PSR B1257+12 C

Phobetor is a super-Earth, an exoplanet that has a radius and mass larger than that of Earth. It has an equilibrium temperature of 567 K (294 °C; 561 °F).^[8] It has a mass of 3.9 M_E and a likely radius of 1.5 R_🜨, based on its mass.

The planet orbits a pulsar named Lich. The stellar remnant has a mass of 1.4 M_☉ and a radius of around 0.000015 R_☉ (10 kilometres). It has a surface temperature of ≤28,856 K and is one billion years old, with a small margin of error. In comparison, the Sun is about 4.6 billion years old^[9] and has a surface temperature of 5,778 K.^[10]

The star's apparent magnitude, or how bright it appears from Earth's perspective, is 12.2. Therefore, it is too dim to be seen with the naked eye.

Phobetor orbits its host star about every 98 days at a distance of 0.46 AU (close to the orbital distance of Mercury from the Sun, which is 0.38 AU).

When Phobetor and its neighbors were discovered, scientists were puzzled on how the planets 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.

Several theories have been proposed for how the planets around Lich 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.^[11]

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 does not explain how the pulsar would reach a spin rate of 6 milliseconds, so the model is still being questioned.^[11]

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 type of pulsar that PSR B1257+12 is. Thus, it is unlikely that this was how the planet formed.^[11]

The most widely accepted model for the planets around Lich 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.^[11]

Phobetor, the common name for the planet, is the name of a character in Ovid's Metamorphoses. In Latin, phobetor literally means "frightener".

On its discovery, the planet was designated PSR 1257+12 C and later PSR B1257+12 C. It was discovered before the convention that extrasolar planets receive designations consisting of the star's name followed by lowercase Roman letters starting from "b" was established.^[12] However, it is listed under the latter convention on astronomical databases such as SIMBAD and the Extrasolar Planets Encyclopedia. Hence the alternative designation PSR B1257+12 d.

The convention that arose for designating pulsars was that of using the letters PSR (Pulsating Source of Radio) followed by the pulsar's right ascension and degrees of declination. The modern convention prefixes the older numbers with a B meaning the coordinates are for the 1950.0 epoch. All new pulsars have a J indicating 2000.0 coordinates and also have declination including minutes. Pulsars that were discovered before 1993 tend to retain their B names rather than use their J names, but all pulsars have a J name that provides more precise coordinates of its location in the sky.^[13]

• List of exoplanets discovered before 2000 - SPR B1257+12 d

• Wolszczan, A.; Frail, D. (1992). "A planetary system around the millisecond pulsar PSR1257 + 12". Nature. 355 (6356): 145–147. Bibcode:1992Natur.355..145W. doi:10.1038/355145a0. S2CID 4260368.

• Pulsar Planets