Hello, friends!
Right now, there’s an interstellar object passing through our Solar System. Astronomers have named this object 3I/ATLAS, and in my last post I explained how 3I/ATLAS is just a comet and NOT an alien spacecraft. But let’s pretend for a moment that an alien spaceship were traveling through the Solar System, perhaps on some sort of survey mission. What would the aliens see when they turn their scientific instruments toward Earth? How much would they learn about us and our planet?
Much depends on how technologically advanced we imagine these aliens to be, of course, but we humans have been observing Earth from space for decades now. We know some things are pretty obvious about our planet, even when viewed from a great distance away. For a start, the aliens would notice that Earth has an abnormally large moon. They’d also notice that Earth has oceans. The glint of sunlight reflecting off water would give that away. And then there’s oxygen. The spectrographic fingerprints of oxygen are all over Earth’s atmosphere.
Do these aliens breathe oxygen like we do? Maybe, maybe not. Either way, Earth’s oxygen-rich atmosphere would make the aliens stop and think, “Huh, that’s weird.” Most planets don’t have atmospheres like that. Even weirder, though, the aliens would also detect traces of methane in our atmosphere. Methane is an easily oxidized chemical, so you’d think all that oxygen would oxidize any atmospheric methane out of existence pretty quick. Something must keep replenishing the methane as quickly as oxygen destroys it. Something alive, perhaps? It’s hard to guess if the aliens would reach that conclusion yet.
As the aliens draw nearer, they’d soon notice this odd green stuff covering much of Earth’s landmasses. To say that in a more technobabbly way, the extraterrestrials would detect a chemical substance with a strong reflection spectrum in green light (and an even stronger reflection spectrum in infrared). You and I know what all that green stuff is, but would the aliens figure it out? Do they have plants back home? Do their plants contain chlorophyll and perform photosynthesis like ours do? Hard to say, but Earth’s green stuff would at least make the aliens think, “Huh, that’s also weird.”
A few more things our hypothetical aliens would notice: a substantial ozone layer, continents shaped by recent (or possibly ongoing) tectonic activity, a complex hydrological cycle with water existing as a solid, a liquid, and a gas… oh, and radio emissions. Lots and lots of narrow-band, amplitude modulated radio emissions, which cannot possibly be a natural phenomenon.
The idea that aliens many lightyears away are watching I Love Lucy, The Brady Bunch, or The Dick Van Dyke Show is probably false. As radio/television broadcasts propagate through space, those signals grow weaker and eventually blend into the background radiation. But for the purposes of this blog post, we’re imagining that an alien spaceship is right here in the Solar System with us. They’re close enough to pick up our broadcasts. At that point, the aliens wouldn’t just think, “Huh, weird.” They’d be forced to conclude not only that life exists on Earth but that intelligent life exists on Earth (unless they start watching our news or listening to our talk radio; if they do that, they might second guess the “intelligent” part).
How much more could the aliens learn about us? Again, it depends on how technologically advanced these aliens are. Consider the stuff I listed in this blog post to be the bare minimum of what they’d know.
WHAT TO LEARN MORE?
This blog post is based off several research papers, which are listed below. Detecting life on a planet—even a planet teeming with life, like Earth—is more of a challenge than you might realize. If that’s a topic you want to learn more about, please check out some of the papers below, especially the first one (the one written by Carl Sagan).
- A Search for Life from the Galileo Spacecraft, by Carl Sagan et al.
- Galileo Multispectral Imaging of Earth, by Paul Geissler et al.
- Earthshine Observation of Vegetation and Implication for Life Detection on Other Planets, by Luc Arnold.
- Detection of Ocean Glint and Ozone Absorption Using LCROSS Earth Observations, by Tyler D. Robinson et al.
- Earth as an Exoplanet: A Two-dimensional Alien Map, by Siteng Fan et al.
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i-love-space.com 
The thing about modern radio waves is that most of it is now digitized and encrypted. Even old school analog VHF / UHF / FM ranges would be very hard to decipher if you didn’t know the way the signal was supposed to work. Although there would still be noticeable patterns, complex patterns that could clue the aliens into the fact that a fellow intelligence was producing them.
What might be a bigger giveaway, aside from the signs of our biosphere, is that the night side of the planet would be lit up. Although I guess they’d have to get fairly close to see that.
Over interstellar distances, my understandings is that a lot of satellite communications happen within the “water hole” range that is very quiet in interstellar space. But again, most of it is going to be heavily protocoled. The Vegans could probably detect us, but watching “I Love Lucy” would require a lot of decoding.
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The Carl Sagan paper mentioned the problem of deciphering our radio signals. Even NASA’s own space probes can’t do that (or at least the Galileo mission couldn’t do it). But it is abundantly obvious that information of some kind is encoded in those signal.
I kind of forgot about the galactic water hole range. My understanding is that we’re still not pumping enough energy into our broadcasts for them to make it too crazy far, but I’ll try to read up on that more, and I’ll try to follow up on that in a future blog post.
I also came across a comment thread about radar pulses being more powerful (and thus more detectable) than ordinary radio broadcasts. This was on Reddit or Quora; I can’t remember which. I couldn’t find a better source to back up that claim, so I left it out of this blog post, but that’s also something I want to follow up on in a future post.
As for Earth’s city lights, I was curious about that, and I read a paper on how to detect city lights on exoplanets. Apparently Earth’s city lights are still very dim. When viewed from a distance, the overall change in Earth’s luminosity is virtually undetectable. However, for the purposes of this blog post, the aliens are supposed to be here in the Solar System with us, and I wasn’t sure how well the information from that paper would apply, so I decided to leave it out. Here’s a link to that paper if your interested:
https://www.semanticscholar.org/paper/The-Detectability-of-Nightside-City-Lights-on-Beatty/4108c43900f96e3303b1dd8e406a78c84ab004ea
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Seth Shostak in a 2011 paper (warning: paywall) mentions the radar thing, but notes that they’re only pointed at a fraction of the sky at any one time. To me, that still means they’d still be detectable intermittently, although maybe not necessarily as originating from anything intelligent.
He also notes that standard TV and other civilian communications are several orders of magnitude too faint to be detected by contemporary SETI style equipment. Although if the aliens have a big enough antenna, they might be able to detect it, but that antenna would have to be four orders of magnitude larger to actually decode it. They’d have to be making a deliberate high resource effort.
The paper is also interesting reading. It indicates that any substantive communications is likely to be by optical lasers due to the bandwidth advantages. But that means the probability of anyone pointing a laser at us right now is vanishingly remote. Although he notes that some societies could choose to “ping” other star systems periodically to prompt emerging civilizations.
Thanks for the link! I’ll take a look.
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Thank you for the link! The radar thing really, really interested me, and I really wanted to read up on that, but I couldn’t find anything.
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