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For the purposes of my story, I have created the following setting.

An Earth-like planet orbits a binary star system made up of a Sun-like star and a neutron star. For the majority of time (hundreds of thousands of years), it orbits the Sun-star, but occasionally, it gets "snatched up" by the neutron star. During these periods, the planet's orbit is erratic and unpredictable - sometimes it's quite far from the star and everything freezes over; at other times it gets so close, the gravitational forces and radiation wreak havoc down on the surface.

Could intelligent life have evolved during one of these "stable" periods (when orbiting the main sequence star + "erratic" orbits around the neutron star that happen to be livable), only to be later wiped out when the planet got too close to the neutron star? If so, what kind of life would have evolved in this setting? If not, what can I change about the setting to make it feasible for intelligent life to have evolved (and later, to have been wiped out by the neutron star)?

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  • $\begingroup$ Welcome to Worldbuilding.SE marmel. Please check out our tour and help center. $\endgroup$ – Cyn says make Monica whole Jul 9 at 4:10
  • $\begingroup$ Folks who have posted answers, consider: if you thought the question was worth your time answering, it is probably also worth an upvote. $\endgroup$ – Willk Jul 9 at 12:30
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For the majority of time (hundreds of thousands of years), it orbits the Sun-star, but occasionally, it gets "snatched up" by the neutron star. During these periods, the planet's orbit is erratic and unpredictable - sometimes it's quite far from the star and everything freezes over; at other times it gets so close, the gravitational forces and radiation wreak havoc down on the surface.

Well, that's your first problem. If the system is chaotic enough to occasionally let the planet switch primaries, you are not going to have nice, well-behaved habitable orbits when it switches back to the regular Sun-like star.

Could intelligent life have evolved during one of these "stable" periods (when orbiting the main sequence star + "erratic" orbits around the neutron star that happen to be livable), only to be later wiped out when the planet got too close to the neutron star?

In merely a few hundred thousand years? No.

What precisely to change depends on what your purpose for having this cataclysm is. In any case, though, I would suggest simply letting your planet always orbit one star or the other, stably. It doesn't really matter which--or, you could have the stars in a very tight binary orbit, with the planet orbiting both of them relatively far out.

The important bit is that the neutron star be close enough to the Sun-like star to slowly accumulate a hydrogen atmosphere by accretion from the solar wind. This will result in periodic novas when the hydrogen atmosphere becomes thick enough to flash-fuse, which should do nicely for sterilizing the surfaces of any planets in the system.

How frequently that occurs will be a function of the distance between the two stars. If you have them in a close binary orbit with the planet orbiting the pair, novas will occur fairly frequently. The only complex surface life that could persist on that planet would be life that can persist itself in highly radiation and temperature resistant spores, probably underwater or underground, which would resprout after the disaster (cf. the alternating Hot Life and Cold Life from Hal Clement's Cycle of Fire).

The farther apart the stars are, the more infrequent novas will be. If the planet permanently orbits the neutron star, that puts a cap on how much the stars can be separated, as the planet+neutron star system must remain in the habitable zone of the Sun-like star. I do not know if that will give you enough separation to make the disasters sufficiently separated for a "normal" civilization to arise or not. If the cyclic nature of the disasters is not actually important, just that at least one happens after the rise of civilization, then I recommend simply letting the planet orbit the Sun-like star, with the neutron star being a distant companion, which accretes new material only very slowly. This gives you the freedom to put the planet exactly where you need it to be habitable over the long term, while not having to specify the exact location of the neutron star; figuring out precisely how far out it needs to be to explode at just the right time in your planet's history can be left as an exercise for the reader.

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Infalling matter on the neutron star

Neutron stars have enormous density and gravity. Things tend to move at relativistic speeds in their vicinity.

Suppose a fairly large asteroid or dwarf planet approaches the partner neutron star obliquely and breaks apart. Because shredding matter down to its constituent atoms is no tall order for the gravity differential of a neutron star, if the infalling matter has any angular momentum at all relative to the neutron star (doesn't collide head-on), it will form an accretion disk: a fast-spinning torus of super-heated plasma.

The matter in that disk will move at relativistic speed, emitting great fractions of its rest mass in the form of high-energy radiation such as x-rays and gamma-rays as it collides with both itself and the neutron star surface.

Your intelligent beings, being near this calamity of penetrating radiation, would suffer an Extinction Level Event as the neutron star shines magnitudes brighter than the main star and irradiates their world aflame—at least for a short period (astronomically speaking) until the whole infall business subsides.

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  • $\begingroup$ Is this the same as a gamma ray burst? One reads about that phenomenon as a handy life extinctor. I think the source need not be so close as to have gravitational impact on the orbit of the planet in question. $\endgroup$ – Willk Jul 9 at 12:34
  • $\begingroup$ @Willk I'm not sure what you mean by a gravitational impact. The author of the question asked for other ways the neutron star could kill off their intelligent life. $\endgroup$ – BMF For Monica Jul 9 at 15:02
  • $\begingroup$ If it is close enough to occasionally gravitationally interact with the planet that makes orbital mechanics difficult. It can kill everything from a great distance with a gamma ray burst. $\endgroup$ – Willk Jul 9 at 15:32
  • $\begingroup$ @Willk I think there's a misunderstanding. The planet isn't the thing that falls into the neutron star. I mean, it could, but like you say that would lend a lot of instability to its orbit prior—not good for developing intelligent life in the first place. A gamma ray burst is essentially what I'm saying bakes the life off the planet, instigated by the infall of a large asteroid or dwarf planet. $\endgroup$ – BMF For Monica Jul 9 at 16:04
  • $\begingroup$ No, I understand you. It makes sense. The premise of the OP is that the neutron star is in the same system as the planet. That complicates orbital mechanics. If the neutron star is in the adjacent system the planet in question can have a normal orbit and still get killed. $\endgroup$ – Willk Jul 9 at 17:05
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I would venture to guess, not likely.

It took ~60 million years for a species to evolve after a cataclysmic event to begin the journey towards intelligence.

The asteroid that ended the Cretaceous Period was fairly traumatic event yet it was not the worse one we have experienced.

Life has been around for the better part of a billion years and "intelligent" creatures have only a million or so. So, I'm assuming your time constraints may be a bit too narrow to allow for an evolutional process to accrue to develop intelligence.

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Others have negatively answered the plausibility issue, so I will jump directly to what needs to be changed...

Why not make the neutron star a recently captured rogue. That would allow the life to have evolved over vast time while the solar system was non-binary. Then after the rogue arrived..., extinction.

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  • $\begingroup$ I would imagine that the neutron star would have to have formed elsewhere, as the event to create a neutron star would most likely kill anything within a couple light years of it $\endgroup$ – Sonvar Jul 11 at 2:50

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