<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>Are their plans to do something like this?</p>

CC BY-SA 4.0
2022-06-06T13:33:34.957Z
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.
antlersoft
CC BY-SA 4.0
2022-06-06T14:19:36.767Z
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?
ProfRob
CC BY-SA 4.0
2022-06-06T14:23:43.080Z
@ProfRob yes. I think the transit method will be one of the first methods that will be feasible for any advancing civilisation.
Hartmut Braun
CC BY-SA 4.0
2022-06-06T18:12:03.103Z
I'm not convinced that advertising our presence is a good idea...
PM 2Ring
CC BY-SA 4.0
2022-06-06T18:54:47.800Z
@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.
Hartmut Braun
CC BY-SA 4.0
2022-06-07T21:01:34.913Z
@antlersoft hmm... An Earth-sized planet with a 10 km atmosphere presents 40 million square kilometers. If observing that against the Sun is the threshold, then a 1 square km set of louvers or filters is going to make a much much weaker signal, unless they record for a long time and the louvers maintain a fixed frequency, so I guess I agree with you. A big powerful radio is a good way. I'm not sure why a big powerful modulated laser isn't also though.
uhoh
CC BY-SA 4.0
2022-06-07T21:18:14.527Z
@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.
Hartmut Braun
CC BY-SA 4.0
2022-06-07T21:23:10.647Z
@HartmutBraun hmm... again :-) The light transmitted through the planet's atmosphere received far away traverses a much longer optical path than when we look up. It's why during a total lunar eclipse the Moon can still have some deep red illumination. That's of course dust and Raleigh scattering not spectral lines, I know, but in the infrared (you did say JWST technology) absorption bands are very very strong - they are after all what trap the Earth's warmth at night. https://commons.wikimedia.org/wiki/File:Solar_spectrum_en.svg
uhoh
CC BY-SA 4.0
2022-06-07T21:31:13.377Z
@uhoh I see, some frequencies are blocked completely, i.e. the atmosphere is optically thick. After all, it was a question *easy* to answer. If you make it an official answer I can give you the credit.
Hartmut Braun
CC BY-SA 4.0
2022-06-07T21:42:11.570Z
@HartmutBraun I'm not really familiar enough with the exo- science to answer, but I think you'll get some good answers in the next few days.
uhoh
<earth><exoplanet>
Could we make ourselves detectable for aliens with JWST-like technology?
Hartmut Braun
# Could we make ourselves detectable for aliens with JWST-like technology?

(Asked by: username_0 on 2022-06-06)

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?

**Tags:** earth, exoplanet
> **Comments:**
> **username_1**: 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.  
> **username_2**: 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?  
> **username_0**: @ProfRob yes. I think the transit method will be one of the first methods that will be feasible for any advancing civilisation.  
> **username_3**: I'm not convinced that advertising our presence is a good idea...  
> **username_0**: @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.  
> **username_4**: @antlersoft hmm... An Earth-sized planet with a 10 km atmosphere presents 40 million square kilometers. If observing that against the Sun is the threshold, then a 1 square km set of louvers or filters is going to make a much much weaker signal, unless they record for a long time and the louvers maintain a fixed frequency, so I guess I agree with you. A big powerful radio is a good way. I'm not sure why a big powerful modulated laser isn't also though.  
> **username_0**: @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.  
> **username_4**: @HartmutBraun hmm... again :-) The light transmitted through the planet's atmosphere received far away traverses a much longer optical path than when we look up. It's why during a total lunar eclipse the Moon can still have some deep red illumination. That's of course dust and Raleigh scattering not spectral lines, I know, but in the infrared (you did say JWST technology) absorption bands are very very strong - they are after all what trap the Earth's warmth at night. https://commons.wikimedia.org/wiki/File:Solar_spectrum_en.svg  
> **username_0**: @uhoh I see, some frequencies are blocked completely, i.e. the atmosphere is optically thick. After all, it was a question *easy* to answer. If you make it an official answer I can give you the credit.  
> **username_4**: @HartmutBraun I'm not really familiar enough with the exo- science to answer, but I think you'll get some good answers in the next few days.  

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## 0 answers

