# How would a neutron star affect the habitability of planets orbiting a companion star?

A star system consisting of two stars forms, with one being more massive. As such, the larger star soon exhausts its fuel and ends up as a neutron star. The secondary star has a semi-major axis of 700 au, with the closest approach being 438 au.

Based on the information, would the secondary star be able to harbor life-supporting planets? The two issues are whether or not it would have been distant enough to survive the death of the primary star, and whether or not the resulting neutron star would produce too much radiation.

• There are different kinds of lifeforms. You can imagine all sorts of things, some people talk about silicon based life, there is the possibility of life in stars. There are lifeforms deep in earth's crust. I think you should specify what you mean by life. – Raditz_35 Jun 20 '17 at 9:22

Not habitable at first, but possibly later.

Life would definitely not survive if one star in a binary system would go supernova. Even in a loose binary with suggested (438+ AU) distance, radiation would be just too strong.

However, after the neutron star is born, things are looking better for the distant companion star and its planetary system. Because the neutron companion is fairly remote, the remaining star would not be suffering from an ablation process. Neutron stars are known for being X-ray and gamma-ray sources, but this kind of activity is caused by accretion process. If there is not much accretion material, neutron star should become very quiet after a while.

So, in a few hundred million years, "normal" star system may produce life and even become suitable for human habitation.

A neutron star always results from the collapse of massive stars (from eight solar masses upwards). Massive stars have very short lifespans in the main sequence. An estimation I've found is that the time in the main sequence for a star is about $10^{10} {M^{-2.5}}$ years (where $M$ = star mass in solar masses) which for an 8-solar-mass star means about 55 million years. At this point an Earth-like planet orbiting the other star would have probably not even cooled enough to have a solid surface or liquid water on it, let alone life. So if the stars were born together, the more massive one will be already a neutron star long before the least massive one has a mature planetary system.

• Would the force of the supernova disrupt planetary formation, or at least delay or alter it? – Dark Martin Jun 20 '17 at 2:45
• I was thinking about that. Wikipedia says "After between three and ten million years, the young Sun's solar wind would have cleared away all the gas and dust in the protoplanetary disc, blowing it into interstellar space, thus ending the growth of the planets." So there should be time enough for planets to form before the supernova goes off. But I don't know, really. – pablodf76 Jun 20 '17 at 14:28

This is surprisingly feasible. In a recent paper about alien civilizations building Dyson-rings around pulsars Zaza Osmanov found their habitable range "should be of the order of (10^−4 to 10^−1) AU with temperature interval (300–600) K for relatively slowly rotating pulsars and (10–350) AU with temperature interval (300–700) K for rapidly spinning neutron stars, respectively." Obviously, for the temperature range specified. So, thermal habitability is fine for the parameters of the secondary star and its planets even for rapidly rotating pulsars.

Provided the secondary star wasn't in the plane of the emitted radiation and, therefore, subject to the pulsar's pulsing it should be safe in terms of radiation.

Since a habitable planet needs a strong geomagnetic field to protect its biosphere from radiation, if the pulsar beam glanced off the planet's magnetic field this produce the most magnificent auroras in the galaxy.

Any life evolving on the planets of the secondary star will be adapted for higher levels of radiation than those of most other planets. Somewhat more extremophile, but definitely more radiation adapted.

Essentially its not implausible that a habitable planet could in the secondary star's planetary system. The odds against it being a safe place to live long term are high. However, if the conditions are right, then it is possible. This means the distance is great enough and its position relative to the pulsar is out of the path of its radiation beam.

Unfortunately, the birth of a neutron star would cause a terrifying radiation pulse.

From one AU, the supernova explosion would be as bright as a hydrogen bomb exploding in front of you... by nine orders of magnitude. From a distance of 700 AU, the explosion would be about six orders of magnitude less powerful than that, which still equals one thousand hydrogen bombs going off in front of you.

While it is possible (if not likely) that the planet may survive, I fear that habitability is out of the question: the planet's atmosphere is going to be shorn away from either the flash or the ensuing super-stellar winds and become part of the scattering planetary nebula.

It is more likely that the planet might form afterwards (the supernova would have happened very early in its companion's life). I seem to remember reading something of the kind in a commentary to Larry Niven's Smoke Ring, where you have a vaguely similar situation - a supernova remnant, LeVoy's Star, with at least one planet (Gold), orbiting around a yellow star.

It might be possible for a part of the supernova's ejecta to condense in an accretion disk around its companion, and finally give birth to a planet. After a couple of billion years you might then get a habitable planet around a star with a neutron companion. The neutron star's jets, if still present, would likely be directed axially (the two stars, being formed from the same original nebula, would have parallel axes of rotation) and not trouble the planet at all.

• Are you sure it's enough to vaporise or otherwise destroy the planet? That kind of assertion could use some math to back it up. (no I didn't do the math to check) – M i ech Jun 20 '17 at 0:38
• The planet can regenerate its atmosphere over millions of years from volcanic outgassing. If the Earth lost its atmosphere tomorrow, it would quickly (by geological standards) rebuild a CO2 atmosphere. – Mike Scott Jun 20 '17 at 6:58