the planets around PSR B1257+12 formed. One theory suggested that the planets actually had existed before the host star exploded in a supernova about 1 billion years ago, however, this is inconsistent as the ejected material from a supernova would be enough to vaporize any planets close to the star. Also, multiple issues arise with this theory that debates nearly-impossible steps on how the planets ended up in their current places. Thus, the scenario has been dropped.
One scenario proposed a massive binary system in which the planets formed around, with the more massive companion exploding in a supernova. The neutron star would then orbit the secondary companion (forming an X-ray binary) until the now-red supergiant exceeded its Roche lobe and began spilling material onto the neutron star, with the transfer being so dramatic that it forms a Thorne–Żytkow object. However this doesn't explain how the pulsar would reach a spin rate of 6 milliseconds, so the model is still being questioned.
Another model stated that the planets might have formed from a fallback disk from the supernova remnant. The main problem is that the resulting pulsar would be a radio pulsar, not the kind of pulsar that PSR B1257+12 is. Thus, it is unlikely that this was how the formed.
The most widely accepted model for the planets around PSR B1257+12 is that they were a result of two white dwarfs merging. The white dwarfs would be in a binary orbit, with the orbit slowly decaying until the lighter white dwarf star filled its Roche lobe. If the mass ratio is large, the lighter companion would be disrupted, forming a disk around the more massive companion. The star would accrete this material, and would result in its mass increasing until it reaches the Chandrasekhar limit, in which it would experience core collapse and turn into a rapidly rotating neutron star, or, to be precise, a pulsar. After the explosion, the disk around the pulsar would still be massive enough (about 0.1 M☉) to form planets, which would likely be terrestrial, due to them being composed of white dwarf material such as carbon and oxygen.