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The James Webb Space Telescope is going to take spectra from planets during transit in front of their central star. By that, spectral changes can identify if the planet possesses an atmosphere and analyse it’s composition.

I imagine we could install technical means, like mirrors in an orbit around earth, that would make it very clear for alien civilisations that we are here, provided they are on a comparable technical level of development like us.

Design sketch: mirrors reflect the sun light, creating a flickering signal, and use colour filters to create a spectral signature that is clearly of artificial origin.

Are their plans to do something like this?

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    $\begingroup$ It's generally easier (lower energy to transmit, easier to distinguish from natural sources) to use a narrow-band radio signal to advertise our presence rather than something in the visible or near-infrared. What's nice about JWST is potential ability to detect non-technological bio-signatures in planetary atmospheres, rather than intentional signals. $\endgroup$
    – antlersoft
    Jun 6, 2022 at 13:33
  • $\begingroup$ Where are the aliens? Do you nean assuming they can see the Earth transit the Sun and that they have identified it as a 1 year period transit? $\endgroup$
    – ProfRob
    Jun 6, 2022 at 14:19
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    $\begingroup$ @ProfRob yes. I think the transit method will be one of the first methods that will be feasible for any advancing civilisation. $\endgroup$ Jun 6, 2022 at 14:23
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    $\begingroup$ @PM2Ring I’m aware of that discussion. But firstly I think that’s a different question and secondly I also doubt that an civilisation capable of doing us harm needs help in finding us. $\endgroup$ Jun 6, 2022 at 18:54
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    $\begingroup$ @uhoh I see your point. But a mirror de/reflects all light while the imprint of a thin, almost transparent, atmosphere on the star‘s light may be orders of magnitude smaller? With the same argument 10 km thickness may be an too optimistic value. In any case, it’s a question more difficult to answer than I originally thought. $\endgroup$ Jun 7, 2022 at 21:18

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