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What feature of Earth would be most likely to attract the interest of a curious alien intelligence searching the Milky Way marking the Earth as an unusual world and how close would they have to be to detect it?

Assume the alien intelligence has the capabilities of today's humanity in detection technology.

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    $\begingroup$ Well, apparently Mars needs women, or so says m.imdb.com/title/tt0060672 $\endgroup$
    – Jon Custer
    Mar 9, 2023 at 2:32
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    $\begingroup$ Emissions from street lighting in the mercury and sodium spectra? They will be looking at street lighting from 20 years back, maybe. $\endgroup$ Mar 9, 2023 at 8:30
  • $\begingroup$ Are you asking what aspect of Earth would be deemed "most interesting" (opinion based), or what would be the first sign observable from space that there's something different about Earth? Please clarify. $\endgroup$
    – Drake P
    Mar 9, 2023 at 22:12
  • $\begingroup$ @Drake By most interesting I mean most unusual and most worthy or a closer look. And as a supplement how far out could that feature be detected from. I would like to create a story where the aliens detect Earth, decide to investigate because of some feature and become more interested as they get closer and more data stacks up. But what do they find out in what sequence as they approach... $\endgroup$
    – Slarty
    Mar 9, 2023 at 22:28
  • $\begingroup$ They love a good comedy? $\endgroup$
    – Mon
    Mar 10, 2023 at 6:00

12 Answers 12

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Life

I'm sure that there are planets with liquid water out there, planets with a human breathable atmosphere out there - regardless of all that - Life would likely be the most interesting thing.

Who are we? How do we act? What do we do? How do we live? How do we die?

There are questions enough about just the Human species that we, ourselves, don't know yet - and so any alien species with a shred of curiosity is going to be interested.

In terms of detection - I'm not sure the distance - but the thing that would most likely cause them to investigate would be non-natural EM spectrum transmissions. Radio waves or similar.

Something with a Mathematical basis.

We can detect EM/F emissions from pretty far away in the Galaxy, so it's possible they could too - the question is isolating our transmissions from naturally occuring noise.

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    $\begingroup$ I actually think they may detect our atmospheric composition before they find any radio signals. But yeah, that may be a good way to find life once we (or they) have the telescopes for it! Look for the tell-tale gasses. $\endgroup$
    – JamieB
    Mar 9, 2023 at 2:26
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    $\begingroup$ Most man-made radio signals are effectively undetectable beyond a distance of very few light-years; only special-purpose focused beams (such as some kinds of military radar) have a chance of being detectable from father away, and even for those farther away is not really all that far away. The signals we are detecting with our radiotelescopes are produced by enormously energetic phenomena. $\endgroup$
    – AlexP
    Mar 9, 2023 at 6:29
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    $\begingroup$ One could specify either 'complex' or 'intelligent' life. Just on Earth we had 5 billion years of life, around 800 million years of complex life and only about 100000 years of intelligent life. We have no idea how representative these numbers are but by definition complex life is a subset of life and intelligent life is a subset of complex life. $\endgroup$
    – quarague
    Mar 9, 2023 at 10:43
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    $\begingroup$ @JamieB if Lrrr, ruler of the planet Omicron Persei 8, can watch Single Female Lawyer, then OP's planet can detect The Honeymooners, Jerry Springer and Ally McBeal... $\endgroup$
    – RonJohn
    Mar 9, 2023 at 15:30
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    $\begingroup$ human breathable atmosphere is impossible without life. You need at least microbial life for there to be free oxygen in the atmosphere. $\endgroup$
    – vsz
    Mar 10, 2023 at 6:31
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Solar eclipse

The sizes of the Sun and the Moon in the sky are almost equal, making solar eplipses possible and especially spectacular. There is no particular reason why this should be the case - we just got remarkably lucky [link]. (I vaguely remember reading a sci-fi novel where that was a plot point, but I can't recall which one that was.)

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    $\begingroup$ You might be thinking of the novel Transition, by Iain M. Banks, which mentions this in the introduction. $\endgroup$
    – David
    Mar 9, 2023 at 18:45
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    $\begingroup$ If our aliens believe in God, they might even see it as a sign of intelligent design. $\endgroup$
    – workerjoe
    Mar 10, 2023 at 5:54
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    $\begingroup$ This may be interesting, curious or funny, but why would they be looking for this? $\endgroup$ Mar 10, 2023 at 12:01
  • $\begingroup$ But how would they be able to tell this at a distance? And by this I mean, on Earth the Moon appears to just barely cover up the sun during an eclipse (or not quite, if it's an annular eclipse) but even from orbit it just looks like a dark circle moving across a small part of the Earth. $\endgroup$
    – Michael
    Mar 12, 2023 at 0:30
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    $\begingroup$ They might be looking for these if they have eclipses on their world, too. Eclipses have played important roles in science: they were used to develop spectrography and discover the periodic table of elements, one was used to test Einstein's theory, and the tradition of Greek philosophy itself began when Thales of Miletus used philosophy to predict an eclipse. If the aliens, similarly, developed science/technology using eclipses, and noted how "lucky" they were, they might be looking for others with the same luck. $\endgroup$
    – workerjoe
    Mar 13, 2023 at 13:10
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Assume the alien intelligence has the capabilities of today's humanity in detection technology.

This means that they hardly can detect us. For comparison, we just started to find exoplanets roughly 30 years ago. And most of we found were gas giants. Very few exoplanets Earth-sized or smaller are known. And almost all of the know exoplanets were detected by indirect methods like transit, radial velocity, astrometry, transit time variation or microlensing.

Very few of them were photographed, and all those that were are gas giants presented no more than a small pixelated blob in an image already severely distorted due to the necessity of filtering out the obfuscating star luminosity.

Even looking at Proxima Centauri, which is the neighbor next door, the first indications of a exoplanet there came only in 2013 with the confirmation only in 2016. The second and third planets were detected only in 2019 and confirmed only in 2020 and 2022.

So, an alien intelligence would probably don't see us at all with our current level of technology. However, after gaining insight from their first observations, what they would be looking for is exactly what we are looking for, a planet that seems to be capable of life.

And the best way to look for planets that have life is to search for planets that are rocky, have the correct temperature range (i.e. Goldilocks planet, not too hot nor too cold) and hint away the presence of water. We know that planets like this can harbor life because Earth is one of those. Gas giants in a Goldilocks position are also candidates for having moons with some life. Surely there could be life out there outside the Goldilocks zone, and even Mars, Europa, Titan and Enceladus here around the sun are considered as possibly harboring microscopic primitive life. But what we knows for sure that works are the Earth-twins.

How close they would have to come? With the current technology, except if they are extremely luck to spot a Earth-transit, too close to be viable or realistic, but the technology is increasing very fast.

In fact, they are likely to detect Jupiter and Saturn first. Then, only after much more detailed studies with a lot of observations and luck, they could see Earth. But they would also likely be investigating millions of stars at once and having limited resources for studying them. So, detecting Jupiter and Saturn might be like "meh" for a few decades until the "meh" turns to be a "wow".

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  • $\begingroup$ I was thinking of things like detecting Earth has oxygen in its atmosphere or liquid water on its surface or its mass and orbit. The alien intelligence only has our level of detection equipment but they have a very high tech ship that is on its way to Earth. I'm trying to figure out what they can find out about us (and in what order) as they approach Earth from very very far out to LEO. $\endgroup$
    – Slarty
    Mar 9, 2023 at 12:22
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    $\begingroup$ @Slarty "has our level of detection equipment but they have a very high tech ship". That makes no sense... if you have the much-higher tech needed for an interstellar ship, then, as a matter of course, you'll have higher tech detection equipment. $\endgroup$
    – RonJohn
    Mar 9, 2023 at 15:21
  • $\begingroup$ Well perhaps your right. The trouble is there needs to be a baseline for the detection equipment else the ordering of things will get hopelessly mixed up by people's assumptions about what the aliens can or can't do with their technology. Lets just assume that these aliens emerged into our spacetime continuum and find our electromagnetism a bit different... and it's out of scope... $\endgroup$
    – Slarty
    Mar 9, 2023 at 17:04
  • $\begingroup$ @Slarty "detecting Earth has oxygen in its atmosphere or liquid water on its surface" - this is beyond our current level of technology - the hopes so far are in measuring the star spectrum during a transit and try to see how it changes other than just having a plain smaller star luminosity. And with that spectrum difference, try to understand the atmospheric composition of the transiting object. But this is a homeric work, has tiny probabilities of being successful and only worked so far for a handful hot-Jupiters with very crude and unreliable results. $\endgroup$ Mar 9, 2023 at 18:05
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    $\begingroup$ I think everyone's taking "current technology" too literally. I'm working on instruments for next generation exoplanet spectroscopic surveys right now for missions within the next few decades. We only need incremental improvements to current tech to look at atmospheric composition, not paradigm-shifting leaps $\endgroup$ Mar 9, 2023 at 22:38
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Free Oxygen in the atmosphere

Oxygen is a pretty reactive chemical, and free oxygen in an atmosphere would tend to get bound up with other stuff -- for example, it would bond with iron to form "rust" as is the case on Mars. In order for our atmosphere to have so much free oxygen, something has to be continually producing or freeing up new oxygen. That's a clue that there's life (specifically, plant life) on this planet.

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    $\begingroup$ Because there are high levels of oxygen in the atmosphere it marks it as unusual. Our atmosphere is far away from the equilibrium state to be expected. $\endgroup$
    – Slarty
    Mar 9, 2023 at 17:07
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    $\begingroup$ @Slarty I'm saying that workerjoe's answer is poor. As to how common advanced (i.e. agriculture and cities) life is... I'd say it's pretty darned rare. Shockingly and stupendously rare, in fact. Specifically, I think that parameter fi in the Drake equation is rrreeeeeeaaaaalllly low, given how many factors there are in the probability chain there are that we exist (spiral galaxy, not a dead globule; far out in a boring arm, so we're not to close to the center and it's lethal radiation; yellow star, not white hot and short-lived, or dwarfish and not enough energy; (cont.) $\endgroup$
    – RonJohn
    Mar 9, 2023 at 19:59
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    $\begingroup$ (part 2) planet in the goldilocks zone; that's not too big or small; another stray proto-planet hits us, forming a moon instead of shattering Earth; the moon doesn't tidally lock the Earth; we don't turn into Venusian hell, and don't get stuck in Snowball; all the "right" mutations happen along at "just" the right time for us to evolve; we're not wiped out by the aftermath of the Toba super-volcano eruption 75,000 years ago.... And don't forget that we and our ancestors were stuck in the paleolithic era for 2.4 MILLION YEARS. Only 10,000 years ago did we enter the Mesolithic. (cont.) $\endgroup$
    – RonJohn
    Mar 9, 2023 at 20:10
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    $\begingroup$ (part 3) Thus, our very existence is reason for them to be curious about us. $\endgroup$
    – RonJohn
    Mar 9, 2023 at 20:11
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    $\begingroup$ Well the Drake equation is basically nonsense, but the OP's question was essentially "What would mark our world as an interesting one worth checking out?" and he doesn't say anything about their motivation, but I would assume signs of life would make us "interesting". An oxygenated atmosphere would not be stable naturally without something happening here, hence it would attract attention. $\endgroup$
    – workerjoe
    Mar 9, 2023 at 20:48
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Plate tectonics

Other answers have covered various aspects of "Earth has life." But what about the big ol' rock herself?

It turns out that Earth is the only planet (that we know of) to have plate tectonics. This would be an irresistible curiosity for alien geologists, as subduction zones would contain naturally formed rocks that would be entirely unfamiliar to them.

We were able to observe the crust structure on Venus with orbital radar to confirm that it lacks subduction zones entirely. But it's much easier to detect a presence than an absence - sufficiently powerful telescopes should be able to show aliens doing a fly-by that there is something funky going on with Earth to give it all these jagged mountains, and a closer look would be required.

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  • $\begingroup$ Plate tectonics might be of interest, but how exactly would they detect such a thing remotely? I imagine studdy from orbit might show some give away signs, but they would have to be very close to detect plate tectonics. $\endgroup$
    – Slarty
    Mar 10, 2023 at 22:19
  • $\begingroup$ The distinctive shape of Earth's mountains is visible from afar with good optics, and has no other explanation $\endgroup$
    – SPavel
    Mar 10, 2023 at 22:54
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This was going to be a response to the answer from @Lyall Stewart in Agreement with @Nosajimiki. but became a bit long. In the galactic population, are predator or prey species the type that generally evolves to sentience? There is a genera(?) of Sci-Fi where humans are referred to as "Deathworlders" because they are the rare exception to sapiants since usually prey species are the ones who commonly evolve, sometime even more rare, Omnivores.

If the earth was much larger, achieving the tech necessary to counteract 9.81 m/s² acceleration to leave its gravity well is approaching impossible with chemical rockets. Thus Just by being born on a ""High G planet" Humans may be among the most physically strong and durable spacefaring sapiants.

We breath one of the most reactive / corrosive elements on the periodic table, Oxygen.

50 to 60 milligrams of nicotine is deadly to a 150lb adult. We consume this substance as regularly and recreationally.

98.6 F (37 C) resting human body temp, is enough to cause serious burns to some species if physically touched.

In less than 8 generations from when humans developed industrialization they were able to destroy the nearly all life on their home planet.

Population expansion ratios are among the highest of native species above 50kg.

Im sure hundreds more examples like these can be suggested for a universe where humans/earth life is exceptional in the universe.

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  • $\begingroup$ Fascinating, but what does this have to do with the question? $\endgroup$
    – Slarty
    Mar 10, 2023 at 22:16
  • $\begingroup$ @Slarty slave labor? Vassal shock troops? Or just squish us before we escape into the wild. $\endgroup$
    – Gillgamesh
    Mar 11, 2023 at 5:19
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I think you are asking what might cause aliens on distant worlds take an interest in Earth, of all planets.

The answer is certainly an atmospheric composition that betrays the existence of life. The atmosphere of exoplanets is examined by spectral analysis, which can be done with the proper telescopes. Humans are at the cusp of deploying telescopes capable enough of doing that routinely, like the JWST. One answer mentions free oxygen; but anything off-balance is a sign for life. This article appears to give a good overview.


If, by contrast, you ask what aliens might find interesting once they are here (or what they may find interesting in alien civilizations in general), a number of ideas have been explored in the past:

  • In Charles Stross' Singularity Sky, an entity called The Festival drops cell phones from orbit and promises through them: "Entertain us and we will give you anything you want." They are a nomadic intelligence absorbing information and stories.

    I think information, in particular cultural information like songs, texts and images, are the most likely items of value Earth could offer spacefaring civilizations. Additionally, actual material artifacts may have value as well.

  • Robe Reid imagines, in his novel Year Zero, that by a twist of fate humans are the only race in the universe which is able to produce good music. Terranean music makes aliens ecstatic. It is cult. They name their heavy metals after rock bands: vanhalium, slayerium, megadeathium.

    "What about bonjovium?" I asked. I've always had a weakness for "You Give Love a Bad Name".
    "Of course it exists. But bonjovium is certainly not a heavy metal by our standards." Özzy sniffed. "It has an atomic number of just fifty. You call it 'tin'."

    The copyright for the Terranean trove of music is of immeasurable value.

  • In the DUST short film Final Offer, featuring Anna Hopkins who also appeared in the Expanse, the aliens are interested in the water on Earth. Given that part of the water on Earth probably comes from asteroids, it seems easier to harvest those than to siphon it up from the bottom of this gravity well. (But the short is still very well done and entertaining.)

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  • $\begingroup$ I do like trying to get a new angle on things. I was trying to develop the idea of something very different from us. Perhaps some sort of complex machine intelligence with a mission to survey star systems, maybe something that emerged into our spacetime from elsewhere with no comprehension of biology. Something that has catalogued hundreds of barren system and then discovers the Earth. The shear novelty and variety of Earth with billions of moving objects swarming over its surface at scales ranging from km (ships) to mm (ants), unnatural transmissions of radio frequencies in all directions. $\endgroup$
    – Slarty
    Mar 11, 2023 at 14:27
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Artificial Computer Intelligence.

We don't know if life is anywhere else in the galaxy, however if there is, there is an argument that such life would usually likely be microbial, non-technological life.

Life has been present on Earth for a long time (over 3 billion years), yet only in the last few dozen years have computers, and artificial life been possible.

So it is conceivable that Artificial Intelligence may be the unique 'turning point' alien civilisations may be looking for to encounter, or satiate curiosity or to conduct reconnaissance.

Such Artificial Intelligence could manifest itself in terms of transmitting radio, communication via light or some other evidence that causes alien civilisations to now take Earth seriously. Communication with our new AI could be at an amazingly fast, iterative, and improved rate, causing essentially the birth of a new era, influenced by / influencing the Alien Intelligence (which, by the way, may be an AI itself).

Plus it is no secret that AI may inevitably become a dominant force on this planet, and can easily also become a dominant force on all planets in the galaxy, so best to now have a look.

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  • An arguably sentient species that discovered the Greenhouse Effect more than a century ago and still argues about environmental protection.
    Consider the uptick of scripted-reality shows and a certain brand of daytime talk show, where people watch watch other people who are even more messed up than themselves to feel better about themselves. So the aliens set up really big antenna arrays and big-data signals processing to catch the annual once-in-a-century hurricane mixed with statements from industry lobbyists. (It is arguable if they would get understandable television signals at any distance, but big antennas may help.)
  • A species whose ability to hold counterfactual scenarios in their mind goes way beyond "if I climb down from this vegetation, the predactor will catch me, so I won't."
    Mankind produced works like Hamlet, the Lord of the Rings, and E.T. Some of them don't translate to alien audiences, but many do.
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    $\begingroup$ People don't argue about environmental protection because they don't understand the greenhouse effect. People argue about it because the damage to the environment is caused by us living a hedonist life style, and saying that lifestyle is how we should judge the goodness of our society. We have literally built a value system that revolves around how much we consume and throw away. $\endgroup$
    – user102593
    Mar 9, 2023 at 6:26
  • $\begingroup$ @RonJohn, yes, but thinking about hypotheticals is on the road to sentience. $\endgroup$
    – o.m.
    Mar 9, 2023 at 16:51
  • $\begingroup$ @o.m. yes, I see your point now. $\endgroup$
    – RonJohn
    Mar 9, 2023 at 17:53
  • $\begingroup$ In order to find out we argue about the greenhouse effect, they would first have to notice our planet at all, notice it has life, then notice it has intelligent life, then notice our transmissions and/or access the internet, and then learn our language ... none of which they would do without their interest being first piqued by something else. $\endgroup$ Mar 10, 2023 at 13:42
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Life on Earth tends towards the aggressive side, where it will destroy rather than assimilate competition. This can be useful if used correctly.

  • We are that strange planet on the edge of the galaxy where monsters live *
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  • $\begingroup$ How does an animal assimilate a different species? $\endgroup$
    – user102593
    Mar 9, 2023 at 12:23
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    $\begingroup$ "Destroy rather than assimilate". Have you seen Britain and the US lately? $\endgroup$
    – RonJohn
    Mar 9, 2023 at 15:13
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    $\begingroup$ Despite downvotes, I actually really like this answer, not so much because it fits any better than other examples but because Human aggression is a really useful plot mechanism. Transitioning humans into a dark forest style space community being who knows how far behind on the space race is hard. But if we are the only ones who spend significant time thinking about how to kill off competition, then we could be the most militarily advanced race in the local cluster and not even know it. $\endgroup$
    – Nosajimiki
    Mar 9, 2023 at 21:12
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Wood is rarer than gold on a cosmic scale.

Interstellar alien loggers use an alien James Webb Space Telescope and alien ramjets to find, and pillage, oxygen rich worlds. Evolution favours tree analogues on earth-like gravity.

Alien culture mines alien asteroid 16-Psyche; alien metal prices plummet and demand for alien wood increases.

Oxygen would attract their interest. Wood would keep it.

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  • $\begingroup$ Why do the aliens care about wood so much? Lots of things are rare without being valuable. In the sense of "Aliens care about unique and diverse life forms for their own sake," that's understandable; but wood implies they care about it as a source of fuel (absurd) or building material (unlikely except as a curiosity). $\endgroup$ Mar 11, 2023 at 9:57
  • $\begingroup$ If I can answer your question with a question: why do humans care about gold so much? wood (analogues) ARE valuable to humanity, and thus, we can assume that any species that evolved on a planet with wood analogues would also find it valuable, especially in a society that has effectively post-scarcity levels of metals. $\endgroup$ Mar 12, 2023 at 1:43
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Water

One feature of Earth that is unique among extraterrestrial planets is its abundance of water. The Blue Planet is covered in the stuff.

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  • $\begingroup$ Yes that might well be of interest lots of liquid water... and how far away would they be able to detect that? $\endgroup$
    – Slarty
    Mar 9, 2023 at 20:29
  • $\begingroup$ Humanity has identified about 1,400 exoplanets with significant water; and many of them have WAY more water than Earth does... so not really that rare or meaningful unto itself. $\endgroup$
    – Nosajimiki
    Mar 9, 2023 at 21:04
  • $\begingroup$ @Nosajimiki You're right, my fault. I didn't see in the question that they specified the scope was galactic. Within our solar system, on the other hand, we are the only planet with stable oceans on our surface. $\endgroup$
    – jtb
    Mar 9, 2023 at 21:41
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    $\begingroup$ Way to ruin my enjoyment of The Ice Pirates. Good job everyone. :-( $\endgroup$
    – T.E.D.
    Mar 10, 2023 at 15:34
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    $\begingroup$ This is underrated. We look for liquid water almost exclusively in exoplanets. It's not unreasonable that that would be the thing they detect from a distance, since it implies a planet in the habitable zone (as we understand it). $\endgroup$ Mar 10, 2023 at 19:46

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