Pulsar planet

Pulsar planets are planets that are orbiting pulsars. The first such planets to be discovered were around a millisecond pulsar in 1992 and were the first extrasolar planets to be confirmed as discovered. Pulsars are extremely precise clocks and even small planets can create detectable variations in pulsar traits; the smallest-known exoplanet is a pulsar planet.

They are extremely rare, with only half a dozen listed by the NASA Exoplanet Archive. Only special processes can give rise to planet-sized companions around pulsars, and many are thought to be exotic bodies, such as planets made of diamond, that were formed through the partial destruction of a companion star. The intense radiation and winds consisting of electron-positron pairs would tend to strip atmospheres away from such planets, thus making them unlikely abodes for life.

The formation of planets requires the existence of a protoplanetary disk, most theories also require a "dead zone" within it where there is no turbulence. There, planetesimals can form and accumulate without falling into the star. Compared to young stars, pulsars have a much higher luminosity and thus the formation of a dead zone is hindered by the ionization of the disk by the pulsar's radiation, which allows the magnetorotational instability to trigger turbulence and thus destroy the dead zone. Thus, a disk needs to have a large mass if it is to give rise to planets.

There are several processes that could give rise to planetary systems:

The formation scenarios have consequences for the planets' composition: A planet formed from supernova debris is likely rich in metals and radioactive isotopes and may contain large quantities of water; one formed through the break-up of a white dwarf would be carbon rich and consist of large amounts of diamond; an actual white dwarf fragment would be extremely dense. As of 2022^[update], the most common type of planet around a pulsar is a "diamond planet", a very low-mass white dwarf. Other objects around pulsars could include asteroids, comets and planetoids. More speculative scenarios are planets consisting of strange matter, which could occur much more close to the pulsars than ordinary matter planets, potentially emitting gravitational waves.

Planets can interact with the magnetic field of a pulsar to produce so-called "Alfvén wings," these are wing-shaped electrical currents around the planet which inject energy into the planet and could produce detectable radio emissions.

Pulsars are extremely precise clocks and pulsar timing is highly regular. It is thus possible to detect very small objects around pulsars, down to the size of large asteroids, from changes in the timing of the pulsar hosting them. The timing needs to be corrected for the effects of the motions of Earth and the Solar System, errors in the position estimates of the pulsar and of the travel times of the radiation across the interstellar medium. Pulsars spin and slow down over time in highly regular fashion; planets alter this pattern through their gravitational attraction on the pulsar, causing a Doppler shift in the pulses. The technique could in theory be also used to detect exomoons around pulsar planets. There are limitations to pulsar planet visibility, however; pulsar glitches and changes in the pulsation mode can mimick the existence of planets.

The first extrasolar planets to be discovered (in 1992 by Dale Frail and Aleksander Wolszczan) were the pulsar planets around PSR B1257+12. The discovery demonstrated that exoplanets can be detected from Earth, and led to the expectation that extrasolar planets might not be uncommon. As of 2016^[update] the least massive known extrasolar planet (PSR B1257+12 A, only 0.02 M_🜨) is a pulsar planet.