Effects of a close approach with a non-pulsar neutron star?

Question

My hard-ish SF story requires an event that would effectively render the entire solar system completely uninhabitable for some time (i.e. retreating underground for a few millennia won't help), but without destroying it.

Currently, my main candidate is a fast moving non-pulsar neutron star passing through the system (any other suggestions are welcome). From a story point of view this should make it harder to detect and therefore give us limited time to prepare, ideally a century at most - I know an encounter like this is astronomically unlikely, but as far as I'm aware there's nothing in physics that outright prevents it from happening, i.e. it's hugely improbable but not impossible.

From what I've read elsewhere, such a high mass object racing through the system should drag a swarm of Oort Cloud objects with it resulting in a lot of planetary impacts. However, what I'd like to know is if there would be other destructive effects besides the gravitational in a situation like this. For example:

• If a gas or ice giant and its moons fell into the neutron star, would this release enough radiation to be bad news for the rest of the system or would it just be a pretty light show?
• Given the magnetic fields of neutron stars, would such an encounter have any appreciable effect on solar activity, or would this be purely gravitational?
• Would radio communications be compromised or be unaffected?

I know I've not provided any numbers such as the speed, distance or mass of the neutron star, but that's deliberate as I'm trying to tailor the outcome of this event to the needs of the story while remaining as scientifically accurate as possible (hugely improbable stellar encounter notwithstanding).

Answer 1

Ok, so with your Neutron Star you've got two primary threats it brings in. The first is the gravitational effects that you've laid out, but the second is radiation. Neutron stars produce a LOT of it, and it's often very focused by the Neutron star's own magnetic field.

The most viable scenario I can think of would that it IS a Pulsar that's emitting its 'pulses' just slightly offset to its direction of travel relative to our solar system. This would make it very difficult to detect until it got close enough that the earth's orbit started intersecting with the pulses. Initially the pulse would only be hitting one spot on Earth's orbit and we'd only be getting the fringe of it but the closer it gets, the more frequently we're getting whacked by the beam, and the more intense the radiation is.

This gets you your hundred years or so of "oh crap. let's get out of here" as the increasing X-ray and Gamma radiation does bad things to our atmosphere. Over the course of a century or two you'd face rapid degradation of the earth's atmosphere, sterilizing radiation, and a constant bombardment of impactors creating sufficient disruption to the Earth's mantle that underground survival is untenable due to earthquakes.

Here's the slick bit though. Once the Neutron star moves far enough, Earth won't be in the path of the pulsar's radiation beam anymore. Earth's magnetic field should recover fairly rapidly since it's internally generated by the rotation of the planet. The atmosphere will ALSO start to recover, primarily due to lingering intense volcanic activity caused by all those earthquakes and meteoric impacts. This would return the Earth's atmosphere to something a lot like it was a couple billion years ago.

By the time your humans return, they'll have a planet again with an atmosphere again, but probably not one they can breathe. This is a problem that could be solved, over time, by seeding the Earth with tailored bacteria much like the ones that originally formed on our planet that consumed the CO2 and ammonia in our atmosphere and created all that lovely Oxygen that we need.