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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. Formation 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: "First generation" planets are planets that orbited the star before it went supernova and became a pulsar: Massive stars tend to lack planets, possibly due to the difficulty in detecting them around very bright stars but also because the radiation from such stars would destroy the protoplanetary disks. Planets orbiting within about 4 astronomical units of the star risk being engulfed and destroyed when it becomes a red giant or red supergiant. During the supernova, the system loses about half of its mass and unless the pulsar is ejected in the same direction as the planet was moving at the time of the supernova, the planets are likely to detach from the system. None of the known pulsar planet systems are likely to have formed in this process. "Second generation" planets from material that falls back on the pulsar after a supernova: The material could theoretically reach a mass comparable to that of a protoplanetary disk, but is likely to dissipate too fast to allow the formation of planets. There are no known examples of planets around young pulsars. "Third generation" planets: A companion star is destroyed through the interaction with a pulsar, forming a low-mass disk. Pulsars can emit energetic radiation that heats the companion star, until it overflows its Roche lobe and is eventually destroyed. Another mechanism is the emission of gravitational waves, which shrink the orbit until the companion star (in these cases often a white dwarf) breaks up. In a third mechanism, the pulsar penetrates the envelope of a larger star, causing it to break up and form a disk around the pulsar. Disks formed in these processes are much more massive than these formed through fallback and thus persist for longer times, allowing the formation of planets. They also contain heavy elements that are essential building blocks for planets, and part of the disk will be accreted by the pulsar and spins it up in the process. Alternatively, a light white dwarf is destroyed by the interaction with a more massive one; the light white dwarf gives rise to a debris disk that generates a planet while the larger white dwarf becomes a pulsar. A companion star may be destroyed during the interaction with a pulsar but leave a planet-sized remnant, such a system is known as a "black widow". Finally, it is possible that planets from companion stars or rogue planets are captured by a pulsar, or that a pulsar merged with the original host star of the planets. The latter process would form a "common envelope" which eventually breaks down to form a disk from which planets can develop. Implications 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, 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. Observability 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 the least massive known extrasolar planet (PSR B1257+12 A, only 0.02 ME) is a pulsar planet. However, the size and particular spectroscopic traits makes actually visualizing such planets very difficult. One potential way to image a planet is to detect its transit in front of the star: in case of pulsar planets, the probability of a planet transiting in front of pulsar is very low because of the small size of pulsars. Spectroscopic analyses of planets are rendered difficult by the complicated spectra of pulsars. Interactions between a planetary magnetic field, the pulsar and the thermal emissions of planets are more likely avenues of getting information on the planets. Occurrence As of 2022 only about half-dozen pulsar plane.... Discover the Jens Schulze popular books. Find the top 100 most popular Jens Schulze books.