Jump to content

How large a space telescope do we need to see exo-civilizations?


Exoscientist
 Share

Recommended Posts

 A team of scientists is investigating ways of detecting exo-civilizations aside from just radio signals as with SETI:

JUNE 19, 2020
Does intelligent life exist on other planets? Technosignatures may hold new clues.
by University of Rochester
https://phys.org/news/2020-06-intelligent-life-planets-technosignatures-clues.html

Two methods of detection mentioned in the article are detection from reflected light from solar panels or detection or pollution such as CFCs.

However, even on our planet the number of solar panels would not be such that they would add appreciably to the Earth light. And CFCs presence might be short lived as it has been on our planet, having been banned.

 Could we instead detect the light on the night side coming from all the artificial lighting that would be used in a civilization? Some of the photos seen from space of the cities alit at night on Earth have been quite striking:

Aug. 14, 2014
Space Station Sharper Images of Earth at Night Crowdsourced For Science

iss038e038300_1.jpg

https://www.nasa.gov/mission_pages/station/research/news/crowdsourcing_night_images

 

 How big would a space scope need to be able to see this in the Alpha Centauri system, for example?

 

  Robert Clark

 

Edited by Exoscientist
Link to comment
Share on other sites

It would need to be large enough to resolve the planet's disk. The answer depends greatly on distance and size of the exoplanet, but it's typically in the category of "pretty darn large". I haven't done the math, but we're probably looking at something much larger than even the biggest terrestrial telescopes currently in use, so 10m+.

If you can resolve the disk, all you need to do is to look for any unusually bright spots on the night side. 

Link to comment
Share on other sites

25 minutes ago, Dragon01 said:

It would need to be large enough to resolve the planet's disk. The answer depends greatly on distance and size of the exoplanet, but it's typically in the category of "pretty darn large". I haven't done the math, but we're probably looking at something much larger than even the biggest terrestrial telescopes currently in use, so 10m+.

If you can resolve the disk, all you need to do is to look for any unusually bright spots on the night side. 

 To be able to directly image an exoplanet the space telescope would not need to be especially large; the James Webb Space Telescope could do it. What would be key though is a starshade to block the stars light. See the discussion here:

https://en.wikipedia.org/wiki/New_Worlds_Mission

 The animation illustrating a proposed starshade is pretty cool on that page. The starshade also would not need to be particularly large, perhaps 10 meters across.

 But the issue with the nightside city lights is that it likely would be much less than the full daytime reflected light of the planet, unless, the civilization is something like Asimov's Trantor, a single city covering the entire surface of the planet.

  Bob Clark

Edited by Exoscientist
Link to comment
Share on other sites

Their artificial light probably matches the spectrum of their sun.

So, we should detect the light flow of this wavelength radiated from the night side of their planet.

So, we should measure the daily cyclic change of luminosity of their planet and check if it never falls below the estimated night level.

Edited by kerbiloid
Link to comment
Share on other sites

3 minutes ago, Exoscientist said:

 To be able to directly image an exoplanet the space telescope would not need to be especially large; the James Webb Space Telescope could do it. What would be key though is a starshade to block the stars light. See the discussion here:

That's not what I meant. Direct imaging, in this context, merely means the ability to resolve the exoplanet as a point light source. Directly imaging the disk is much harder. In fact, I just done the math (plugging the numbers into the Rayleigh criterion), and it's not encouraging. To resolve the disk of an Earth-sized planet from 1pc (about the distance to Alpha Centauri) would require a visible light telescope with an aperture diameter of 1.6km! Talk about big, and most promising exoplanets are further away. 

The sunshade, while a cool idea, is mostly to block out the star so that it doesn't obscure the light from exoplanets, BTW. It's proposed exactly because JWST is barely even capable of separating the star and the planet as two point sources, nevermind imaging the disk of either.

Link to comment
Share on other sites

24 minutes ago, Dragon01 said:

That's not what I meant. Direct imaging, in this context, merely means the ability to resolve the exoplanet as a point light source. Directly imaging the disk is much harder. In fact, I just done the math (plugging the numbers into the Rayleigh criterion), and it's not encouraging. To resolve the disk of an Earth-sized planet from 1pc (about the distance to Alpha Centauri) would require a visible light telescope with an aperture diameter of 1.6km! Talk about big, and most promising exoplanets are further away. 

The sunshade, while a cool idea, is mostly to block out the star so that it doesn't obscure the light from exoplanets, BTW. It's proposed exactly because JWST is barely even capable of separating the star and the planet as two point sources, nevermind imaging the disk of either.

 Thanks for that. Perhaps we could just observe the variation in light between the day and night side as a point source. Say, for reasonable surface materials for a rocky planet, the night side brightness is too high and would be best explained by artificial illumination.

   Bob Clark

Link to comment
Share on other sites

Well, it would be more within our range, but it's hard to say what it would take to measure that. It would require very precise brightness values, including when the planet is close to the star, and you won't be able to catch only the nightside, as then the star would be directly behind. The sunshade might help with that method, but even then, it'd still be hard.

Another method would be to use an orbital telescope array. An aperture of 1.6km could possibly be achieved with such an arrangement, but this would be a major project, to say the least. Astronomical interferometry is mostly good for stars, since the amount of light gathered is be limited, but for this particular application, it could work, provided there were enough telescopes to register the light. 

Link to comment
Share on other sites

I think that if the planet were covered in an appreciable amount of city lights which made the night side less dark, in a solar transit we would simply undershoot the radius of the planet, thinking it had less area to block the normal solar light. If it was one of the few planets we're capable of directly imaging, we'd probably overshoot the size of the planet or associate that tiny extra brightness with microscopically brighter rocks or clouds. I don't think city lights in other star systems can be detected reliably.

Link to comment
Share on other sites

Right now they can't. The only way city lights could be detected without a multi-km telescope is by measuring variations in brightness. Since planets changing radius is an unlikely event, unexplained patterns of brightness changes could be a clue.

Of course, that'd require a measurement precise enough so that the variations are outside the normal error bars, which is a tall order. Even then, we'd have to exclude all other possibilities, and there are plenty. The only way to say for sure is to see the disk.

Link to comment
Share on other sites

2 hours ago, cubinator said:

I think that if the planet were covered in an appreciable amount of city lights which made the night side less dark, in a solar transit we would simply undershoot the radius of the planet, thinking it had less area to block the normal solar light. If it was one of the few planets we're capable of directly imaging, we'd probably overshoot the size of the planet or associate that tiny extra brightness with microscopically brighter rocks or clouds. I don't think city lights in other star systems can be detected reliably.

Problem is that city light don't produce much energy, or rather how far out could you detect this on earth using say Hubble? 
Now how about an small moon or two like mars have, they reflect light and are pretty hard to spot unless you see an dish. 
I say radio noise and perhaps chemicals in the atmosphere would be an better marker.

Now the first thing you want is to detect life who is easier, unless planet has descent with oxygen its not relevant as its very unlikely it has advanced life.  
Then you watch it closely. Now its time to build the big one or send something who use sun as an gravity lens. 

Link to comment
Share on other sites

17 hours ago, magnemoe said:

Problem is that city light don't produce much energy, or rather how far out could you detect this on earth using say Hubble? 
... 

 

 Yes, that is a good question. Here's a NASA page showing the full disk at night of some regions on Earth. The lighting is pretty spotty.

Earthatnight.jpg

https://earth.app.goo.gl/rDX9W5

 

  Bob Clark

 

Link to comment
Share on other sites

  • 9 months later...

 

 These articles discuss the issue of detecting nighttime light signatures in exoplanets:

Proxima Centauri b: Artificial Illumination as a Technosignature

by PAUL GILSTER on MAY 21, 2021

https://www.centauri-dreams.org/2021/05/21/proxima-centauri-b-artificial-illumination-as-a-technosignature/

 

Lights of the Nightside City

by PAUL GILSTER on MAY 24, 2021

https://www.centauri-dreams.org/2021/05/24/lights-of-the-nightside-city/

 They link also to research articles on the topic. A proposed upcoming mission LUVOIR may be able to detect nighttime lighting  at perhaps 10 times higher density than on Earth on an exoplanet of Próxima Centauri:

Large Ultraviolet Optical Infrared Surveyor.

Comparison_between_mirrors_of_HST,_JWST_

https://en.m.wikipedia.org/wiki/Large_Ultraviolet_Optical_Infrared_Surveyor

  Robert Clark

Edited by Exoscientist
Link to comment
Share on other sites

On 7/4/2021 at 7:15 PM, JoeSchmuckatelli said:

Large Optical Ultraviolet Infrared Surveyor (LOUIS)? 

Used for Location of ANthropogenic Emissions.

Link to comment
Share on other sites

  • 1 year later...

 

Alien hunters should look for city lights from 'urbanized planets,' study suggests.

By Leonard David published about 21 hours ago

Lights from alien cities are an intriguing potential technosignature.

https://www.leonarddavid.com/search-for-extraterrestrial-technology-city-lights-from-urbanized-planets/

 

 Bob Clark

Link to comment
Share on other sites

Yep, and iron airships between moon and planet. Could be a sign of advanced entertainment, and highly speculative of a civilization that has passed the crest of its development and is now on the downhill side, destroying one another. The "window of communication" (S. Lem) is closing and maybe a message sent now arrives over a pile of smouldering rubble.

 

Seriously, here's the paper (open access):

https://www.sciencedirect.com/science/article/pii/S0094576522002594?via%3Dihub

There's nothing concrete in there but a lot of conditionals and suggestions of what could may be might be worth to risk an eye.

For now, I think we can put our observation power to better use.

Edited by Pixophir
Link to comment
Share on other sites

This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...