habitability of neutron star systems
{{Short description|Possible factors for life on planets or moons around neutron stars}}
File:Artist's_concept_of_PSR_B1257+12_system.jpg.]]
{{Life in the Universe}}
The habitability of neutron star systems is the potential of planets and moons orbiting a neutron star to provide suitable habitats for life. Of the roughly 3000 neutron stars known, only a handful have sub-stellar companions. The most famous of these are the low-mass planets around the millisecond pulsar PSR B1257+12.
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. Pulsars 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 pulsar emits and how much of it can actually reach a hypothetical planet's surface; of the known pulsar planets, only those of PSR B1257+12 are close to the habitable zone and as of 2015, no known pulsar planet is likely to be habitable.
A habitable planet orbiting a neutron star must be between one and 10 times the mass of the Earth. If the planet were lighter, its atmosphere would be lost. Its atmosphere must also be thick enough to convert the intense X-ray radiation from the neutron star into heat on its surface allowing it to have a temperature suitable for life.{{Cite web |url=https://www.avaruus.fi/uutiset/astrobiologia/tutkimus-spekuloi-pulsaria-ymparoivien-planeettojen-elinkelpoisuudella.html |title=Tutkimus spekuloi pulsaria ympäröivien planeettojen elinkelpoisuudella |website=Tähdet ja avaruus; in translation "Stars and space" |last=Suominen |first=Mikko |archive-url=https://archive.today/20180328214430/https://www.avaruus.fi/uutiset/astrobiologia/tutkimus-spekuloi-pulsaria-ymparoivien-planeettojen-elinkelpoisuudella.html |archive-date=2018-03-28 |access-date=2022-10-07 |publisher=Tähtitieteellinen yhdistys Ursa (Ursa Astronomical Association) |publication-place=Helsinki |language=fi |trans-title=The study speculates on the viability of the planets surrounding the pulsar |issn=0355-9467 |quote=According to a Dutch research team, a planet orbiting a neutron star may be surprisingly suitable for life. In other words, liquid water could exist on the planet's surface. The original text: Hollantilaisen tutkimusryhmän mukaan neutronitähteä kiertävä planeetta saattaa hämmästyttävää kyllä sopia elämälle. Toisin sanoen planeetan pinnalla voisi esiintyä nestemäistä vettä.}}
A magnetic field strong enough — the magnetosphere — would protect the planet from the strong solar winds. This could preserve the planet's atmosphere for several billion years. Such a planet could have liquid water on its surface.
A Dutch research team published an article on the subject in the journal Astronomy & Astrophysics in December 2017.{{Cite journal |title=Neutron star planets: Atmospheric processes and irradiation |journal=Astronomy & Astrophysics |url=https://www.aanda.org/articles/aa/abs/2017/12/aa31102-17/aa31102-17.html |last1=Patruno |first1=Alessandro |issue=A147 |volume=608 |last2=Kama |first2=Mihkel |publisher=EDP Sciences. |doi=10.1051/0004-6361/201731102 |year=2017 |pages=A147 |issn=1432-0746 |quote=p. 9: It is thus possible to speculate that the wind could completely miss the planets orbiting around the pulsar because of a geometric misalignment of the orbital plane with respect to the ion current sheet or negative charge flow. |archive-url=https://web.archive.org/web/20180727034734/https://www.aanda.org/articles/aa/pdf/2017/12/aa31102-17.pdf |archive-date=2018-07-27|doi-access=free |arxiv=1705.07688 |bibcode=2017A&A...608A.147P }}{{Cite web |url=https://www.astron.nl/habitable-planets-around-pulsars-theoretically-possible/ |title=Habitable planets around pulsars theoretically possible |date=2017-12-19 |access-date=2022-10-07 |website=astron.nl |publisher=Netherlands Institute for Radio Astronomy (ASTRON) |quote=In the future, the astronomers would love to observe the pulsar in more detail and compare it with other pulsars. The ALMA telescope of the European Southern Observatory would be able to show dust discs around neutron stars. |archive-date=2022-10-07 |archive-url=https://web.archive.org/web/20221007151914/https://www.astron.nl/habitable-planets-around-pulsars-theoretically-possible/ |url-status=live }}