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Say a star system in the local neighborhood (~15 light years away max) has a species that did not survive its own version of the cold war. The result is the usage of tens of thousands of nuclear weapons from plain old fission bombs all the way up to cobalt-salted multi-megaton thermonuclear devices. These weapons were deployed not only on the surface of the planet, but in the high atmosphere, low orbit, and points well beyond the planet.

Assuming a clear line of sight, would such an exchange be detectable here on Earth?

For the question assume the maximum total yield of any single weapon is no greater than the theoretical maximum yield of the Tzar Bomba (100 Mt). The Entire exchange took place over the course of one hour. Considerations for exotic weapons are just fine, but they need to be practical. A weapon that releases most of its energy in the form of neutrinos would not be an effective weapon for example.

Answers or links to resources that could help me find an answer are perfectly acceptable.

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In short: Yes, but you'd have to either be very lucky or be carrying out a continuous, 24/7 all-sky survey to catch it, and you would only see the chemical aftereffects of the war on the planet's spectroscopic signature, not the exchange itself (which would be too faint to be detectable at anywhere near Earthlike stockpiles of nukes). There was a study undertaken on the matter last year, along with several other apocalyptic scenarios, which was summarized at surface level by The Atlantic and is available here on arxiv.org. The relevant sections, emphasis mine:

Given that the world’s nuclear arsenal is equivalent to around 10^19 J of energy, the resulting radiation from its combined detonation would be much fainter than a typical GRB. If we assume that the energy is released on a similar timescale and with a similar spectrum to a GRB, a nuclear apocalypse is equivalent in bolometric flux to a GRB detonating around a trillion times closer than its typical distance. If we take a nearby GRB such as GRB 980425 (Galama et al 1998) which is thought to have detonated around 40 Mpc away, then we would expect a global nuclear detonation event to produce a similar amount of bolometric flux only 8 AU away!

Therefore, for us to be able to detect nuclear detonation outside the Solar system, the total energy of detonation must be at least nine orders of magnitude larger [than Earth global nuclear arsenal -- ed.], i.e. the ETIs responsible for the event must engage in massive weapon proliferation and concurrent usage. However, the production of fallout from terrestrial size payloads, which persists for much longer timescales, may make itself visible in studies of extrasolar planet atmospheres.

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Global nuclear war therefore potentially offers several spectral signatures that could be observed: a gamma flash, followed by UV/visible airglow and the depletion of ozone signatures. However, the aftermath of a global nuclear war will also act to obscure these spectral signatures. Ground­burst nuclear explosions generate a significant amount of dust that will be lofted into the atmosphere. Air­burst explosions do not generate dust, but still introduce particulates into the atmosphere. Atmospheric effects of nuclear warfare have been extensively modelled in climate simulations, the global consequences being known as “nuclear winter”. Recent simulations have shown that even with reduced modern nuclear arsenals severe climate effects are felt for at least ten years after a global conflict, especially due to the long lifetime of aerosols lofted into the stratosphere (Robock et al. 2007). They show that the atmospheric optical depth is increased several times for several years. The worst effects are confined to the northern hemisphere given that the model includes conflict over the US and Russia, though the entire planet is affected to a lesser extent.

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Hence, to confirm that a planet had been subject to a global nuclear catastrophe would require the observation of several independent signatures in short succession. One on its own is unlikely to be sufficient, and could easily be caused by any number of other processes on planets with potentially no biological activity whatsoever. There are cases beyond a global nuclear catastrophe that a space­faring civilisation might be able to inflict on itself, given that the destructive energy at their disposal would be far greater than nuclear weapons (Crawford and Baxter 2015), including redirecting asteroids. These would be far more destructive than nuclear warfare but would generate observable signatures different than those of a naturally occurring impact event.

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  • $\begingroup$ It looks like the actual exchange itself would not be visible. Nine orders of magnitude greater than the total global nuclear arsenal means extraordinarily good luck or sensitive equipment to detect the detonation of weapons at all. Far less likely for any single detonation given that all that energy would not be released at once. I figured the aftereffects could be detected, provided the planet had a transit that allowed the spectrographic analysis of the atmosphere. $\endgroup$ – Joe Kissling Nov 2 '16 at 13:43

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