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The Aargh are a humanoid alien race a couple solar systems away. Their home planet is quite earth-like (oxygen atmosphere, a lot of water, traditional organic chemistry even if a few proteins are different to earth), and in the habitable zone of their solar system. They have colonized several planets and moons in their own system, and have started to branch out to uninhabited solar systems.

They haven't found another sentient species yet, however, nor discovered traces of one. This, they want to change. They deliberately send out scientific discovery missions to solar systems with at least one planet in the habitable zone. Since they are very careful and do not want to provoke a hostile first contact (especially when they don't know the technological level of their potential adversary) they stop several times along the way to check if they can't find a hint of sentient life on the other planet.

One of those planets is earth, in the year of 2020 (with current technology levels).

Question: How far away do the Aargh have to be to get unmistakable proof of sentient / intelligent life on earth?

Can they distinguish that from their home planet some 10 lightyears away by running a spectral analysis on our atmosphere and discover that there are X gases that should not be that prevalent in a non-technological but habitable planet? If so - which gases or changes in gas concentration?

Or do they need to be close enough to receive the radio / electromagnetic emissions that our technology radiates into space? If so, how far away is that if they want to make sure that it is deliberate and not some sort of strange natural radiation?

Or can they detect the light from earth's night side from further away?

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    $\begingroup$ Having spent my morning fighting with the office printer, I can say that it is straightfoward to find examples of hostile intelligence on Earth, but I've no idea where you might find sentience as such. $\endgroup$ – Starfish Prime Jan 2 at 11:08
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    $\begingroup$ Related sister site question How big is Earth's sphere of broadcast influence? $\endgroup$ – James Jenkins Jan 2 at 19:14
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    $\begingroup$ Go to Washington, D.C. Stand outside 1600 Pennsylvania Avenue. Observe the activity. No sign of intelligent life. OK - CALL IN THE CONSTRUCTOR SHIPS!!!!! Hyperspace bypasses must be built, and if these creatures can't be bothered to check at their local office..? I've got no sympathy, really I haven't... $\endgroup$ – Bob Jarvis - Reinstate Monica Jan 2 at 23:53
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    $\begingroup$ @StarfishPrime Alternatively there's the Monty Python "Galaxy song". youtube.com/watch?v=buqtdpuZxvk $\endgroup$ – Graham Jan 3 at 12:58
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    $\begingroup$ "have started to branch out to uninhabited solar systems" - please note that being able to do this requires technology and resources so many orders of magnitude higher than what we do and what we know, if you compare the Moon landings to a caveman coming to the idea that he can cut a tree down and use it to swim across a lake, the technological difference is smaller than between the Moon landings and interstellar colonization. $\endgroup$ – vsz Jan 3 at 15:06
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Not really an answer but a fun fact: There is a thin layer of high-power transmissions in the longwave and various other AM and FM bands, around 100 LY away and a few dozen LY 'thick'.

In the early twentieth century most long range communication was done through massive transmittters and equally massive antennas. Directional broadcasting was still high-tech so many of these transmitters just blasted full power in all directions.

For example between 1934 and 1939 one american AM radio station broadcast at a stunning 500 kilowatt and some miltary transmitters could pump out a solid megawatt of radio power.

But as soon as high-capacity undersea cables and satellite links became available the popularity of these transmitters dropped off a cliff and from somewhere in the mid-1960s earth is a lot 'quieter'.

So if your aliens stop to check in this shell they'll probably receive 1930's radio loud and clear* but only a few years later on the same source... goes quiet.

Omnious John Williams score swells.

* this is assuming that a civilisation who have fairly casual FTL also have a very senistive radio receiver.

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    $\begingroup$ Wow, basically, we stopped to send detectable signals toward the space at the beginning of the nuclear age... If an alien civilization can detect that we can build nukes, then ceases to receive any more broadcasts, they would probably make 2+2 and think we self blasted from existence! :D $\endgroup$ – McTroopers Jan 2 at 20:45
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    $\begingroup$ And how does one detect a puny (and omnidirectional!) 500 kW longwave radio signal at 1 lightyear distance, not to say 10 light-years? We still have powerful long-wave radio transmitters all over Europe: can they be easily received in America? (And no, the curvature of the Earth doesn't count: long-wave radio signals go around the Earth as if it wasn't there; that's why they are long-wave.) Short-wave radio (which is reflected by the ionosphere) was introduced for a very good reason... $\endgroup$ – AlexP Jan 2 at 21:09
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    $\begingroup$ A large radio telescope like the Arecebo telescope could definitely detect such a signal. $\endgroup$ – Skyler Jan 2 at 21:49
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    $\begingroup$ There's an episode of PBS Space Time about this. Basically, our early radio transmissions would not be detectable by any race we can reasonably imagine because at even at a distance of few light years, the temporal and spatial resolution needed to detect them would require a radio telescope of some ridiculously improbable size, like solar system scale. $\endgroup$ – user560822 Jan 2 at 21:55
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    $\begingroup$ Even though our civil broadcasts have decreased in power over time, there are still some intentionally "loud" transmitters that are just as powerful as the most wasteful radio antennas of a century ago: Radar. Notably, the Duga radar (nicknamed the Russian Woodpecker) in the late 1970's and '80's, and several other Over-The-Horizon radars dispersed around the world. $\endgroup$ – Ghedipunk Jan 2 at 23:49
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There's a few ways this can be done, but all of them require the aliens to have very high-precision high-accuracy equipment:

  • Light: You were on the right track with E&M emissions. We've been radiating 'noticeable' amounts of it for less than a hundred years. That being said, with our own equipment if we noticed such light on a foreign body we would most likely assume it to be an error with the equipment that we just have to deal with (or just the effect of our sun's solar radiation).
  • Our probes: Voyager 1 and 2 may in some point in the future exit our solar system completely. If this alien race somehow spots these tiny machines and don't mistake them for comets, they may have confirmation that something is going on in our solar system. This would be a very conclusive piece of evidence, but definitely the hardest to identify.
  • Monitoring gas levels: Again, you were on the right track with this. We can estimate the gas content of planets not only in our solar system but in others as well. Perhaps they can do the same. Just knowing the gas content isn't enough though. Thanks to human-caused climate change, one can monitor gas levels in our atmosphere over time. If they notice how rapidly CO2 levels are increasing versus other gases, they may deduce that some life-form is doing some heavy pollution on our planet. They would have to do this over 50 years or so at least for them to deduce anything concrete though. That is assuming that polluting one's planet is a trademark of intelligent species in every planet.

Although the human race has come very far very quickly, our solar system as a whole isn't a 'busy' place because of us. We have no megastructures that modify entire planets significantly, we aren't testing massive weapons in gas giants - arguably, we aren't doing anything massive outside of our tiny planet. So, if all three of the points I listed are confirmed, that still might not be enough for your aliens to deduce that there's any intelligent life on Earth - at best, they may become a little suspicious.

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    $\begingroup$ Something I'd like to add: We have artificial satellites in orbit. That's sure to produce some effect when the Aargh try to measure the light given off by our planet. Perhaps the glint of metal would be a far more concrete piece of evidence to them than the presence of GHGs and other atmospheric pollutants? $\endgroup$ – KaiGuyMBK Jan 2 at 13:50
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    $\begingroup$ If you're looking for life on another planet, one that is unusually bright on the night side is a potentially interesting indicator. However, there might also be natural causes, and aliens don't necessarily light up their worlds the way humans do... from a distance, it would be an interesting possible indicator, but not a certainty. I'd still look for it, though; that was actually my first thought as well. $\endgroup$ – Matthew Jan 2 at 18:25
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    $\begingroup$ It's interesting that we look at CO2 being a sign of pollution when we release it, but not when you go back in time 5-10 million years. Previous levels were not correlated with what I assume OP meant by "sentience", and certainly not with technology. $\endgroup$ – Mad Physicist Jan 2 at 19:23
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    $\begingroup$ CO2's no good. You can also get sudden rises from a flood-basalt or supervolcano eruption, or from an asteroid impact triggering planet-wide forest fires. I'd look for something more exotic, such as fluorcarbons. $\endgroup$ – Mark Jan 3 at 0:52
  • $\begingroup$ @Mark but we have made fluorocarbons less prevalent in our atmosphere in the last 10 years alone. That would make it less detectable within just a decade of observation. $\endgroup$ – cyber101 Jan 3 at 11:02
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A civilization doing exoplanet analysis using a level of technology similar to our own would be able to detect CFC's in our atmosphere. CFC's do not occur naturally, so their presence would be a strong confirmation of industrial activity.

We have detected exoplanets up to 2500 light years away, so that would be the current practical distance limit.

Detecting industrial pollution in the atmospheres of earth-like exoplanets

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    $\begingroup$ Ahh, but wouldn't that limit be severely reduced since CFCs have not been present in our atmosphere for very long? If they looked at us from several million light years away, would they not see the state of the planet in the distant past? $\endgroup$ – KaiGuyMBK Jan 2 at 23:37
  • $\begingroup$ @Kai, the question says 10 light years away, not millions. $\endgroup$ – prl Jan 3 at 4:00
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    $\begingroup$ How do we know CFCs don't occur naturally on alien planets? (either from geological processes, or non-sentient life) $\endgroup$ – user253751 Jan 3 at 13:20
  • $\begingroup$ @user253751 - They do not occur naturally on Earth. I have to concede that I do not know enough about chemistry to know if the same would be true of all hypothetical alien environments. So I guess this solution applies only to planets with atmospheres like Earth's. $\endgroup$ – tbrookside Jan 3 at 16:11
  • $\begingroup$ Probably this is messed up by the requirement that it is unmistakable. I wonder if that is what the asker really meant. A planet with a bunch of CFC's might not be unmistakably hosting sentients, but what odds do you want to give it? 10% at least, right? Worth a flyby if life is as rare as it seems to be. $\endgroup$ – Zwuwdz Jan 3 at 21:24
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We believe that in the next 50 years or so, we'll be able to resolve continents on extrasolar planets through a variety of new telescope technology (the bulk of which is just putting those telescopes into space, and stitching together imagery from multiple ones that sit at great distances to each other).

If this were used on us, they would definitely see artificial lighting at night, and in patterns that suggest widespread technology. It would not be a slam dunk, mind you, but I'd be convinced... those forest fires are in the same place, night after night, for months on end? They don't cause discoloration during daytime? Hard to imagine what else it might be.

So they don't have to leave their solar system necessarily.

If they are looking directly at our planet, they might detect various broadcast radio signals. There are also at least a few dozen nuclear detonations they could spot (if they're looking for them), but these last at most a second or two before the signal's gone, and would have to be on the nearside of our planet.

If they fail to do all these things, then they have to enter our solar system itself, probably to within the orbit of Jupiter, and will be able to both visibly detect surface indications or find the remaining radio transmissions that we still broadcast.

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    $\begingroup$ I'm suddenly interested in this, do you have some documentation about these future telescop ? this looks fucking rad if you excuse my language $\endgroup$ – GlorfSf Jan 3 at 8:48
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    $\begingroup$ @GlorfSf check aperture synthesis for starters, from there you can follow for specifics: en.m.wikipedia.org/wiki/Aperture_synthesis $\endgroup$ – Gnudiff Jan 3 at 15:54
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    $\begingroup$ @GlorfSf: Also do a search for CHARA if you are interested in the current state-of-the-art for terrestrial optical array telescopes. $\endgroup$ – Eric Lippert Jan 3 at 23:45
  • $\begingroup$ thanks a lot to both of you $\endgroup$ – GlorfSf Jan 6 at 8:00
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They can easily see our cities from the orbit of Mars. They might be able todetect our radio noise from the orbit of Pluto. Basically, once their scout ship is in our Solar System they will notice something's strange with Earth. From 10 ly away, not so much and not so easily.

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  • $\begingroup$ Voyager 2 can detect our radio noise from well beyond the orbit of Pluto, with a puny 3.7m dish. Someone who's actively looking would be using an antenna a fair bit bigger. $\endgroup$ – Mark Jan 3 at 0:55
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    $\begingroup$ @Mark: Voyager 2 can detect signals intended for it, radiated in a very narrow directional beam. Of course, if the aliens are lucky and find themselves straight in the path of, for example, a powerful military radar signal, then they can detect its artificial nature from quite a way away. $\endgroup$ – AlexP Jan 3 at 2:15
  • $\begingroup$ Anti-ICBM radars like RAF Fylingdales and the Russian Woodpecker use(d) phased arrays to scan in both horizontal and vertical planes. They would likely be detectable at interstellar distances, although the Woodpecker used shortwave to work over the horizon, so it would depend a lot on ionospheric conditions. $\endgroup$ – Paul Johnson Jan 3 at 14:48
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I'm going to have to challenge this question a bit, or at least most of the other answers.

When people ask these kinds of things, we are automatically assuming the alien technology and cultures are paralleling our own, in that they see in what we call the visible spectrum, they use what we call radio frequencies, that they form cities, they live above ground and not in the water or in tunnels, breathe oxygen, aren't deathly allergic to nitrogen, are also deathly "allergic" to high levels of radiation, and so much more.

Granted, there will probably be a high level of this parallelism, but it's not necessarily 100% true.

Even if they are looking for some of these things, will they be able to understand them? All of the radio and TV broadcasts are encoded in a way that we understand (or at least our engineers). That doesn't mean an alien race will be able to figure out that these are radio and TV broadcasts, and that they have some meaning to it. If they know to look for this type of radiation, all they'll likely see if a bunch of noise. They won't even be able to likely pinpoint a source more closely than a continent, until they get relatively close. They might be able to determine the general location, since the signal appears and disappears according to the rotation of the earth, but that's about it. What that point is, I don't know, since I'm not a radio frequency buff.

Also, our definitions of sentient life might be significantly different. They might be a hive mind and decide that anything different isn't sentient. They might decide that anything living on the land surface can't be sentient, by their definition. Maybe a dolphin is sentient and anything that enslaves them and makes them jump through hoops for entertainment can't be sentient. Or anything that breathes mere oxygen can't sentient due to not having the right metabolism to have the brainpower to be sentient. There's probably thousands of differing reasons why an alien species wouldn't consider us sentient. I mean, what sentient being would destroy their own environment with trash and chemicals when they can't even vacate the world before it destroys them back?

Then there's technology itself. Maybe they don't use it as we do. Maybe because we use technology, instead of pure brainpower to reshape reality around ourselves, we aren't sentient. Maybe they've harnessed protons for their power and communication, and our electron emissions are not even noticed? Maybe they communicate only in light/color or pheromones, instead of aurally.

As humans that are fairly open about how we communicate and use technology, we like to think that every other species would work like this. Due to them being aliens, as in "not from this world", they are by definition not going to be us and highly likely they won't think like us, regardless what Star Trek, Star Wars, or even FarScape show us. They are far more likely to be something like Predator or Aliens, even if they aren't hostile. We don't/can't communicate with them, only assume some basic thought patterns from their behaviors and decor. They may be so alien that what they consider to be a handshake or the most polite greeting is considered to be horrifying and disgusting to us.

What I'm trying to say is that we can't rely on technological nuances for a species to recognize us as sentient. They may realize technology of our's exists and therefore realize we exist because of it, but not that we are sentient because we have technology.

An alien culture might only recognize individuals who are kind and helping others to be sentient, while those who are greedy and self-serving or violent to not be sentient. Maybe they consider your pet to be sentient, since it's loving and kind. (Ok, so cats are probably not going to count here. :-) )

Again, what I'm trying to say here is that we might get judged to be sentient based on how we act, instead of what we know. Is a gorilla sentient because it uses a stick to break open a coconut to get to the milk and flesh inside, or is it sentient because it learned sign language, or is it sentient because it prevented a predator from killing a defenseless baby? What are the requirements for humans to believe another species is sentient? How homo-centric is that?

I realize I'm probably going way beyond that the OP is asking, since they are asking more as a story or plot point, but it might be interesting to explore some of the questions I've asked, rather than rehashing the same old plots of "We've detected life on this planet 1 parsec that way, and it has a colony, so they must be humanoid sentient creatures like us." Maybe what I've asked is exactly what the OP wants or what the OP already has questions about. Maybe I'm just showing how OCD I am. If the latter is true, sorry about that. Truly.

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  • $\begingroup$ First couple of sentences of the question show they are fairly similar to us: "The Aargh are a humanoid alien race [...] Their home planet is quite earth-like". (emphasis mine) $\endgroup$ – Artemis still doesn't trust SE Jan 3 at 23:29
  • $\begingroup$ @Artemisstilldoesn'ttrustSE, that still doesn't mean they have a parallel evolution, nor does it mean they have the exact same atmosphere. It doesn't take much difference in evolution early on to make a completely different being. It also doesn't take much of a change in atmosphere to make it unbearable to live in. Simply having a high pollen day makes life for some human completely miserable, not to mention that still gives leeway for underground and under water aliens. $\endgroup$ – computercarguy Jan 4 at 0:47
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If they are monitoring X and Gamma radiation they will probably (depending on the orientation of the Earth at the relevant times) notice some pulses from atmospheric nuclear explosions. Hopefully they will also notice that these stopped after about 1990.

You might also want to read "Contact" by Carl Sagan, which explores this question in some detail. He identifies the first sufficiently powerful TV broadcast to be the 1936 Olympics in Nazi Germany. Audio and Morse transmissions were being made back in the 1910s, but whether your aliens could pick those up will depend on the power used, the sensitivity of the alien radio detectors, and the details of noise and propagation in the interstellar medium; its quite possible that the Nazi Olympic broadcast was the first one sufficiently powerful to be detectable over interstellar distances.

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Something around 50 light years away
We spread bobble around us of radiowaves transmissions. In theory we've stared using radios in late XIX century, I don't think that those were strong enough to be heard from space, another thing when we stared using it to connect to vessels on orbit.

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