Knowledge

If a planet about 10 light years away from Earth had an all-out nuclear war (10 years ago), would we be able to detect it?

No.

I’m not sure if this is still true or not, but a number of years ago, I heard from one astronomer who said “If there is an planetary system identical at the distance of our nearest stellar neighbor, and I mean identical in every way, same planets with same masses, and on the third planet there was a species identical to ours, emitting the exact same signatures, we couldn’t even tell that any of the planets existed.

(Note that this doesn’t quite work in practice, because Alpha Centauri is a binary star system. Also note that there are at least likely detected planets in that system, but all are either substantially closer to the sun and/or larger than earth. Most planets that we detect are either very close to their parent stars, very large, and often both. This was also well over a decade ago, and the science has evolved. Nonetheless, the fact remains that that our ability to detect and understand exoplanets is very, very limited. )

Even a major nuclear exchange isn’t, in the scheme of galactic energy, much of anything at all.

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