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

[Exoscientist]

 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

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 September 28, 2020 by Exoscientist

[Guest]

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. 

[Exoscientist]

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 September 27, 2020 by Exoscientist

[kerbiloid]

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 September 27, 2020 by kerbiloid

[Guest]

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.

[Exoscientist]

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

[Guest]

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. 

[cubinator]

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.

[Guest]

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.

[magnemoe]

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. 

[Exoscientist]

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.

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

 

  Bob Clark

 

[Exoscientist]

 

 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.

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

  Robert Clark

Edited July 4, 2021 by Exoscientist

[JoeSchmuckatelli]

Why is Large Ultraviolet Optical Infrared Surveyor (LUVOIR) 

 

Not

 

Large Optical Ultraviolet Infrared Surveyor (LOUIS)? 

 

Weak Naming by the space agency. 

[kerbiloid]

Large UltraViolet (LUV) Machine.

[DDE]

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

Large Optical Ultraviolet Infrared Surveyor (LOUIS)? 

Used for Location of ANthropogenic Emissions.

[Exoscientist]

 

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

[kerbiloid]

Strong lines of neon in spectrum.

[Pixophir]

They're probably in their 1960's right now, drivng around in piles of chrome, listening to Beatles or Elvis on distorted AM frequencies ...

Edited July 27, 2022 by Pixophir

[kerbiloid]

In their turn, they should detect pink in the spectrum of Earth.

[Pixophir]

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 July 27, 2022 by Pixophir

[kerbiloid]

I also still insist on the Orion bursts dot line X-ray radiation.

[Exoscientist]

 Interesting article here about researchers who had for a moment thought they might have detected evidence of  intelligent extraterrestrial signals:

https://astronomy.com/news/2021/12/technosignature-from-proxima-centauri--and-why-astronomers-rejected-it

 However, it is notable in their search that they excluded radio frequencies commonly used for communications on Earth so as not to accidentally detect those signals. But if those frequencies are commonly selected for use on Earth they also may be commonly selected on extraterrestrial systems.

 Then it is quite notable there are multiple plans worldwide for megaconstellations containing tens of thousands of satellites each. The question: how radio loud would the Earth be then when these tens of thousands of satellites are in place? How far away would the Earth be detectable then in radio frequencies from other systems? Would the Earth then have a peculiar radio spectrum that would be unlikely to be produced naturally?

 Because of the intereference from the Earth communications we couldn’t reliably detect such signals from other systems using the radio observatories on Earth. But how about the lunar far side? Astronomers want to maintain the radio silence on the lunar far side for scientific research in the radio spectrum:

Astronomers call for radio silence on the far side of the moon.
News
By Leonard David( space.com-leonard-david ) published March 21, 2024
https://www.space.com/the-moon-far-side-radio-silence

 But it may also provide an ideal means to search for radio SETI.

   Bob Clark

[magnemoe]

6 minutes ago, Exoscientist said:

 Interesting article here about researchers who had for a moment thought they might have detected evidence of  intelligent extraterrestrial signals:

https://astronomy.com/news/2021/12/technosignature-from-proxima-centauri--and-why-astronomers-rejected-it

 However, it is notable in their search that they excluded radio frequencies commonly used for communications on Earth so as not to accidentally detect those signals. But if those frequencies are commonly selected for use on Earth they also may be commonly selected on extraterrestrial systems.

 Then it is quite notable there are multiple plans worldwide for megaconstellations containing tens of thousands of satellites each. The question: how radio loud would the Earth be then when these tens of thousands of satellites are in place? How far away would the Earth be detectable then in radio frequencies from other systems? Would the Earth then have a peculiar radio spectrum that would be unlikely to be produced naturally?

 Because of the intereference from the Earth communications we couldn’t reliably detect such signals from other systems using the radio observatories on Earth. But how about the lunar far side? Astronomers want to maintain the radio silence on the lunar far side for scientific research in the radio spectrum:

Astronomers call for radio silence on the far side of the moon.
News
By Leonard David( space.com-leonard-david ) published March 21, 2024
https://www.space.com/the-moon-far-side-radio-silence

 But it may also provide an ideal means to search for radio SETI.

   Bob Clark

The satellite constellations run pretty low power and its pointed downward. We are moving away from the large TV antennas and over to low power cellphone, wifi and low earth satellites so earth will be harder to detect. The thing you could see of starlink is the up-link stations. Now these signals are broad band compressed and encrypted so hard to differentiate from noise. 

[kerbiloid]

Let's refer to the basic concepts, well-known to the conspiracy alarmists, but for unknown reason rarely widely mentioned by other educated public.

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

It's a set of resonant frequencies of the Earth planet/atmosphere electromagnetic system, working as a resonant cavity.
From 7 to 40 Hz.

This is the range of frequencies, which the planet is buzzing at.
So, by listening them, we can detect the electromagnetic activities.

As probably only Earth-like planets deserve the name of habitable, so probably their Schumann resonances will be more or less same.

***

Compare them to the human brain waves.
https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/brain-waves

https://ru.wikipedia.org/wiki/Ритмы_головного_мозга

(In Russian, the online translators fail to translate.)

https://en.wikipedia.org/wiki/Gamma_wave
https://en.wikipedia.org/wiki/Beta_wave
https://en.wikipedia.org/wiki/Alpha_wave
https://en.wikipedia.org/wiki/Delta_wave
https://en.wikipedia.org/wiki/Theta_wave
https://en.wikipedia.org/wiki/Mu_wave

As we can see, the human brain wave frequencies almost match the Schumann resonance frequencies.

(That's why some accuse the innocent HAARP and other ionospheric communication systems, including but not limited with the ultra-low frequencies for submarines, and partially the 5G networks (the low-frequency beats between a pair of close high-frequencies, say 8 Hz beats caused by 99 996 Hz and 100 004 Hz), in ability to affect the mass human behaviour and climatic/seismic activity at the selected region on the planet.
Btw, about the Starlink network and its coming analogs from other countries as well.
And no, a tin cap won't help. At least you need a dyno suit with a grounding in the tail.)

***

Thus, hypothetically it's also possible not just listen to the other planet activity, but also to affect that planet population and nature, by modulating waves in its ionosphere.

[Exoscientist]

22 hours ago, magnemoe said:

The satellite constellations run pretty low power and its pointed downward. We are moving away from the large TV antennas and over to low power cellphone, wifi and low earth satellites so earth will be harder to detect. The thing you could see of starlink is the up-link stations. Now these signals are broad band compressed and encrypted so hard to differentiate from noise. 

 
 Thanks. I didn’t think of the uplinks. Even though it might not be possible to decrypt the signals, it might possible the spectrum is so unusual to be unlikely to be natural.

   Bob Clark