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The James Webb Space Telescope, after decades of development, has finally launched. It stands to reveal great knowledge about the dawn of the universe, and great insights into extrasolar planets.

With that thought in mind, I started thinking on what would be needed to not only detect life on an extrasolar planet, but detect intelligent life, that is, some kind of civilization.

I've excluded radio for the purposes of this question, as it is the obvious one - I'm wondering more of if we can see enough about a planet to determine if there is some kind of civilization living on it. With non-intelligent life, we can do things like spectroscopy for detecting elements in an atmosphere. Finding certain molecules in said atmospheres can lead us to hypothesize the existence of life, as we are currently investigating on Venus.

But I'm looking further - to maybe James Webb, but more likely even past LUVOIR. All I can think is being able to see some kind of unnatural light on a planet's surface, and that would probably be difficult for detecting any preindustrial society. For a civilization that uses some form of communication beyond our own however, imaging a planet to find an intelligent species might be the only way.

So, in short, how could we do this? I'm looking for specific types of evidence we could gather, that sort of thing.

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Gravitational lensing presents a potentially interesting way to make a telescope with a truly vast pre-fabricated optical element in the form of the Sun's gravity well, which gives rise to interesting project ideas like this: Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravitational Lens Mission.

Here's an artist's impression of how an image from such a telescope might look:

Artist's impression of the image returned by a gravitational lensing optical telescope, showing 25km scale surface features.

This would obviously be enough to see things like weather patterns, vegetation patterns and quite plausibly night-side lighting of areas inhabited by a suitably advanced species. Plenty of scope for all sorts of clever science that could be used to spot industrial civilisations.

The tricky bit is that you need to put your observatory at the appropriate lensing point, 546 AU away. And once in place, it can't realistically move very far, so if you wanted to observe a bunch of different star systems, you'd need one observatory per system.

Whilst the idea isn't without its potential problems, it certainly has amazing potential.

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    $\begingroup$ AU, not lightyears $\endgroup$
    – ths
    Jan 10, 2022 at 12:01
  • $\begingroup$ @ths blurp, don't know how I managed that one. $\endgroup$ Jan 10, 2022 at 13:16
  • $\begingroup$ For perspective on the technical challenges involved, the Voyager Space Probes, the furthest-flown and some of the fastest-moving man-made objects, have so far managed a little over 140 AU in the course of 44 years. They are considered to be in interstellar space now. $\endgroup$
    – Ruadhan
    Jan 10, 2022 at 15:30
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There are only 2 ways, I think, that life can readily be detected by scientific means, at such remote exoplanetary distances.

  1. By chemical signatures of life-byproducts. Detecting elements, molecules, reactions, or complex chemicals which indicate life exists. Problem - can't distinguish intelligent from non intelligent, can't perhaps detect life in unfamiliar forms (with unfamiliar processes).
  2. By findings that are unlikely to result other than from intelligent life. EM emissions that we find to be non random data or technologically mediated (polarised lasing?), elements and isotopes that can't exist other than by technological processes, accidentally having earth in the way of some beaming output (neutrino byproducts, gamma rays, etc), planets orbiting in ways and locations that don't semi to have a natural explanation, time based events such as lightening and darkening not caused by nature...
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Powerful outer space nuclear tests might give a specific fingerprint: a gamma/x ray flash followed shortly after by a bright emission of visible light, very close to the planet so to not be confused with another astronomical phenomena.

This would require having different telescopes (high energy and visible light, at least) synchronized and looking at the same field when the test is observable.

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