Pulsar planets are planets that are found orbiting pulsars, or rapidly rotating neutron stars. 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 electrons-positrons would tend to strip atmospheres away from such planets, thus making them unlikely abodes for life.
Formation
It is thought that the formation of planets requires the existence of a protoplanetary disk with a "dead zone" where there is no turbulence. There, planetesimals can form and accumulate without falling into the star. Compared to young stars, neutron stars have a much higher luminosity and thus the formation of a dead zone is hindered by the ionization of the disk by the neutron star'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 neutron star: 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. Planet within about 4 astronomical units distance from the star risk being engulfed and destroyed when it becomes a red giant/red supergiant. During the supernova, the system loses about half of its mass and unless the neutron star 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 neutron star 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 neutron stars.
- "Third generation planets": A companion star is destroyed through the interaction with a neutron star, forming a low-mass disk. Neutron stars 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 neutron star penetrates the envelope of a larger star, causing it to break up and form a disk around the neutron star. 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 neutron star 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 neutron star.
- A companion star may be destroyed during the interaction with a neutron star 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 neutron star, or that a neutron star 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 neutron star planet is a "diamond planet", a very low mass white dwarf. Other objects around neutron stars could include asteroids, comets and planetoids. More speculative scenarios are planets consisting of strange matter, which could occur much more close to the neutron stars than ordinary matter planets, potentially emitting gravitational waves.
Planets can interact with the magnetic field of a neutron star 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 known extrasolar planets to be discovered (in 1992) 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 (only 0.02 MEarth) is a pulsar planet.
However, the size and particular spectroscopic traits makes actually visualizing such planets very difficult. Pulsars are very small and thus the probability of a planet transiting in front of the pulsar - one potential way to image them - is very low. Spectroscopic analyses of planets are rendered difficult by the complicated spectra of pulsars. Interactions between a planetary magnetic field and the pulsar and the thermal emissions of planets are more likely avenues of getting information on the planets.
Occurrence
As of 2022 only half a dozen pulsar planets are known, implying an occurrence rate of no more than one planetary system per 200 neutron stars. Most of the planet formation scenarios require that the precursor be a binary star with one star much more massive than the other, and that the system survives the supernova that generated the pulsar. Both these conditions are rarely met and thus the formation of pulsar planets is a rare process. Additionally, planets and their orbits would have to survive the energetic radiation emitted by pulsars, including X-rays, gamma rays and energetic particles ("pulsar wind"). This would be particularly important for millisecond pulsars that were spun up by accretion, while they formed X-ray binaries; the radiation emitted under these circumstances would evaporate any planet. Pulsars remain visible for only a few million years, less than the time it takes for a planet to form, thus limiting the chance of observing one.
Based on the known occurrence rate of pulsar planets, there might be as many as 10 millions of them in the Milky Way. All known pulsar planets are found around millisecond pulsars, these are old pulsars that were spun up through the accrual of mass from a companion. As of 2015 there are no known planets around young pulsars; they are less regular than millisecond pulsars and thus detecting planets is more difficult.
Examples
Among the better known pulsar planets are:
- The pulsar PSR B1257+12, 1300 lightyears away in the constellation Virgo, was confirmed to have planets in 1992 based on observations made with the Arecibo Observatory. The system consists of one tiny planet with a mass of 0.02±0.002 Earth masses and two Super-Earths with masses 4.3±0.2 and 3.9±0.2 times that of Earth, assuming that the pulsar has a mass of 1.4 solar masses. They most likely formed from a protoplanetary disk, probably generated from the destruction of a companion star. Computer simulations have shown that the system should be stable for at least one billion years and that exomoons could survive in the system. The system resembles the inner Solar System; the planets orbit the pulsar at distances comparable to that of Mercury to the Sun and may have comparable surface temperatures. Reports of additional bodies in this system might be due to solar disturbances.
- A cthonian planet with a mass comparable to Jupiter but less than 40% of its radius orbits the pulsar PSR J1719-1438. This planet is probably the remnant of a companion star that was evaporated by the pulsar's radiation and has been described as a "diamond planet".
- A circumbinary planet with a mass of 2.5±1 Jupiter masses orbits around PSR B1620-26, a binary star consisting of a pulsar and a white dwarf in the globular cluster M4. This planet may have been captured into the pulsar's orbit, a process which is particularly likely within the packed environment of a globular cluster, and may be about 12.6 billion years old, making it the oldest known planet. Its existence may demonstrate that planets can form in low-metallicity globular clusters.
- PSR J2322-2650 seems to have a roughly Jupiter-mass companion. The radiation from the pulsar could be heating it to temperatures of about 2,300 K (2,030 °C); an light source observed close to the pulsar may be the planet. This pulsar is considerably less luminous than many, which may explain why the planet has survived to this day.
| Companion (in order from star) |
Mass | Semimajor axis (AU) |
Orbital period (days) |
Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| PSR B0329+54b | 1.97±0.19 M🜨 | 10.26±0.07 | 10140±11 | 0.236±0.011 | — | — |
| PSR B1257+12b | 0.02±0.002 M🜨 | 0.19 | 25.262±0.003 | 0 | — | — |
| PSR B1257+12c | 4.3±0.2 M🜨 | 0.36 | 66.5419±0.0001 | 0.0186±0.0002 | — | — |
| PSR B1257+12d | 3.9±0.2 M🜨 | 0.46 | 98.2114±0.0002 | — | — | — |
| PSR B1620-26b | 2.5±1.0 MJ | 23 | — | — | — | — |
| PSR J1719-1438b | >1.2 MJ | — | 0.090706293±0.000000002 | <0.06 | — | — |
| PSR J2322-2650b | 0.7949±0.0002 MJ | — | 0.322963997±0.000000006 | <0.0017 | — | — |
Debris disks and precursors
Timing variations of the pulsars PSR J1937+21 and PSR J0738-4042 may reflect the existence of an asteroid belt around the pulsars, and collisions between asteroids/comets and neutron stars have been proposed as an explanation for the phenomenon of fast radio bursts, the gamma ray burst GRB 101225A and other types of pulsar variability. There are no known debris disks around pulsars, although the magnetars 4U 0142+61 and 1E 2259+586 have been suggested to harbour them.
The white dwarf-pulsar binary PSR J0348+0432 may be a system that could develop pulsar planets in the future. The existence of a dust cloud at the pulsar Geminga that may be a precursor to planets has been proposed.
Discredited candidates
There were earlier reports of pulsar planets which were either retracted or considered unconvincing, such as the 1991 "discovery" of a planet around PSR B1829-10 which turned out to be an artifact caused by the motion of the Earth. The existence of planets around the pulsar PSR B0329+54 has been debated since 1979 and is still unresolved as of 2017. PSR B1828-11 has been conclusively established to display magnetospheric activity that mimicks planets, without having any, and a planet candidate around the pulsar Geminga was later attributed to timing noise.
Habitability
Pulsars emit a very different radiation than regular stars, with very little optical or infrared radiation but plenty of ionizing radiation and electron-positron pairs that are generated by the pulsar's magnetic field as it spins. Additionally, remnant heat from before the neutron star's birth, heating of the pulsar's poles from its own radiation and from mass accretion processes drives the emission of thermal radiation and neutrinos. The electron-positron pairs and X-rays are absorbed by planetary atmospheres and heat them, driving intense atmospheric escape that can strip them away. The presence of a planetary magnetic field could mitigate the impact of the electron-positron pairs.
Habitability is conventionally defined by the equilibrium temperature of a planet, which is a function of the amount of incoming radiation; a planet is defined "habitable" if liquid water can exist on its surface although even planets with little external energy can harbour underground life. Neutron stars do not emit large quantities of radiation given their small size; the habitable zone can easily end up lying so close to the star that tidal effects destroy the planets. Additionally, it is often unclear how much radiation a given neutron star emits and how much of it can actually reach a hypothetical planet's surface; of the known pulsar planets, only these of PSR B1257+12 are close to the habitable zone and as of 2015, no known pulsar planet is likely to be habitable. Additional heat sources may be radioactive isotopes such as potassium-40 formed during the supernova that gave rise to the pulsar and tidal heating for planets with close orbits. Radiation from outside sources such as companion stars would also add to the energy budget.
